101-696: 2; see text Niopa ( Benth. ) Britton & Rose Anadenanthera is a genus of South American trees in the Legume family, Fabaceae . The genus contains two species, A. colubrina and A. peregrina . These trees are known to the western world primarily as sources of the hallucinogenic snuffs vilca /cebil and yopo /cohoba. The main active constituent of Anadenanthera is bufotenin . Chemical compounds contained in Anadenanthera include: Benth. George Bentham CMG FRS FLS (22 September 1800 – 10 September 1884)
202-420: A synonymous substitution . However, many mutations in non-coding DNA have deleterious effects. Although both mutation rates and average fitness effects of mutations are dependent on the organism, a majority of mutations in humans are slightly deleterious. Some mutations occur in "toolkit" or regulatory genes . Changes in these often have large effects on the phenotype of the individual because they regulate
303-457: A "Nature" which would do the selection. At the time, other mechanisms of evolution such as evolution by genetic drift were not yet explicitly formulated, and Darwin believed that selection was likely only part of the story: "I am convinced that Natural Selection has been the main but not exclusive means of modification." In a letter to Charles Lyell in September 1860, Darwin regretted the use of
404-519: A chromosome. During the formation of gametes, recombination reshuffles the alleles. The chance that such a reshuffle occurs between two alleles is inversely related to the distance between them. Selective sweeps occur when an allele becomes more common in a population as a result of positive selection. As the prevalence of one allele increases, closely linked alleles can also become more common by " genetic hitchhiking ", whether they are neutral or even slightly deleterious. A strong selective sweep results in
505-554: A different, incompatible allele of the same gene. Selection against the heterozygote would then directly create reproductive isolation, leading to the Bateson–Dobzhansky–Muller model , further elaborated by H. Allen Orr and Sergey Gavrilets . With reinforcement , however, natural selection can favor an increase in pre-zygotic isolation, influencing the process of speciation directly. Natural selection acts on an organism's phenotype, or physical characteristics. Phenotype
606-422: A large estate. While studying at Angoulême , Bentham came across a copy of A. P. de Candolle 's Flore française , and became interested in the analytical tables for identifying plants. He immediately tested them on the first plant he saw. The result was successful and he applied it to every plant he came across. In London in 1823, he met English botanists. His uncle pushed him to study law at Lincoln's Inn . He
707-479: A precursor to speciation . Alternatively, selection can be divided according to its effect on genetic diversity . Purifying or negative selection acts to remove genetic variation from the population (and is opposed by de novo mutation , which introduces new variation. In contrast, balancing selection acts to maintain genetic variation in a population, even in the absence of de novo mutation, by negative frequency-dependent selection . One mechanism for this
808-429: A process by which animals and plants with traits considered desirable by human breeders are systematically favoured for reproduction. The concept of natural selection originally developed in the absence of a valid theory of heredity; at the time of Darwin's writing, science had yet to develop modern theories of genetics. The union of traditional Darwinian evolution with subsequent discoveries in classical genetics formed
909-441: A region of the genome where the positively selected haplotype (the allele and its neighbours) are in essence the only ones that exist in the population. Selective sweeps can be detected by measuring linkage disequilibrium , or whether a given haplotype is overrepresented in the population. Since a selective sweep also results in selection of neighbouring alleles, the presence of a block of strong linkage disequilibrium might indicate
1010-430: A relatively high probability of surviving to adulthood. Natural selection can act on any heritable phenotypic trait , and selective pressure can be produced by any aspect of the environment, including sexual selection and competition with members of the same or other species. However, this does not imply that natural selection is always directional and results in adaptive evolution; natural selection often results in
1111-545: A remarkable linguistic aptitude. By the age of seven, he could speak French, German and Russian, and he learned Swedish during a short residence in Sweden while still a child. The family made a long tour through France, staying two years at Montauban , where Bentham studied Hebrew and mathematics in the Protestant Theological School. They eventually settled near Montpellier where Sir Samuel bought
SECTION 10
#17327804845271212-458: A reproductive advantage regardless of whether or not the trait is heritable. Following Darwin's primary usage, the term is used to refer both to the evolutionary consequence of blind selection and to its mechanisms. It is sometimes helpful to explicitly distinguish between selection's mechanisms and its effects; when this distinction is important, scientists define "(phenotypic) natural selection" specifically as "those mechanisms that contribute to
