202-427: The Selfish Gene is a 1976 book on evolution by ethologist Richard Dawkins that promotes the gene-centred view of evolution , as opposed to views focused on the organism and the group . The book builds upon the thesis of George C. Williams 's Adaptation and Natural Selection (1966); it also popularized ideas developed during the 1960s by W. D. Hamilton and others. From the gene-centred view, it follows that
404-428: A "Tit-for-Tat" strategy (shown to be successful in game theory) as the most common, simple, and profitable choice. More generally, the objection has been made that The Selfish Gene discusses philosophical and moral questions that go beyond biological arguments, relying upon anthropomorphisms and careless analogies. The Selfish Gene was first published by Oxford University Press in 1976 in eleven chapters with
606-460: A gland . When the signal is received and acted on, negative feedback is provided to the receptor that stops the need for further signaling. The cannabinoid receptor type 1 (CB1), located at the presynaptic neuron , is a receptor that can stop stressful neurotransmitter release to the postsynaptic neuron; it is activated by endocannabinoids (ECs) such as anandamide ( N -arachidonoylethanolamide; AEA) and 2-arachidonoylglycerol (2-AG) via
808-551: A retrograde signaling process in which these compounds are synthesized by and released from postsynaptic neurons, and travel back to the presynaptic terminal to bind to the CB1 receptor for modulation of neurotransmitter release to obtain homeostasis. The polyunsaturated fatty acids (PUFAs) are lipid derivatives of omega-3 (docosahexaenoic acid, DHA , and eicosapentaenoic acid, EPA ) or of omega-6 (arachidonic acid, ARA ) are synthesized from membrane phospholipids and used as
1010-490: A DNA sequence within a chromosome is known as a locus . If the DNA sequence at a locus varies between individuals, the different forms of this sequence are called alleles. DNA sequences can change through mutations, producing new alleles. If a mutation occurs within a gene, the new allele may affect the trait that the gene controls, altering the phenotype of the organism. However, while this simple correspondence between an allele and
1212-570: A GC-biased E. coli mutator strain in 1967, along with the proposal of the neutral theory , established the plausibility of mutational explanations for molecular patterns, which are now common in the molecular evolution literature. For instance, mutation biases are frequently invoked in models of codon usage. Such models also include effects of selection, following the mutation-selection-drift model, which allows both for mutation biases and differential selection based on effects on translation. Hypotheses of mutation bias have played an important role in
1414-446: A change in lottery rules (for example, inheriting a defective gene "responsible" for a disorder) "cause" differences in outcome that might or might not occur? It certainly alters the likelihood of events, but a concatenation of contingencies decides what actually occurs. Dawkins thinks the use of "cause" as a statistical weighting is acceptable in common usage. Like Gould, Gabriel Dover in criticizing The Selfish Gene says: However, from
1616-514: A chromosome becoming duplicated (usually by genetic recombination ), which can introduce extra copies of a gene into a genome. Extra copies of genes are a major source of the raw material needed for new genes to evolve. This is important because most new genes evolve within gene families from pre-existing genes that share common ancestors. For example, the human eye uses four genes to make structures that sense light: three for colour vision and one for night vision ; all four are descended from
1818-478: A comparison with Dawkins's discussion of this very same point, it would seem both Gould's and Dover's comments are more a critique of his sloppy usage than a difference of views. Hull suggested a resolution based upon a distinction between replicators and interactors. The term "replicator" includes genes as the most fundamental replicators but possibly other agents, and interactor includes organisms but maybe other agents, much as do Dawkins's 'vehicles'. The distinction
2020-494: A constant and sufficient supply of the micronutrient while simultaneously avoiding excess levels. If an insufficient amount of copper is ingested for a short period of time, copper stores in the liver will be depleted. Should this depletion continue, a copper health deficiency condition may develop. If too much copper is ingested, an excess condition can result. Both of these conditions, deficiency and excess, can lead to tissue injury and disease. However, due to homeostatic regulation,
2222-491: A detailed examination of the concept of DNA as a look-up-table and the role of the cell in orchestrating the DNA-to-RNA transcription , indicating that by anyone's account the DNA is hardly the whole story. Thompson goes on to suggest that the cell-environment interrelationship has much to do with reproduction and inheritance, and a focus on the gene as a form of "information [that] passes through bodies and affects them but
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#17327649165082424-478: A form of "information [that] passes through bodies and affects them, but is not affected by them on its way through"". The philosophical/biological field of enactivism stresses the interaction of the living agent with its environment and the relation of probing the environment to cognition and adaptation. Gene activation depends upon the cellular milieu. An extended discussion of the contrasts between enactivism and Dawkins's views, and with their support by Dennett ,
2626-467: A gene can achieve its own selfish goals best by fostering a limited form of altruism at the level of individual animals." Gene selection provides one explanation for kin selection and eusociality , where organisms act altruistically, against their individual interests (in the sense of health, safety or personal reproduction), namely the argument that by helping related organisms reproduce, a gene succeeds in "helping" copies of themselves (or sequences with
2828-422: A gene, evidence is inconclusive whether this success is selected for at a group or individual level. In fact, Dawkins has proposed that it is at the level of the extended phenotype : Although Dawkins agrees that groups can assist survival, they rank as a "vehicle" for survival only if the group activity is replicated in descendants, recorded in the gene, the gene being the only true replicator. An improvement in
3030-403: A high plasma pH stimulates the kidneys to secrete hydrogen ions into the blood and to excrete bicarbonate into the urine. The hydrogen ions combine with the excess bicarbonate ions in the plasma, once again forming an excess of carbonic acid which can be exhaled, as carbon dioxide, in the lungs, keeping the plasma bicarbonate ion concentration, the partial pressure of carbon dioxide and, therefore,
3232-400: A hormone released into the blood by the kidney in response to high PTH levels in the blood, the duodenum and jejunum . Parathyroid hormone (in high concentrations in the blood) causes bone resorption , releasing calcium into the plasma. This is a very rapid action which can correct a threatening hypocalcemia within minutes. High PTH concentrations cause the excretion of phosphate ions via
3434-400: A lowering of the plasma sodium concentration, or to a fall in the arterial blood pressure, the juxtaglomerular cells release renin into the blood. Renin is an enzyme which cleaves a decapeptide (a short protein chain, 10 amino acids long) from a plasma α-2-globulin called angiotensinogen . This decapeptide is known as angiotensin I . It has no known biological activity. However, when
3636-402: A manner similar to a sequence of letters spelling out a sentence. Before a cell divides, the DNA is copied, so that each of the resulting two cells will inherit the DNA sequence. Portions of a DNA molecule that specify a single functional unit are called genes; different genes have different sequences of bases. Within cells, each long strand of DNA is called a chromosome . The specific location of
3838-405: A nutrient in a long-term laboratory experiment , Flavobacterium evolving a novel enzyme that allows these bacteria to grow on the by-products of nylon manufacturing, and the soil bacterium Sphingobium evolving an entirely new metabolic pathway that degrades the synthetic pesticide pentachlorophenol . An interesting but still controversial idea is that some adaptations might increase
4040-607: A person using birth control might also be enhancing the survival chances of their genes by limiting family size to conform with available resources, thus avoiding extinction as predicted under the Malthusian model of population growth. Dawkins says that his "purpose" in writing The Selfish Gene is "to examine the biology of selfishness and altruism." He does this by supporting the claim that "gene selfishness will usually give rise to selfishness in individual behaviour. However, as we shall see, there are special circumstances in which
4242-611: A phenomenon termed de novo gene birth . The generation of new genes can also involve small parts of several genes being duplicated, with these fragments then recombining to form new combinations with new functions ( exon shuffling ). When new genes are assembled from shuffling pre-existing parts, domains act as modules with simple independent functions, which can be mixed together to produce new combinations with new and complex functions. For example, polyketide synthases are large enzymes that make antibiotics ; they contain up to 100 independent domains that each catalyse one step in
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#17327649165084444-538: A population are therefore more likely to be replaced by the offspring of parents with favourable characteristics for that environment. In the early 20th century, competing ideas of evolution were refuted and evolution was combined with Mendelian inheritance and population genetics to give rise to modern evolutionary theory. In this synthesis the basis for heredity is in DNA molecules that pass information from generation to generation. The processes that change DNA in
4646-575: A population include natural selection, genetic drift, mutation , and gene flow . All life on Earth—including humanity —shares a last universal common ancestor (LUCA), which lived approximately 3.5–3.8 billion years ago. The fossil record includes a progression from early biogenic graphite to microbial mat fossils to fossilised multicellular organisms . Existing patterns of biodiversity have been shaped by repeated formations of new species ( speciation ), changes within species ( anagenesis ), and loss of species ( extinction ) throughout
4848-430: A population is not critical, but instead a measure known as the effective population size. The effective population is usually smaller than the total population since it takes into account factors such as the level of inbreeding and the stage of the lifecycle in which the population is the smallest. The effective population size may not be the same for every gene in the same population. It is usually difficult to measure
5050-433: A population through neutral transitions due to the principles of excess capacity, presuppression, and ratcheting, and it has been applied in areas ranging from the origins of the spliceosome to the complex interdependence of microbial communities . The time it takes a neutral allele to become fixed by genetic drift depends on population size; fixation is more rapid in smaller populations. The number of individuals in