1313-611: A scheme for the preparation of a series of Floras or descriptions in the English language of the indigenous plants of British colonies and possessions. Bentham began with the Flora Hongkongensis in 1861, which was the first comprehensive work on any part of the little-known flora of China and Hong Kong, including Hong Kong croton . This was followed by the Flora Australiensis , in seven volumes (1863–1878),
1414-454: A species were uninteresting departures from their Platonic ideals (or typus ) of created kinds . However, the theory of uniformitarianism in geology promoted the idea that simple, weak forces could act continuously over long periods of time to produce radical changes in the Earth's landscape. The success of this theory raised awareness of the vast scale of geological time and made plausible
1515-483: A well-documented example involves the spread of a gene in the butterfly Hypolimnas bolina suppressing male-killing activity by Wolbachia bacteria parasites on the island of Samoa , where the spread of the gene is known to have occurred over a period of just five years. A prerequisite for natural selection to result in adaptive evolution, novel traits and speciation is the presence of heritable genetic variation that results in fitness differences. Genetic variation
1616-532: Is heterozygote advantage , where individuals with two different alleles have a selective advantage over individuals with just one allele. The polymorphism at the human ABO blood group locus has been explained in this way. Another option is to classify selection by the life cycle stage at which it acts. Some biologists recognise just two types: viability (or survival) selection , which acts to increase an organism's probability of survival, and fecundity (or fertility or reproductive) selection, which acts to increase
1717-405: Is all the parts of one whole) happened like as if they were made for the sake of something, these were preserved, having been appropriately constituted by an internal spontaneity, and whatsoever things were not thus constituted, perished, and still perish. But Aristotle rejected this possibility in the next paragraph, making clear that he is talking about the development of animals as embryos with
1818-564: Is best known for his taxonomic classification of plants in collaboration with Joseph Dalton Hooker , his Genera Plantarum (1862–1883). He died in London in 1884. Bentham was born in Stoke, Plymouth , on 22 September 1800. His father, Sir Samuel Bentham , a naval architect , was the only brother of Jeremy Bentham to survive into adulthood. His mother, Mary Sophia Bentham , was a botanist and author. Bentham had no formal education but had
1919-500: Is central to natural selection. In broad terms, individuals that are more "fit" have better potential for survival, as in the well-known phrase " survival of the fittest ", but the precise meaning of the term is much more subtle. Modern evolutionary theory defines fitness not by how long an organism lives, but by how successful it is at reproducing. If an organism lives half as long as others of its species, but has twice as many offspring surviving to adulthood, its genes become more common in
2020-455: Is determined by an organism's genetic make-up (genotype) and the environment in which the organism lives. When different organisms in a population possess different versions of a gene for a certain trait, each of these versions is known as an allele . It is this genetic variation that underlies differences in phenotype. An example is the ABO blood type antigens in humans, where three alleles govern
2121-456: Is increased, which means that it leaves more offspring. If the traits that give these individuals a reproductive advantage are also heritable , that is, passed from parent to offspring, then there will be differential reproduction, that is, a slightly higher proportion of fast rabbits or efficient algae in the next generation. Even if the reproductive advantage is very slight, over many generations any advantageous heritable trait becomes dominant in
SECTION 20
#17327804845272222-438: Is intrinsic to the concept of a species that hybrids are selected against, opposing the evolution of reproductive isolation, a problem that was recognised by Darwin. The problem does not occur in allopatric speciation with geographically separated populations, which can diverge with different sets of mutations. E. B. Poulton realized in 1903 that reproductive isolation could evolve through divergence, if each lineage acquired
2323-663: Is known as fecundity selection . Natural selection is a cornerstone of modern biology . The concept, published by Darwin and Alfred Russel Wallace in a joint presentation of papers in 1858 , was elaborated in Darwin's influential 1859 book On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life . He described natural selection as analogous to artificial selection,
2424-483: Is lowered by the presence of another. This may be because both rely on a limited supply of a resource such as food, water, or territory . Competition may be within or between species , and may be direct or indirect. Species less suited to compete should in theory either adapt or die out , since competition plays a powerful role in natural selection, but according to the "room to roam" theory it may be less important than expansion among larger clades . Competition