5252-399: A precursor for endocannabinoids (ECs) mediate significant effects in the fine-tuning adjustment of body homeostasis. The word homeostasis ( / ˌ h oʊ m i oʊ ˈ s t eɪ s ɪ s / ) uses combining forms of homeo- and -stasis , Neo-Latin from Greek : ὅμοιος homoios , "similar" and στάσις stasis , "standing still", yielding the idea of "staying the same". The concept of
5454-487: A preface by the author and a foreword by Robert Trivers . A second edition was published in 1989. This edition added two extra chapters, and substantial endnotes to the preceding chapters, reflecting new findings and thoughts. It also added a second preface by the author, but the original foreword by Trivers was dropped. The book contains no illustrations. The book has been translated into at least 23 languages including Arabic, Thai and Turkish. Evolution Evolution
5656-402: A process of acclimatisation . Homeostasis does not govern every activity in the body. For instance, the signal (be it via neurons or hormones ) from the sensor to the effector is, of necessity, highly variable in order to convey information about the direction and magnitude of the error detected by the sensor. Similarly, the effector's response needs to be highly adjustable to reverse
5858-435: A range of blood pressure values by vasoconstriction and vasodilation of the arteries. High pressure receptors called baroreceptors in the walls of the aortic arch and carotid sinus (at the beginning of the internal carotid artery ) monitor the arterial blood pressure . Rising pressure is detected when the walls of the arteries stretch due to an increase in blood volume . This causes heart muscle cells to secrete
6060-499: A range of genes from bacteria, fungi and plants. Viruses can also carry DNA between organisms, allowing transfer of genes even across biological domains . Large-scale gene transfer has also occurred between the ancestors of eukaryotic cells and bacteria, during the acquisition of chloroplasts and mitochondria . It is possible that eukaryotes themselves originated from horizontal gene transfers between bacteria and archaea . Some heritable changes cannot be explained by changes to
6262-430: A reduction in scope when it was discovered that (1) GC-biased gene conversion makes an important contribution to composition in diploid organisms such as mammals and (2) bacterial genomes frequently have AT-biased mutation. Contemporary thinking about the role of mutation biases reflects a different theory from that of Haldane and Fisher. More recent work showed that the original "pressures" theory assumes that evolution
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6464-452: A single ancestral gene. New genes can be generated from an ancestral gene when a duplicate copy mutates and acquires a new function. This process is easier once a gene has been duplicated because it increases the redundancy of the system; one gene in the pair can acquire a new function while the other copy continues to perform its original function. Other types of mutations can even generate entirely new genes from previously noncoding DNA,
6666-419: A single chromosome compared to expectations , which is called their linkage disequilibrium . A set of alleles that is usually inherited in a group is called a haplotype . This can be important when one allele in a particular haplotype is strongly beneficial: natural selection can drive a selective sweep that will also cause the other alleles in the haplotype to become more common in the population; this effect
6868-431: A single individual with a tendency towards more selfish behaviour to undermine a population otherwise filled only with the gene for altruism towards non-kin. The Selfish Gene was extremely popular when first published, causing "a silent and almost immediate revolution in biology", and it continues to be widely read. It has sold over a million copies and has been translated into more than 25 languages. Proponents argue that
7070-420: A species or population, in particular shifts in allele frequency and adaptation. Macroevolution is the outcome of long periods of microevolution. Thus, the distinction between micro- and macroevolution is not a fundamental one—the difference is simply the time involved. However, in macroevolution, the traits of the entire species may be important. For instance, a large amount of variation among individuals allows
7272-544: A species to rapidly adapt to new habitats , lessening the chance of it going extinct, while a wide geographic range increases the chance of speciation, by making it more likely that part of the population will become isolated. In this sense, microevolution and macroevolution might involve selection at different levels—with microevolution acting on genes and organisms, versus macroevolutionary processes such as species selection acting on entire species and affecting their rates of speciation and extinction. A common misconception
7474-476: A step further to suggest that our genetic programming can be overcome by what amounts to an extreme version of free will . Part of Mary Midgley's concern is that Richard Dawkins's account of The Selfish Gene serves as a moral and ideological justification for selfishness to be adopted by modern human societies as simply following "nature", providing an excuse for behavior with bad consequences for future human society. Dawkins's major concluding theme, that humanity
7676-411: A striking example are people with the inherited trait of albinism , who do not tan at all and are very sensitive to sunburn . Heritable characteristics are passed from one generation to the next via DNA , a molecule that encodes genetic information. DNA is a long biopolymer composed of four types of bases. The sequence of bases along a particular DNA molecule specifies the genetic information, in
7878-424: A subsequent increase in hemoglobin that increases the oxygen carrying capacity. This is the mechanism whereby high altitude dwellers have higher hematocrits than sea-level residents, and also why persons with pulmonary insufficiency or right-to-left shunts in the heart (through which venous blood by-passes the lungs and goes directly into the systemic circulation) have similarly high hematocrits. Regardless of
8080-472: A system in which organisms interact with every other element, physical as well as biological , in their local environment. Eugene Odum , a founder of ecology, defined an ecosystem as: "Any unit that includes all of the organisms...in a given area interacting with the physical environment so that a flow of energy leads to clearly defined trophic structure, biotic diversity, and material cycles (i.e., exchange of materials between living and nonliving parts) within
8282-428: A trait works in some cases, most traits are influenced by multiple genes in a quantitative or epistatic manner. Evolution can occur if there is genetic variation within a population. Variation comes from mutations in the genome, reshuffling of genes through sexual reproduction and migration between populations ( gene flow ). Despite the constant introduction of new variation through mutation and gene flow, most of
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8484-454: Is directional selection , which is a shift in the average value of a trait over time—for example, organisms slowly getting taller. Secondly, disruptive selection is selection for extreme trait values and often results in two different values becoming most common, with selection against the average value. This would be when either short or tall organisms had an advantage, but not those of medium height. Finally, in stabilising selection there
8686-442: Is more noticeable . Indeed, the evolution of microorganisms is particularly important to evolutionary research since their rapid reproduction allows the study of experimental evolution and the observation of evolution and adaptation in real time. Adaptation is the process that makes organisms better suited to their habitat. Also, the term adaptation may refer to a trait that is important for an organism's survival. For example,
8888-427: Is a byproduct of this process that may sometimes be adaptively beneficial. Gene flow is the exchange of genes between populations and between species. It can therefore be a source of variation that is new to a population or to a species. Gene flow can be caused by the movement of individuals between separate populations of organisms, as might be caused by the movement of mice between inland and coastal populations, or
9090-417: Is an inherited characteristic and an individual might inherit the "brown-eye trait" from one of their parents. Inherited traits are controlled by genes and the complete set of genes within an organism's genome (genetic material) is called its genotype . The complete set of observable traits that make up the structure and behaviour of an organism is called its phenotype . Some of these traits come from
9292-433: Is an integral part of the antioxidant enzyme copper-zinc superoxide dismutase, and has a role in iron homeostasis as a cofactor in ceruloplasmin. Changes in the levels of oxygen, carbon dioxide, and plasma pH are sent to the respiratory center , in the brainstem where they are regulated. The partial pressure of oxygen and carbon dioxide in the arterial blood is monitored by the peripheral chemoreceptors ( PNS ) in
9494-432: Is as follows: Hull suggests that, despite some similarities, Dawkins takes too narrow a view of these terms, engendering some of the objections to his views. According to Godfrey-Smith, this more careful vocabulary has cleared up "misunderstandings in the "units of selection" debates." Behavioural genetics entertains the view: This view from 1970 is still espoused today, and conflicts with Dawkins's view of "the gene as
9696-464: Is based on standing variation: when evolution depends on events of mutation that introduce new alleles, mutational and developmental biases in the introduction of variation (arrival biases) can impose biases on evolution without requiring neutral evolution or high mutation rates. Several studies report that the mutations implicated in adaptation reflect common mutation biases though others dispute this interpretation. Recombination allows alleles on
9898-407: Is best for the survival of the parents' genes is not always best for individual children. Similarly, Dawkins argues, there are conflicts of interest between males and females, but he notes that R. A. Fisher showed that the optimal sex ratio is 50:50. He explains that this is true even in an extreme case like the harem-keeping elephant seal , where 4% of the males get 88% of copulations. In that case,
10100-407: Is called deep homology . During evolution, some structures may lose their original function and become vestigial structures. Such structures may have little or no function in a current species, yet have a clear function in ancestral species, or other closely related species. Examples include pseudogenes , the non-functional remains of eyes in blind cave-dwelling fish, wings in flightless birds,
10302-692: Is called genetic hitchhiking or genetic draft. Genetic draft caused by the fact that some neutral genes are genetically linked to others that are under selection can be partially captured by an appropriate effective population size. A special case of natural selection is sexual selection, which is selection for any trait that increases mating success by increasing the attractiveness of an organism to potential mates. Traits that evolved through sexual selection are particularly prominent among males of several animal species. Although sexually favoured, traits such as cumbersome antlers, mating calls, large body size and bright colours often attract predation, which compromises
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#173276491650810504-510: Is confusing, misleading, or simply silly to ascribe mental attributes to something that is mindless. For example, Andrew Brown has written: Donald Symons also finds it inappropriate to use anthropomorphism in conveying scientific meaning in general, and particularly in this instance. He writes in The Evolution of Human Sexuality (1979): Dawkins proposes the idea of the "replicator": The original replicator (Dawkins R eplicator )
10706-424: Is finally gaining power over the "selfish replicators" by virtue of their intelligence, is criticized also by primatologist Frans de Waal , who refers to it as an example of a " veneer theory " (the idea that morality is not fundamental, but is laid over a brutal foundation). Dawkins claims he merely describes how things are under evolution, and makes no moral arguments. On BBC-2 TV, Dawkins pointed to evidence for
10908-404: Is homeostatically controlled it does not imply that its value is necessarily absolutely steady in health. Core body temperature is, for instance, regulated by a homeostatic mechanism with temperature sensors in, amongst others, the hypothalamus of the brain . However, the set point of the regulator is regularly reset. For instance, core body temperature in humans varies during the course of
11110-433: Is limited in its capacity to respond to extreme temperatures. When the core temperature falls, the blood supply to the skin is reduced by intense vasoconstriction . The blood flow to the limbs (which have a large surface area) is similarly reduced and returned to the trunk via the deep veins which lie alongside the arteries (forming venae comitantes ). This acts as a counter-current exchange system that short-circuits
11312-413: Is made that if the organism becomes intelligent enough to understand its own interests, as distinct from those of its genes, there can be true conflict. An example of such a conflict might be a person using birth control to prevent fertilisation, thereby inhibiting the replication of his or her genes. But this action might not be a conflict of the 'self-interest' of the organism with his or her genes, since
11514-594: Is measured by an organism's ability to survive and reproduce, which determines the size of its genetic contribution to the next generation. However, fitness is not the same as the total number of offspring: instead fitness is indicated by the proportion of subsequent generations that carry an organism's genes. For example, if an organism could survive well and reproduce rapidly, but its offspring were all too small and weak to survive, this organism would make little genetic contribution to future generations and would thus have low fitness. If an allele increases fitness more than
11716-454: Is more of a deciding factor in the monitoring of pH. However, at high altitude (above 2500 m) the monitoring of the partial pressure of oxygen takes priority, and hyperventilation keeps the oxygen level constant. With the lower level of carbon dioxide, to keep the pH at 7.4 the kidneys secrete hydrogen ions into the blood and excrete bicarbonate into the urine. This is important in acclimatization to high altitude . The kidneys measure
11918-502: Is no change in the osmolality of the ECF, and therefore no change in the ADH concentration of the plasma. However, low aldosterone levels cause a loss of sodium ions from the ECF, which could potentially cause a change in extracellular osmolality and therefore of ADH levels in the blood. High potassium concentrations in the plasma cause depolarization of the zona glomerulosa cells' membranes in
12120-403: Is not affected by them on its way through" is tantamount to adoption of a form of material-informational dualism that has no explanatory value and no scientific basis. (Thomson, p. 187) The enactivist view, however, is that information results from the probing and experimentation of the agent with the agent's environment subject to the limitations of the agent's abilities to probe and process
12322-435: Is not the species, nor the group, nor even strictly the individual. It is the gene, the unit of heredity." However, he continues in a later chapter: Dawkins's later formulation is in his book The Extended Phenotype (1982), where the process of selection is taken to involve every possible phenotypical effect of a gene. Stephen Jay Gould finds Dawkins's position tries to have it both ways: The view of The Selfish Gene
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#173276491650812524-409: Is not whether the behavior is to the benefit of the behaver, but whether it is to the benefit of the gene G ...With altruism this will happen only if the affected individual is a relative of the altruist, therefore having an increased chance of carrying the gene. Wilkins and Hull (2014) provide an extended discussion of Dawkins's views and of his book The Selfish Gene . Dawkins begins by discussing
12726-508: Is provided by Thompson. In Mind in Life , the philosopher Evan Thompson has assembled a multi-sourced objection to the "selfish gene" idea. Thompson takes issue with Dawkin's reduction of "life" to "genes" and "information": Thompson objects that the gene cannot operate by itself, since it requires an environment such as a cell, and life is "the creative outcome of highly structured contingencies". Thompson quotes Sarkar: Thompson follows with
12928-421: Is selection against extreme trait values on both ends, which causes a decrease in variance around the average value and less diversity. This would, for example, cause organisms to eventually have a similar height. Natural selection most generally makes nature the measure against which individuals and individual traits, are more or less likely to survive. "Nature" in this sense refers to an ecosystem , that is,
13130-402: Is stimulated to rise ( tachycardia ) when the arterial blood pressure falls, or to slow down ( bradycardia ) when the pressure rises above the set point. Thus the heart rate (for which there is no sensor in the body) is not homeostatically controlled but is one of the effector responses to errors in arterial blood pressure. Another example is the rate of sweating . This is one of the effectors in
13332-429: Is that evolution has goals, long-term plans, or an innate tendency for "progress", as expressed in beliefs such as orthogenesis and evolutionism; realistically, however, evolution has no long-term goal and does not necessarily produce greater complexity. Although complex species have evolved, they occur as a side effect of the overall number of organisms increasing, and simple forms of life still remain more common in
13534-525: Is that selection based upon groups and populations is rare compared to selection on individuals. Although supported by Dawkins and by many others, this claim continues to be disputed. While naïve versions of group selectionism have been disproved, more sophisticated formulations make accurate predictions in some cases while positing selection at higher levels. Both sides agree that very favourable genes are likely to prosper and replicate if they arise and both sides agree that living in groups can be an advantage to
13736-417: Is that the genes that are passed on are the ones whose evolutionary consequences serve their own implicit interest (to continue the anthropomorphism ) in being replicated, not necessarily those of the organism. In later work, Dawkins brings evolutionary "selfishness" down to creation of a widely proliferated extended phenotype . For some, the metaphor of "selfishness" is entirely clear, while to others it
13938-447: Is the nearly neutral theory , according to which a mutation that would be effectively neutral in a small population is not necessarily neutral in a large population. Other theories propose that genetic drift is dwarfed by other stochastic forces in evolution, such as genetic hitchhiking, also known as genetic draft. Another concept is constructive neutral evolution (CNE), which explains that complex systems can emerge and spread into
14140-500: Is the change in the heritable characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, resulting in certain characteristics becoming more or less common within a population over successive generations. The process of evolution has given rise to biodiversity at every level of biological organisation . The scientific theory of evolution by natural selection
14342-495: Is the more common means of reproduction among eukaryotes and multicellular organisms. The Red Queen hypothesis has been used to explain the significance of sexual reproduction as a means to enable continual evolution and adaptation in response to coevolution with other species in an ever-changing environment. Another hypothesis is that sexual reproduction is primarily an adaptation for promoting accurate recombinational repair of damage in germline DNA, and that increased diversity
14544-439: Is the process by which traits that enhance survival and reproduction become more common in successive generations of a population. It embodies three principles: More offspring are produced than can possibly survive, and these conditions produce competition between organisms for survival and reproduction. Consequently, organisms with traits that give them an advantage over their competitors are more likely to pass on their traits to
14746-439: Is the state of steady internal physical and chemical conditions maintained by living systems . This is the condition of optimal functioning for the organism and includes many variables, such as body temperature and fluid balance , being kept within certain pre-set limits (homeostatic range). Other variables include the pH of extracellular fluid , the concentrations of sodium , potassium , and calcium ions , as well as
14948-438: Is therefore that hydrogen ions are lost in the urine when the pH of the plasma falls. The concomitant rise in the plasma bicarbonate mops up the increased hydrogen ions (caused by the fall in plasma pH) and the resulting excess carbonic acid is disposed of in the lungs as carbon dioxide. This restores the normal ratio between bicarbonate and the partial pressure of carbon dioxide and therefore the plasma pH. The converse happens when
15150-435: Is thought to be the central motivation for all organic action. All homeostatic control mechanisms have at least three interdependent components for the variable being regulated: a receptor, a control center, and an effector. The receptor is the sensing component that monitors and responds to changes in the environment, either external or internal. Receptors include thermoreceptors and mechanoreceptors . Control centers include
15352-508: Is to define a problem arising during empirical research, then select or devise the theory that is needed to solve it. Almost all research in inclusive-fitness theory has been the opposite: hypothesize the key roles of kinship and kin selection, then look for evidence to test that hypothesis." According to Wilson: "People must have a tribe...Experiments conducted over many years by social psychologists have revealed how swiftly and decisively people divide into groups, and then discriminate in favor of