2525-555: Is modelled by r/K selection theory , which is based on Robert MacArthur and E. O. Wilson 's work on island biogeography . In this theory, selective pressures drive evolution in one of two stereotyped directions: r - or K -selection. These terms, r and K , can be illustrated in a logistic model of population dynamics : d N d t = r N ( 1 − N K ) {\displaystyle {\frac {dN}{dt}}=rN\left(1-{\frac {N}{K}}\right)\qquad \!} where r
2626-461: Is natural selection via any means other than sexual selection, such as kin selection, competition, and infanticide . Following Darwin, natural selection is sometimes defined as ecological selection, in which case sexual selection is considered a separate mechanism. Sexual selection as first articulated by Darwin (using the example of the peacock 's tail) refers specifically to competition for mates, which can be intrasexual , between individuals of
2727-410: Is selection favoring extreme trait values over intermediate trait values. Disruptive selection may cause sympatric speciation through niche partitioning . Some forms of balancing selection do not result in fixation, but maintain an allele at intermediate frequencies in a population. This can occur in diploid species (with pairs of chromosomes) when heterozygous individuals (with just one copy of
2828-412: Is slowed by the competition. The variant which is a candidate for a beneficial mutation in this limited carrying capacity environment must first out-compete the "less fit" variants in order to accumulate the requisite number of replications for there to be a reasonable probability of that beneficial mutation occurring. In biology, competition is an interaction between organisms in which the fitness of one
2929-461: Is still often used by non-biologists, modern biologists avoid it because it is tautological if "fittest" is read to mean "functionally superior" and is applied to individuals rather than considered as an averaged quantity over populations. Natural selection relies crucially on the idea of heredity, but developed before the basic concepts of genetics . Although the Moravian monk Gregor Mendel ,
3030-405: Is the growth rate of the population ( N ), and K is the carrying capacity of its local environmental setting. Typically, r -selected species exploit empty niches , and produce many offspring, each with a relatively low probability of surviving to adulthood. In contrast, K -selected species are strong competitors in crowded niches, and invest more heavily in much fewer offspring, each with
3131-611: Is the differential survival and reproduction of individuals due to differences in phenotype . It is a key mechanism of evolution , the change in the heritable traits characteristic of a population over generations. Charles Darwin popularised the term "natural selection", contrasting it with artificial selection , which is intentional, whereas natural selection is not. Variation of traits, both genotypic and phenotypic , exists within all populations of organisms . However, some traits are more likely to facilitate survival and reproductive success . Thus, these traits are passed onto
Anadenanthera - Misplaced Pages Continue
3232-435: Is the result of mutations, genetic recombinations and alterations in the karyotype (the number, shape, size and internal arrangement of the chromosomes ). Any of these changes might have an effect that is highly advantageous or highly disadvantageous, but large effects are rare. In the past, most changes in the genetic material were considered neutral or close to neutral because they occurred in noncoding DNA or resulted in
3333-474: The Industrial Revolution , many of the trees on which the moths rested became blackened by soot , giving the dark-coloured moths an advantage in hiding from predators. This gave dark-coloured moths a better chance of surviving to produce dark-coloured offspring, and in just fifty years from the first dark moth being caught, nearly all of the moths in industrial Manchester were dark. The balance
3434-692: The Linnean Society of London from 1861 to 1874. He was elected a Foreign Honorary Member of the American Academy of Arts and Sciences in 1866. He was appointed CMG (Companion of St Michael & St George) in 1878. His foreign awards included the Clarke Medal of the Royal Society of New South Wales in 1879. Bentham's first publication was his Catalogue des plantes indigènes des Pyrénées et du Bas Languedoc (Paris 1826),
3535-551: The Roman poet Lucretius , expressed the idea that nature produces a huge variety of creatures, randomly, and that only those creatures that manage to provide for themselves and reproduce successfully persist. Empedocles' idea that organisms arose entirely by the incidental workings of causes such as heat and cold was criticised by Aristotle in Book II of Physics . He posited natural teleology in its place, and believed that form
3636-491: The Royal Botanic Gardens, Kew . At the same time, he contemplated the abandonment of botanical work. However, he yielded to the persuasion of Sir William Jackson Hooker , John Lindley and other scientific friends. In 1855 he took up his residence in London, and worked at Kew for five days a week, with a brief summer holiday, from this time onwards until the end of his life. In 1857, the government sanctioned