15554-427: The autonomic nervous system are stimulated to influence the activity of chiefly the heart and the smallest diameter arteries, called arterioles . The arterioles are the main resistance vessels in the arterial tree , and small changes in diameter cause large changes in the resistance to flow through them. When the arterial blood pressure rises the arterioles are stimulated to dilate making it easier for blood to leave
15756-406: The blood sugar level , and these need to be regulated despite changes in the environment, diet, or level of activity. Each of these variables is controlled by one or more regulators or homeostatic mechanisms, which together maintain life. Homeostasis is brought about by a natural resistance to change when already in optimal conditions, and equilibrium is maintained by many regulatory mechanisms; it
15958-457: The brain 's ability to simulate the world with subjective consciousness , and signalling between species . He then introduces the idea of the evolutionarily stable strategy , and uses it to explain why alternative competitive strategies like bullying and retaliating exist. This allows him to consider what selfishness in a gene might actually mean, describing W. D. Hamilton 's argument for kin selection , that genes for behaviour that improves
16160-433: The carotid artery and aortic arch . A change in the partial pressure of carbon dioxide is detected as altered pH in the cerebrospinal fluid by central chemoreceptors ( CNS ) in the medulla oblongata of the brainstem . Information from these sets of sensors is sent to the respiratory center which activates the effector organs – the diaphragm and other muscles of respiration . An increased level of carbon dioxide in
16362-400: The central nervous system play a homeostatic role in the balance of neuronal activity between excitation and inhibition. Inhibitory neurons using GABA , make compensating changes in the neuronal networks preventing runaway levels of excitation. An imbalance between excitation and inhibition is seen to be implicated in a number of neuropsychiatric disorders . The neuroendocrine system is
16564-448: The hypothalamus detects a hypertonic extracellular environment, it causes the secretion of an antidiuretic hormone (ADH) called vasopressin which acts on the effector organ, which in this case is the kidney . The effect of vasopressin on the kidney tubules is to reabsorb water from the distal convoluted tubules and collecting ducts , thus preventing aggravation of the water loss via the urine. The hypothalamus simultaneously stimulates
16766-411: The medulla oblongata of the brain indicating whether the blood pressure has fallen or risen, and by how much. The medulla oblongata then distributes messages along motor or efferent nerves belonging to the autonomic nervous system to a wide variety of effector organs, whose activity is consequently changed to reverse the error in the blood pressure. One of the effector organs is the heart whose rate
16968-425: The neutral theory of molecular evolution most evolutionary changes are the result of the fixation of neutral mutations by genetic drift. In this model, most genetic changes in a population are thus the result of constant mutation pressure and genetic drift. This form of the neutral theory has been debated since it does not seem to fit some genetic variation seen in nature. A better-supported version of this model
17170-454: The product of a gene , or prevent the gene from functioning, or have no effect. About half of the mutations in the coding regions of protein-coding genes are deleterious — the other half are neutral. A small percentage of the total mutations in this region confer a fitness benefit. Some of the mutations in other parts of the genome are deleterious but the vast majority are neutral. A few are beneficial. Mutations can involve large sections of
17372-408: The renal tubular fluid after it has already undergone a certain amount of modification in the proximal convoluted tubule and loop of Henle . These cells also respond to rate of blood flow through the juxtaglomerular apparatus, which, under normal circumstances, is directly proportional to the arterial blood pressure , making this tissue an ancillary arterial blood pressure sensor. In response to
17574-460: The renin–angiotensin system , control more than one variable. When the receptor senses a stimulus, it reacts by sending action potentials to a control center. The control center sets the maintenance range—the acceptable upper and lower limits—for the particular variable, such as temperature. The control center responds to the signal by determining an appropriate response and sending signals to an effector , which can be one or more muscles, an organ, or
17776-433: The respiratory center and the renin-angiotensin system . An effector is the target acted on, to bring about the change back to the normal state. At the cellular level, effectors include nuclear receptors that bring about changes in gene expression through up-regulation or down-regulation and act in negative feedback mechanisms. An example of this is in the control of bile acids in the liver . Some centers, such as
17978-416: The zona glomerulosa of the adrenal glands has an effect on particularly the epithelial cells of the distal convoluted tubules and collecting ducts of the kidneys. Here it causes the reabsorption of sodium ions from the renal tubular fluid , in exchange for potassium ions which are secreted from the blood plasma into the tubular fluid to exit the body via the urine. The reabsorption of sodium ions from
18180-519: The ability of organisms to generate genetic diversity and adapt by natural selection (increasing organisms' evolvability). Adaptation occurs through the gradual modification of existing structures. Consequently, structures with similar internal organisation may have different functions in related organisms. This is the result of a single ancestral structure being adapted to function in different ways. The bones within bat wings, for example, are very similar to those in mice feet and primate hands, due to
18382-402: The adaptation of horses' teeth to the grinding of grass. By using the term adaptation for the evolutionary process and adaptive trait for the product (the bodily part or function), the two senses of the word may be distinguished. Adaptations are produced by natural selection. The following definitions are due to Theodosius Dobzhansky: Adaptation may cause either the gain of a new feature, or
18584-518: The addition of Dawkins's book to the country's consciousness, the term "meme" entered popular culture. Yuzuru Tanaka of Hokkaido University wrote a book, Meme Media and Meme Market Architectures , while the psychologist Susan Blackmore wrote The Meme Machine (2000), with a foreword by Dawkins. The information scientist Osamu Sakura has published a book in Japanese and several papers in English on
18786-507: The alleles are subject to sampling error . This drift halts when an allele eventually becomes fixed, either by disappearing from the population or by replacing the other alleles entirely. Genetic drift may therefore eliminate some alleles from a population due to chance alone. Even in the absence of selective forces, genetic drift can cause two separate populations that begin with the same genetic structure to drift apart into two divergent populations with different sets of alleles. According to
18988-399: The alpha cells into the blood. This inhibits the uptake of glucose from the blood by the liver, fats cells, and muscle. Instead the liver is strongly stimulated to manufacture glucose from glycogen (through glycogenolysis ) and from non-carbohydrate sources (such as lactate and de-aminated amino acids ) using a process known as gluconeogenesis . The glucose thus produced is discharged into
19190-562: The altruism that people display, indicating that he will argue it is explained by gene selfishness, and attacking group selection as an explanation. He considers the origin of life with the arrival of molecules able to replicate themselves. From there, he looks at DNA 's role in evolution , and its organisation into chromosomes and genes , which in his view behave selfishly. He describes organisms as apparently purposive but fundamentally simple survival machines, which use negative feedback to achieve control . This extends, he argues, to
19392-436: The arteries, thus deflating them, and bringing the blood pressure down, back to normal. At the same time, the heart is stimulated via cholinergic parasympathetic nerves to beat more slowly (called bradycardia ), ensuring that the inflow of blood into the arteries is reduced, thus adding to the reduction in pressure, and correcting the original error. Low pressure in the arteries, causes the opposite reflex of constriction of
19594-412: The arterioles, and a speeding up of the heart rate (called tachycardia ). If the drop in blood pressure is very rapid or excessive, the medulla oblongata stimulates the adrenal medulla , via "preganglionic" sympathetic nerves , to secrete epinephrine (adrenaline) into the blood. This hormone enhances the tachycardia and causes severe vasoconstriction of the arterioles to all but the essential organ in
19796-402: The basis that it is not subject to selection. These unselected for but transmitted DNA variations connect the individual genetically to its parents but confer no survival benefit. These examples might suggest that there is a power struggle between genes and their interactor. In fact, the claim is that there isn't much of a struggle because the genes usually win without a fight. However, the claim
19998-406: The benefits to the organism or even population were supposed to account for the popularity of the genes responsible for the tendency towards that behaviour. Modern versions of "multilevel selection" claim to have overcome the original objections, namely, that at that time no known form of group selection led to an evolutionarily stable strategy. The claim still is made by some that it would take only
20200-407: The biological term of homeostasis. The metabolic processes of all organisms can only take place in very specific physical and chemical environments. The conditions vary with each organism, and with whether the chemical processes take place inside the cell or in the interstitial fluid bathing the cells. The best-known homeostatic mechanisms in humans and other mammals are regulators that keep
20402-441: The biosphere. For example, the overwhelming majority of species are microscopic prokaryotes , which form about half the world's biomass despite their small size and constitute the vast majority of Earth's biodiversity. Simple organisms have therefore been the dominant form of life on Earth throughout its history and continue to be the main form of life up to the present day, with complex life only appearing more diverse because it
20604-467: The blood circulates through the lungs a pulmonary capillary endothelial enzyme called angiotensin-converting enzyme (ACE) cleaves a further two amino acids from angiotensin I to form an octapeptide known as angiotensin II . Angiotensin II is a hormone which acts on the adrenal cortex , causing the release into the blood of the steroid hormone , aldosterone . Angiotensin II also acts on the smooth muscle in
20806-399: The blood correcting the detected error ( hypoglycemia ). The glycogen stored in muscles remains in the muscles, and is only broken down, during exercise, to glucose-6-phosphate and thence to pyruvate to be fed into the citric acid cycle or turned into lactate . It is only the lactate and the waste products of the citric acid cycle that are returned to the blood. The liver can take up only
21008-419: The blood, or a decreased level of oxygen, will result in a deeper breathing pattern and increased respiratory rate to bring the blood gases back to equilibrium. Too little carbon dioxide, and, to a lesser extent, too much oxygen in the blood can temporarily halt breathing, a condition known as apnea , which freedivers use to prolong the time they can stay underwater. The partial pressure of carbon dioxide
21210-401: The blood, the latter combines with the excess hydrogen ions in the plasma that stimulated the kidneys to perform this operation. The resulting reaction in the plasma is the formation of carbonic acid which is in equilibrium with the plasma partial pressure of carbon dioxide. This is tightly regulated to ensure that there is no excessive build-up of carbonic acid or bicarbonate. The overall effect
21412-598: The blood. This combination (high blood insulin levels and low glucagon levels) act on effector tissues, the chief of which is the liver , fat cells , and muscle cells . The liver is inhibited from producing glucose , taking it up instead, and converting it to glycogen and triglycerides . The glycogen is stored in the liver, but the triglycerides are secreted into the blood as very low-density lipoprotein (VLDL) particles which are taken up by adipose tissue , there to be stored as fats. The fat cells take up glucose through special glucose transporters ( GLUT4 ), whose numbers in
21614-502: The body (especially the heart, lungs, and brain). These reactions usually correct the low arterial blood pressure ( hypotension ) very effectively. The plasma ionized calcium (Ca ) concentration is very tightly controlled by a pair of homeostatic mechanisms. The sensor for the first one is situated in the parathyroid glands , where the chief cells sense the Ca level by means of specialized calcium receptors in their membranes. The sensors for
21816-479: The book The Immortal Gene . In July 2017, a poll to celebrate the 30th anniversary of the Royal Society science book prize listed The Selfish Gene as the most influential science book of all time. Dawkins builds upon George C. Williams 's book Adaptation and Natural Selection (1966), which argued that altruism is not based upon group benefit per se , but results from selection that occurs "at
22018-407: The brain. Homeostasis is an almost exclusively biological term, referring to the concepts described by Bernard and Cannon, concerning the constancy of the internal environment in which the cells of the body live and survive. The term cybernetics is applied to technological control systems such as thermostats , which function as homeostatic mechanisms but are often defined much more broadly than
22220-618: The cell wall are increased as a direct effect of insulin acting on these cells. The glucose that enters the fat cells in this manner is converted into triglycerides (via the same metabolic pathways as are used by the liver) and then stored in those fat cells together with the VLDL-derived triglycerides that were made in the liver. Muscle cells also take glucose up through insulin-sensitive GLUT4 glucose channels, and convert it into muscle glycogen. A fall in blood glucose, causes insulin secretion to be stopped, and glucagon to be secreted from
22422-442: The cell, into the interstitial fluid and two potassium ions into the cell from the interstitial fluid. This creates an ionic concentration gradient which results in the reabsorption of sodium (Na ) ions from the tubular fluid into the blood, and secreting potassium (K ) ions from the blood into the urine (lumen of collecting duct). The total amount of water in the body needs to be kept in balance. Fluid balance involves keeping
22624-518: The central point, that replicating the gene is the object of selection, usefully completes and extends the explanation of evolution given by Charles Darwin before the basic mechanisms of genetics were understood. According to the ethologist Alan Grafen , acceptance of adaptionist theories is hampered by a lack of a mathematical unifying theory and a belief that anything in words alone must be suspect. According to Grafen, these difficulties along with an initial conflict with population genetics models at
22826-488: The change over time in this genetic variation. The frequency of one particular allele will become more or less prevalent relative to other forms of that gene. Variation disappears when a new allele reaches the point of fixation —when it either disappears from the population or replaces the ancestral allele entirely. Mutations are changes in the DNA sequence of a cell's genome and are the ultimate source of genetic variation in all organisms. When mutations occur, they may alter
23028-507: The comparison, [but] knew that Ayer had recanted much of his first book and [he] could hardly miss Cain's pointed implication that [he] should, in the fullness of time, do the same." This point also was made by the philosopher Mary Midgley : "The same thing happened to AJ Ayer, she says, but he spent the rest of his career taking back what he'd written in Language, Truth and Logic . "This hasn't occurred to Dawkins", she says. "He goes on saying
23230-607: The composition of the extracellular fluid (or the "internal environment") constant, especially with regard to the temperature , pH , osmolality , and the concentrations of sodium , potassium , glucose , carbon dioxide , and oxygen . However, a great many other homeostatic mechanisms, encompassing many aspects of human physiology , control other entities in the body. Where the levels of variables are higher or lower than those needed, they are often prefixed with hyper- and hypo- , respectively such as hyperthermia and hypothermia or hypertension and hypotension . If an entity
23432-427: The day (i.e. has a circadian rhythm ), with the lowest temperatures occurring at night, and the highest in the afternoons. Other normal temperature variations include those related to the menstrual cycle . The temperature regulator's set point is reset during infections to produce a fever. Organisms are capable of adjusting somewhat to varied conditions such as temperature changes or oxygen levels at altitude, by
23634-416: The deficiencies of kin selection and outlines group selection, which Wilson argues is a more realistic model of social evolution. He criticises earlier approaches to social evolution, saying: "unwarranted faith in the central role of kinship in social evolution has led to the reversal of the usual order in which biological research is conducted. The proven best way in evolutionary biology, as in most of science,
23836-403: The descent of all these structures from a common mammalian ancestor. However, since all living organisms are related to some extent, even organs that appear to have little or no structural similarity, such as arthropod , squid and vertebrate eyes, or the limbs and wings of arthropods and vertebrates, can depend on a common set of homologous genes that control their assembly and function; this
24038-400: The development of thinking about the evolution of genome composition, including isochores. Different insertion vs. deletion biases in different taxa can lead to the evolution of different genome sizes. The hypothesis of Lynch regarding genome size relies on mutational biases toward increase or decrease in genome size. However, mutational hypotheses for the evolution of composition suffered
24240-490: The direct control of genes include the inheritance of cultural traits and symbiogenesis . From a neo-Darwinian perspective, evolution occurs when there are changes in the frequencies of alleles within a population of interbreeding organisms, for example, the allele for black colour in a population of moths becoming more common. Mechanisms that can lead to changes in allele frequencies include natural selection, genetic drift, and mutation bias. Evolution by natural selection
24442-411: The distal convoluted tubules and collecting ducts is impermeable to water in the absence of antidiuretic hormone (ADH) in the blood. ADH is part of the control of fluid balance . Its levels in the blood vary with the osmolality of the plasma, which is measured in the hypothalamus of the brain. Aldosterone's action on the kidney tubules prevents sodium loss to the extracellular fluid (ECF). So there
24644-413: The earlier reactions are insufficient to correct the hypothermia . When core temperature rises are detected by thermoreceptors , the sweat glands in the skin are stimulated via cholinergic sympathetic nerves to secrete sweat onto the skin, which, when it evaporates, cools the skin and the blood flowing through it. Panting is an alternative effector in many vertebrates, which cools the body also by
24846-429: The error – in fact it should be very nearly in proportion (but in the opposite direction) to the error that is threatening the internal environment. For instance, arterial blood pressure in mammals is homeostatically controlled and measured by stretch receptors in the walls of the aortic arch and carotid sinuses at the beginnings of the internal carotid arteries . The sensors send messages via sensory nerves to
25048-444: The evaporation of water, but this time from the mucous membranes of the throat and mouth. Blood sugar levels are regulated within fairly narrow limits. In mammals, the primary sensors for this are the beta cells of the pancreatic islets . The beta cells respond to a rise in the blood sugar level by secreting insulin into the blood and simultaneously inhibiting their neighboring alpha cells from secreting glucagon into
25250-489: The evolutionary history of life on Earth. Morphological and biochemical traits tend to be more similar among species that share a more recent common ancestor , which historically was used to reconstruct phylogenetic trees , although direct comparison of genetic sequences is a more common method today. Evolutionary biologists have continued to study various aspects of evolution by forming and testing hypotheses as well as constructing theories based on evidence from
25452-414: The excess water in the body. Urinary water loss, when the body water homeostat is intact, is a compensatory water loss, correcting any water excess in the body. However, since the kidneys cannot generate water, the thirst reflex is the all-important second effector mechanism of the body water homeostat, correcting any water deficit in the body. The plasma pH can be altered by respiratory changes in
25654-424: The fact that water losses from the body, (through unavoidable water loss through the skin which is not entirely waterproof and therefore always slightly moist, water vapor in the exhaled air , sweating , vomiting , normal feces and especially diarrhea ) are all hypotonic , meaning that they are less salty than the body fluids (compare, for instance, the taste of saliva with that of tears. The latter has almost
25856-414: The field or laboratory and on data generated by the methods of mathematical and theoretical biology . Their discoveries have influenced not just the development of biology but also other fields including agriculture, medicine, and computer science . Evolution in organisms occurs through changes in heritable characteristics—the inherited characteristics of an organism. In humans, for example, eye colour
26058-427: The fitness of an allele is not a fixed characteristic; if the environment changes, previously neutral or harmful traits may become beneficial and previously beneficial traits become harmful. However, even if the direction of selection does reverse in this way, traits that were lost in the past may not re-evolve in an identical form. However, a re-activation of dormant genes, as long as they have not been eliminated from
26260-400: The fluid volume stabilized, and also keeping the levels of electrolytes in the extracellular fluid stable. Fluid balance is maintained by the process of osmoregulation and by behavior. Osmotic pressure is detected by osmoreceptors in the median preoptic nucleus in the hypothalamus . Measurement of the plasma osmolality to give an indication of the water content of the body, relies on
26462-467: The gene is the fundamental unit of selection, and then points out that selection does not act directly upon the gene, but upon "vehicles" or '"extended phenotypes". Stephen Jay Gould took exception to calling the gene a 'unit of selection' because selection acted only upon phenotypes . Summarizing the Dawkins-Gould difference of view, Sterelny says: The word "cause" here is somewhat tricky: does
26664-455: The gene's own chances of being replicated, and, as a result, "successful" genes frequently provide a benefit to the organism. An example of this might be a gene that protects the organism against a disease. This helps the gene spread, and also helps the organism. There are other times when the implicit interests of the vehicle and replicator are in conflict, such as the genes behind certain male spiders' instinctive mating behaviour, which increase
26866-472: The genome and were only suppressed perhaps for hundreds of generations, can lead to the re-occurrence of traits thought to be lost like hindlegs in dolphins, teeth in chickens, wings in wingless stick insects, tails and additional nipples in humans etc. "Throwbacks" such as these are known as atavisms . Natural selection within a population for a trait that can vary across a range of values, such as height, can be categorised into three different types. The first
27068-479: The genome of a species is very similar among all individuals of that species. However, discoveries in the field of evolutionary developmental biology have demonstrated that even relatively small differences in genotype can lead to dramatic differences in phenotype both within and between species. An individual organism's phenotype results from both its genotype and the influence of the environment it has lived in. The modern evolutionary synthesis defines evolution as
27270-504: The group members. The conflict arises in part over defining concepts: In The Social Conquest of Earth (2012), the entomologist E. O. Wilson contends that although the selfish-gene approach was accepted "until 2010 [when] Martin Nowak, Corina Tarnita, and I demonstrated that inclusive fitness theory, often called kin selection theory, is both mathematically and biologically incorrect." Chapter 18 of The Social Conquest of Earth describes
27472-567: The gut. Another example are the most well-characterised endocannabinoids like anandamide ( N -arachidonoylethanolamide; AEA) and 2-arachidonoylglycerol (2-AG), whose synthesis occurs through the action of a series of intracellular enzymes activated in response to a rise in intracellular calcium levels to introduce homeostasis and prevention of tumor development through putative protective mechanisms that prevent cell growth and migration by activation of CB1 and/or CB2 and adjoining receptors . The homeostatic mechanism which controls
27674-412: The homeostatic control of body temperature, and therefore highly variable in rough proportion to the heat load that threatens to destabilize the body's core temperature, for which there is a sensor in the hypothalamus of the brain. Mammals regulate their core temperature using input from thermoreceptors in the hypothalamus , brain, spinal cord , internal organs , and great veins. Apart from
27876-400: The hormone atrial natriuretic peptide (ANP) into the blood. This acts on the kidneys to inhibit the secretion of renin and aldosterone causing the release of sodium, and accompanying water into the urine, thereby reducing the blood volume. This information is then conveyed, via afferent nerve fibers , to the solitary nucleus in the medulla oblongata . From here motor nerves belonging to
28078-591: The human body is capable of balancing a wide range of copper intakes for the needs of healthy individuals. Many aspects of copper homeostasis are known at the molecular level. Copper's essentiality is due to its ability to act as an electron donor or acceptor as its oxidation state fluxes between Cu ( cuprous ) and Cu ( cupric ). As a component of about a dozen cuproenzymes, copper is involved in key redox (i.e., oxidation-reduction) reactions in essential metabolic processes such as mitochondrial respiration, synthesis of melanin , and cross-linking of collagen . Copper
28280-640: The idea of developmental bias . Haldane and Fisher argued that, because mutation is a weak pressure easily overcome by selection, tendencies of mutation would be ineffectual except under conditions of neutral evolution or extraordinarily high mutation rates. This opposing-pressures argument was long used to dismiss the possibility of internal tendencies in evolution, until the molecular era prompted renewed interest in neutral evolution. Noboru Sueoka and Ernst Freese proposed that systematic biases in mutation might be responsible for systematic differences in genomic GC composition between species. The identification of
28482-591: The idea of the meme , a culturally-transmitted entity such as a hummable tune, by analogy to genetic transmission. Dawkins describes God as an old idea which probably arose many times, and which has sufficient psychological appeal to survive effectively in the meme pool. The second edition (1989) added two more chapters. In describing genes as being "selfish", Dawkins states unequivocally that he does not intend to imply that they are driven by any motives or will , but merely that their effects can be metaphorically and pedagogically described as if they were. His contention
28684-450: The interaction of its genotype with the environment while others are neutral. Some observable characteristics are not inherited. For example, suntanned skin comes from the interaction between a person's genotype and sunlight; thus, suntans are not passed on to people's children. The phenotype is the ability of the skin to tan when exposed to sunlight. However, some people tan more easily than others, due to differences in genotypic variation;
28886-495: The internal regulation of temperature, a process called allostasis can come into play that adjusts behaviour to adapt to the challenge of very hot or cold extremes (and to other challenges). These adjustments may include seeking shade and reducing activity, seeking warmer conditions and increasing activity, or huddling. Behavioral thermoregulation takes precedence over physiological thermoregulation since necessary changes can be affected more quickly and physiological thermoregulation
29088-424: The lactate, and, by the process of energy-consuming gluconeogenesis , convert it back to glucose. Controlling iron levels in the body is a critically important part of many aspects of human health and disease. In humans iron is both necessary to the body and potentially harmful. Copper is absorbed, transported, distributed, stored, and excreted in the body according to complex homeostatic processes which ensure
29290-456: The level of the gene mediated by the phenotype " and that any selection at the group level occurred only under rare circumstances. W. D. Hamilton and others developed this approach further during the 1960s; they opposed the concepts of group selection and of selection aimed directly at benefit to the individual organism: Despite the principle of ' survival of the fittest ' the ultimate criterion which determines whether [a gene] G will spread
29492-512: The level of the individual organism are genes called transposons , which can replicate and spread throughout a genome. Selection at a level above the individual, such as group selection , may allow the evolution of cooperation. Genetic drift is the random fluctuation of allele frequencies within a population from one generation to the next. When selective forces are absent or relatively weak, allele frequencies are equally likely to drift upward or downward in each successive generation because
29694-452: The longer term, evolution produces new species through splitting ancestral populations of organisms into new groups that cannot or will not interbreed. These outcomes of evolution are distinguished based on time scale as macroevolution versus microevolution. Macroevolution refers to evolution that occurs at or above the level of species, in particular speciation and extinction, whereas microevolution refers to smaller evolutionary changes within
29896-405: The loss of an ancestral feature. An example that shows both types of change is bacterial adaptation to antibiotic selection, with genetic changes causing antibiotic resistance by both modifying the target of the drug, or increasing the activity of transporters that pump the drug out of the cell. Other striking examples are the bacteria Escherichia coli evolving the ability to use citric acid as
30098-458: The mechanics in developmental plasticity and canalisation . Heritability may also occur at even larger scales. For example, ecological inheritance through the process of niche construction is defined by the regular and repeated activities of organisms in their environment. This generates a legacy of effects that modify and feed back into the selection regime of subsequent generations. Other examples of heritability in evolution that are not under
30300-423: The more two individuals are genetically related, the more sense (at the level of the genes) it makes for them to behave cooperatively with each other. A lineage is expected to evolve to maximise its inclusive fitness —the number of copies of its genes passed on globally (rather than by a particular individual). As a result, populations will tend towards an evolutionarily stable strategy . The book also introduces
30502-408: The movement of pollen between heavy-metal-tolerant and heavy-metal-sensitive populations of grasses. Gene transfer between species includes the formation of hybrid organisms and horizontal gene transfer . Horizontal gene transfer is the transfer of genetic material from one organism to another organism that is not its offspring; this is most common among bacteria. In medicine, this contributes to
30704-469: The nearby thirst center causing an almost irresistible (if the hypertonicity is severe enough) urge to drink water. The cessation of urine flow prevents the hypovolemia and hypertonicity from getting worse; the drinking of water corrects the defect. Hypo-osmolality results in very low plasma ADH levels. This results in the inhibition of water reabsorption from the kidney tubules, causing high volumes of very dilute urine to be excreted, thus getting rid of
30906-427: The next generation than those with traits that do not confer an advantage. This teleonomy is the quality whereby the process of natural selection creates and preserves traits that are seemingly fitted for the functional roles they perform. Consequences of selection include nonrandom mating and genetic hitchhiking . The central concept of natural selection is the evolutionary fitness of an organism. Fitness
31108-499: The offspring of sexual organisms contain random mixtures of their parents' chromosomes that are produced through independent assortment. In a related process called homologous recombination , sexual organisms exchange DNA between two matching chromosomes. Recombination and reassortment do not alter allele frequencies, but instead change which alleles are associated with each other, producing offspring with new combinations of alleles. Sex usually increases genetic variation and may increase
31310-435: The one hand, and calcitonin on the other can very rapidly correct any impending error in the plasma ionized calcium level by either removing calcium from the blood and depositing it in the skeleton, or by removing calcium from it. The skeleton acts as an extremely large calcium store (about 1 kg) compared with the plasma calcium store (about 180 mg). Longer term regulation occurs through calcium absorption or loss from
31512-635: The one to which they belong." (pp. 57, 59) According to Wilson: "Different parts of the brain have evolved by group selection to create groupishness." (p. 61) Some authors consider facets of this debate between Dawkins and his critics about the level of selection to be blather: Other authors say Dawkins has failed to make some critical distinctions, in particular, the difference between group selection for group advantage and group selection conveying individual advantage. A good deal of objection to The Selfish Gene stemmed from its failure to be always clear about "selection" and "replication". Dawkins says
31714-440: The organism's inclusive fitness by allowing it to reproduce but shorten its life by exposing it to the risk of being eaten by the cannibalistic female . Another example is the existence of segregation distorter genes that are detrimental to their host, but nonetheless propagate themselves at its expense. Likewise, the persistence of junk DNA that [Dawkins believed at that time] provides no benefit to its host can be explained on
31916-456: The other alleles of that gene, then with each generation this allele has a higher probability of becoming common within the population. These traits are said to be "selected for ." Examples of traits that can increase fitness are enhanced survival and increased fecundity . Conversely, the lower fitness caused by having a less beneficial or deleterious allele results in this allele likely becoming rarer—they are "selected against ." Importantly,
32118-427: The outer layer of the adrenal cortex . This causes the release of aldosterone into the blood. Aldosterone acts primarily on the distal convoluted tubules and collecting ducts of the kidneys, stimulating the excretion of potassium ions into the urine. It does so, however, by activating the basolateral Na /K pumps of the tubular epithelial cells. These sodium/potassium exchangers pump three sodium ions out of
32320-578: The overall process, like a step in an assembly line. One example of mutation is wild boar piglets. They are camouflage coloured and show a characteristic pattern of dark and light longitudinal stripes. However, mutations in the melanocortin 1 receptor ( MC1R ) disrupt the pattern. The majority of pig breeds carry MC1R mutations disrupting wild-type colour and different mutations causing dominant black colouring. In asexual organisms, genes are inherited together, or linked , as they cannot mix with genes of other organisms during reproduction. In contrast,
32522-458: The oxygen content rather than the partial pressure of oxygen in the arterial blood. When the oxygen content of the blood is chronically low, oxygen-sensitive cells secrete erythropoietin (EPO) into the blood. The effector tissue is the red bone marrow which produces red blood cells (RBCs, also called erythrocytes ). The increase in RBCs leads to an increased hematocrit in the blood, and
32724-425: The pH. Respiratory compensation a mechanism of the respiratory center , adjusts the partial pressure of carbon dioxide by changing the rate and depth of breathing, to bring the pH back to normal. The partial pressure of carbon dioxide also determines the concentration of carbonic acid, and the bicarbonate buffer system can also come into play. Renal compensation can help the bicarbonate buffer system. The sensor for
32926-515: The partial pressure of carbon dioxide; or altered by metabolic changes in the carbonic acid to bicarbonate ion ratio. The bicarbonate buffer system regulates the ratio of carbonic acid to bicarbonate to be equal to 1:20, at which ratio the blood pH is 7.4 (as explained in the Henderson–Hasselbalch equation ). A change in the plasma pH gives an acid–base imbalance . In acid–base homeostasis there are two mechanisms that can help regulate
33128-532: The partial pressure of oxygen in the blood, the amount of oxygen that can be carried, depends on the hemoglobin content. The partial pressure of oxygen may be sufficient for example in anemia , but the hemoglobin content will be insufficient and subsequently as will be the oxygen content. Given enough supply of iron, vitamin B12 and folic acid , EPO can stimulate RBC production, and hemoglobin and oxygen content restored to normal. The brain can regulate blood flow over
33330-446: The plasma bicarbonate concentration is not known for certain. It is very probable that the renal tubular cells of the distal convoluted tubules are themselves sensitive to the pH of the plasma. The metabolism of these cells produces carbon dioxide, which is rapidly converted to hydrogen and bicarbonate through the action of carbonic anhydrase . When the ECF pH falls (becoming more acidic) the renal tubular cells excrete hydrogen ions into
33532-417: The plasma pH, constant. Cerebrospinal fluid (CSF) allows for regulation of the distribution of substances between cells of the brain, and neuroendocrine factors, to which slight changes can cause problems or damage to the nervous system. For example, high glycine concentration disrupts temperature and blood pressure control, and high CSF pH causes dizziness and syncope . Inhibitory neurons in
33734-415: The plasma sodium concentration is rather more complex than most of the other homeostatic mechanisms described on this page. The sensor is situated in the juxtaglomerular apparatus of kidneys, which senses the plasma sodium concentration in a surprisingly indirect manner. Instead of measuring it directly in the blood flowing past the juxtaglomerular cells , these cells respond to the sodium concentration in
33936-527: The presence of hip bones in whales and snakes, and sexual traits in organisms that reproduce via asexual reproduction. Examples of vestigial structures in humans include wisdom teeth , the coccyx , the vermiform appendix , and other behavioural vestiges such as goose bumps and primitive reflexes . However, many traits that appear to be simple adaptations are in fact exaptations : structures originally adapted for one function, but which coincidentally became somewhat useful for some other function in
34138-469: The process. One example is the African lizard Holaspis guentheri , which developed an extremely flat head for hiding in crevices, as can be seen by looking at its near relatives. However, in this species, the head has become so flattened that it assists in gliding from tree to tree—an exaptation. Within cells, molecular machines such as the bacterial flagella and protein sorting machinery evolved by
34340-494: The rate of evolution. The two-fold cost of sex was first described by John Maynard Smith . The first cost is that in sexually dimorphic species only one of the two sexes can bear young. This cost does not apply to hermaphroditic species, like most plants and many invertebrates . The second cost is that any individual who reproduces sexually can only pass on 50% of its genes to any individual offspring, with even less passed on as each new generation passes. Yet sexual reproduction
34542-420: The recruitment of several pre-existing proteins that previously had different functions. Another example is the recruitment of enzymes from glycolysis and xenobiotic metabolism to serve as structural proteins called crystallins within the lenses of organisms' eyes. Homeostasis In biology , homeostasis ( British also homoeostasis ; / h ɒ m i oʊ ˈ s t eɪ s ɪ s , - m i ə -/ )
34744-415: The regulation of the internal environment was described by French physiologist Claude Bernard in 1849, and the word homeostasis was coined by Walter Bradford Cannon in 1926. In 1932, Joseph Barcroft a British physiologist, was the first to say that higher brain function required the most stable internal environment. Thus, to Barcroft homeostasis was not only organized by the brain—homeostasis served
34946-408: The relative importance of selection and neutral processes, including drift. The comparative importance of adaptive and non-adaptive forces in driving evolutionary change is an area of current research . Mutation bias is usually conceived as a difference in expected rates for two different kinds of mutation, e.g., transition-transversion bias, GC-AT bias, deletion-insertion bias. This is related to
35148-436: The release of renin from the juxtaglomerular apparatus is halted, ceasing the production of angiotensin II, and its consequent aldosterone-release into the blood. The kidneys respond by excreting sodium ions into the urine, thereby normalizing the plasma sodium ion concentration. The low angiotensin II levels in the blood lower the arterial blood pressure as an inevitable concomitant response. The reabsorption of sodium ions from
35350-422: The renal tubular fluid halts further sodium ion losses from the body, and therefore preventing the worsening of hyponatremia . The hyponatremia can only be corrected by the consumption of salt in the diet. However, it is not certain whether a "salt hunger" can be initiated by hyponatremia, or by what mechanism this might come about. When the plasma sodium ion concentration is higher than normal ( hypernatremia ),
35552-404: The result is herd behaviour. Altruism too can evolve, as in the social insects such as ants and bees, where workers give up the right to reproduce in favour of a sister, the queen; in their case, the unusual ( haplodiploid ) system of sex determination may have helped to bring this about, as females in a nest are exceptionally closely related. The final chapter of the first edition introduced
35754-401: The result of probing, and DNA is simply one mechanism the agent brings to bear upon its activity. Another criticism of the book is its treatment of morality, and more particularly altruism, as existing only as a form of selfishness: The philosopher Mary Midgley has suggested this position is a variant of Hobbes's explanation of altruism as enlightened self-interest , and that Dawkins goes
35956-556: The same phenotypic effect ) in other bodies to replicate. The claim is made that these "selfish" actions of genes lead to unselfish actions by organisms. A requirement upon this claim, supported by Dawkins in Chapter 10: "You scratch my back, I'll ride on yours" by examples from nature, is the need to explain how genes achieve kin recognition , or manage to orchestrate mutualism and coevolution . Although Dawkins (and biologists in general) recognize these phenomena result in more copies of
36158-440: The same salt content as the extracellular fluid, whereas the former is hypotonic with respect to the plasma. Saliva does not taste salty, whereas tears are decidedly salty). Nearly all normal and abnormal losses of body water therefore cause the extracellular fluid to become hypertonic . Conversely, excessive fluid intake dilutes the extracellular fluid causing the hypothalamus to register hypotonic hyponatremia conditions. When
36360-417: The same strand of DNA to become separated. However, the rate of recombination is low (approximately two events per chromosome per generation). As a result, genes close together on a chromosome may not always be shuffled away from each other and genes that are close together tend to be inherited together, a phenomenon known as linkage . This tendency is measured by finding how often two alleles occur together on
36562-533: The same thing."" However, according to Wilkins and Hull, Dawkins's thinking has developed, although perhaps not defusing this criticism: As to the unit of selection : "One internally consistent logical picture is that the unit of replication is the gene,...and the organism is one kind of ...entity on which selection acts directly." Dawkins proposed the matter without a distinction between 'unit of replication' and 'unit of selection' that he made elsewhere: "the fundamental unit of selection, and therefore of self-interest,
36764-413: The second are the parafollicular cells in the thyroid gland . The parathyroid chief cells secrete parathyroid hormone (PTH) in response to a fall in the plasma ionized calcium level; the parafollicular cells of the thyroid gland secrete calcitonin in response to a rise in the plasma ionized calcium level. The effector organs of the first homeostatic mechanism are the bones , the kidney , and, via
36966-552: The sequence of nucleotides in the DNA. These phenomena are classed as epigenetic inheritance systems. DNA methylation marking chromatin , self-sustaining metabolic loops, gene silencing by RNA interference and the three-dimensional conformation of proteins (such as prions ) are areas where epigenetic inheritance systems have been discovered at the organismic level. Developmental biologists suggest that complex interactions in genetic networks and communication among cells can lead to heritable variations that may underlay some of
37168-406: The spread of antibiotic resistance , as when one bacteria acquires resistance genes it can rapidly transfer them to other species. Horizontal transfer of genes from bacteria to eukaryotes such as the yeast Saccharomyces cerevisiae and the adzuki bean weevil Callosobruchus chinensis has occurred. An example of larger-scale transfers are the eukaryotic bdelloid rotifers , which have received
37370-590: The strategy of having a female offspring is safe, as she'll have a pup, but the strategy of having a male can bring a large return (dozens of pups), even though many males live out their lives as bachelors. Amotz Zahavi 's theory of honest signalling explains stotting as a selfish act, he argues, improving the springbok's chances of escaping from a predator by indicating how difficult the chase would be. Dawkins discusses why many species live in groups, achieving mutual benefits through mechanisms such as Hamilton's selfish herd model: each individual behaves selfishly but
37572-487: The survival chances of close relatives can spread in a population, because those relatives carry the same genes. Dawkins examines childbearing and raising children as evolutionary strategies. He attacks the idea of group selection for the good of the species as proposed by V. C. Wynne-Edwards , arguing instead that each parent necessarily behaves selfishly. A question is whether parents should invest in their offspring equally or should favour some of them and explains that what
37774-422: The survival lottery for the group must improve that for the gene for sufficient replication to occur. Dawkins argues qualitatively that the lottery for the gene is based upon a very long and broad record of events, and group advantages are usually too specific, too brief, and too fortuitous to change the gene lottery: Prior to the 1960s, it was common for altruism to be explained in terms of group selection , where
37976-640: The survival of individual males. This survival disadvantage is balanced by higher reproductive success in males that show these hard-to-fake , sexually selected traits. Evolution influences every aspect of the form and behaviour of organisms. Most prominent are the specific behavioural and physical adaptations that are the outcome of natural selection. These adaptations increase fitness by aiding activities such as finding food, avoiding predators or attracting mates. Organisms can also respond to selection by cooperating with each other, usually by aiding their relatives or engaging in mutually beneficial symbiosis . In
38178-642: The system...." Each population within an ecosystem occupies a distinct niche , or position, with distinct relationships to other parts of the system. These relationships involve the life history of the organism, its position in the food chain and its geographic range. This broad understanding of nature enables scientists to delineate specific forces which, together, comprise natural selection. Natural selection can act at different levels of organisation , such as genes, cells, individual organisms, groups of organisms and species. Selection can act at multiple levels simultaneously. An example of selection occurring below
38380-449: The term meme for a unit of human cultural evolution analogous to the gene, suggesting that such "selfish" replication may also model human culture, in a different sense. Memetics has become the subject of many studies since the publication of the book. In raising awareness of Hamilton's ideas, as well as making its own valuable contributions to the field, the book has also stimulated research on human inclusive fitness . Dawkins uses
38582-499: The term "selfish gene" as a way of expressing the gene-centred view of evolution . As such, the book is not about a particular gene that causes selfish behaviour; in fact, much of the book's content is devoted to explaining the evolution of altruism. In the foreword to the book's 30th-anniversary edition, Dawkins said he "can readily see that [the book's title] might give an inadequate impression of its contents" and in retrospect thinks he should have taken Tom Maschler 's advice and called
38784-402: The time of its introduction "explains why within biology the considerable scientific contributions it [ The Selfish Gene ] makes are seriously underestimated, and why it is viewed mainly as a work of exposition." According to comparative psychologist Nicky Hayes, "Dawkins presented a version of sociobiology that rested heavily on metaphors drawn from animal behavior, and extrapolated these...One of
38986-493: The topic. Nippon Animation produced an educational television program titled The Many Journeys of Meme . In 1976, the ecologist Arthur Cain , one of Dawkins's tutors at Oxford in the 1960s, called it a "young man's book" (which Dawkins points out was a deliberate quote of a commentator on the New College, Oxford philosopher A. J. Ayer 's Language, Truth, and Logic (1936)). Dawkins noted that he had been "flattered by
39188-407: The tubular fluid as a result of high aldosterone levels in the blood does not, of itself, cause renal tubular water to be returned to the blood from the distal convoluted tubules or collecting ducts . This is because sodium is reabsorbed in exchange for potassium and therefore causes only a modest change in the osmotic gradient between the blood and the tubular fluid. Furthermore, the epithelium of
39390-402: The tubular fluid to leave the body via urine. Bicarbonate ions are simultaneously secreted into the blood that decreases the carbonic acid, and consequently raises the plasma pH. The converse happens when the plasma pH rises above normal: bicarbonate ions are excreted into the urine, and hydrogen ions released into the plasma. When hydrogen ions are excreted into the urine, and bicarbonate into
39592-455: The upper small intestine, increasing their capacity to absorb calcium from the gut contents into the blood. The second homeostatic mechanism, with its sensors in the thyroid gland, releases calcitonin into the blood when the blood ionized calcium rises. This hormone acts primarily on bone, causing the rapid removal of calcium from the blood and depositing it, in insoluble form, in the bones. The two homeostatic mechanisms working through PTH on
39794-407: The urine. Since phosphates combine with calcium ions to form insoluble salts (see also bone mineral ), a decrease in the level of phosphates in the blood, releases free calcium ions into the plasma ionized calcium pool. PTH has a second action on the kidneys. It stimulates the manufacture and release, by the kidneys, of calcitriol into the blood. This steroid hormone acts on the epithelial cells of
39996-502: The walls of the arterioles causing these small diameter vessels to constrict, thereby restricting the outflow of blood from the arterial tree, causing the arterial blood pressure to rise. This, therefore, reinforces the measures described above (under the heading of "Arterial blood pressure"), which defend the arterial blood pressure against changes, especially hypotension . The angiotensin II-stimulated aldosterone released from
40198-472: The warmth from the arterial blood directly into the venous blood returning into the trunk, causing minimal heat loss from the extremities in cold weather. The subcutaneous limb veins are tightly constricted, not only reducing heat loss from this source but also forcing the venous blood into the counter-current system in the depths of the limbs. The metabolic rate is increased, initially by non-shivering thermogenesis , followed by shivering thermogenesis if
40400-493: The weaknesses of the sociological approach is that it tends only to seek confirmatory examples from among the huge diversity of animal behavior. Dawkins did not deviate from this tradition." More generally, critics argue that The Selfish Gene oversimplifies the relationship between genes and the organism. (As an example, see Thompson.) The Selfish Gene further popularised sociobiology in Japan after its translation in 1980. With
40602-895: Was conceived independently by two British naturalists, Charles Darwin and Alfred Russel Wallace , in the mid-19th century as an explanation for why organisms are adapted to their physical and biological environments. The theory was first set out in detail in Darwin's book On the Origin of Species . Evolution by natural selection is established by observable facts about living organisms: (1) more offspring are often produced than can possibly survive; (2) traits vary among individuals with respect to their morphology , physiology , and behaviour; (3) different traits confer different rates of survival and reproduction (differential fitness ); and (4) traits can be passed from generation to generation ( heritability of fitness). In successive generations, members of
40804-457: Was the initial molecule which first managed to reproduce itself and thus gained an advantage over other molecules within the primordial soup . As replicating molecules became more complex, Dawkins postulates, the replicators became the genes within organisms, with each organism's body serving the purpose of a 'survival machine' for its genes. Dawkins writes that gene combinations which help an organism to survive and reproduce tend to also improve
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