3737-524: The founder effect in initially small new populations. When genetic variation does not result in differences in fitness, selection cannot directly affect the frequency of such variation. As a result, the genetic variation at those sites is higher than at sites where variation does influence fitness. However, after a period with no new mutations, the genetic variation at these sites is eliminated due to genetic drift. Natural selection reduces genetic variation by eliminating maladapted individuals, and consequently
3838-434: The genetic regulatory programs which control the development of the embryo at molecular level. Natural selection is here understood to act on embryonic development to change the morphology of the adult body. The term natural selection is most often defined to operate on heritable traits, because these directly participate in evolution. However, natural selection is "blind" in the sense that changes in phenotype can give
3939-455: The law of real property . But the most remarkable production of this period was the Outline of a New System of Logic: With a Critical Examination of Dr. Whately's Elements of Logic (1827). In this the principle of the quantification of the predicate was first explicitly stated. This Stanley Jevons declared to be undoubtedly the most fruitful discovery made in abstract logical science since
4040-424: The modern synthesis of the mid-20th century. The addition of molecular genetics has led to evolutionary developmental biology , which explains evolution at the molecular level. While genotypes can slowly change by random genetic drift , natural selection remains the primary explanation for adaptive evolution . Several philosophers of the classical era , including Empedocles and his intellectual successor,
4141-468: The "principle by which each slight variation [of a trait], if useful, is preserved". The concept was simple but powerful: individuals best adapted to their environments are more likely to survive and reproduce. As long as there is some variation between them and that variation is heritable , there will be an inevitable selection of individuals with the most advantageous variations. If the variations are heritable, then differential reproductive success leads to
Anadenanthera - Misplaced Pages Continue
4242-496: The "upper power" but instead are generated in different forms naturally and then selected for reproduction by their compatibility with the environment. The more recent classical arguments were reintroduced in the 18th century by Pierre Louis Maupertuis and others, including Darwin's grandfather, Erasmus Darwin . Until the early 19th century, the prevailing view in Western societies was that differences between individuals of
4343-433: The Origin of Species (1942). W. D. Hamilton conceived of kin selection in 1964. This synthesis cemented natural selection as the foundation of evolutionary theory, where it remains today. A second synthesis was brought about at the end of the 20th century by advances in molecular genetics , creating the field of evolutionary developmental biology ("evo-devo"), which seeks to explain the evolution of form in terms of
4444-441: The adult population of the next generation. Though natural selection acts on individuals, the effects of chance mean that fitness can only really be defined "on average" for the individuals within a population. The fitness of a particular genotype corresponds to the average effect on all individuals with that genotype. A distinction must be made between the concept of "survival of the fittest" and "improvement in fitness". "Survival of
4545-551: The allele is fixed and the entire population shares the fitter phenotype. Far more common is stabilizing selection, which lowers the frequency of alleles that have a deleterious effect on the phenotype—that is, produce organisms of lower fitness. This process can continue until the allele is eliminated from the population. Stabilizing selection conserves functional genetic features, such as protein-coding genes or regulatory sequences , over time by selective pressure against deleterious variants. Disruptive (or diversifying) selection
4646-427: The allele) have a higher fitness than homozygous individuals (with two copies). This is called heterozygote advantage or over-dominance, of which the best-known example is the resistance to malaria in humans heterozygous for sickle-cell anaemia . Maintenance of allelic variation can also occur through disruptive or diversifying selection , which favours genotypes that depart from the average in either direction (that is,
4747-544: The antlers of stags , which are used in combat with other stags. More generally, intrasexual selection is often associated with sexual dimorphism , including differences in body size between males and females of a species. Natural selection is seen in action in the development of antibiotic resistance in microorganisms . Since the discovery of penicillin in 1928, antibiotics have been used to fight bacterial diseases. The widespread misuse of antibiotics has selected for microbial resistance to antibiotics in clinical use, to
4848-404: The apparently vestigial structure may retain a limited functionality, or may be co-opted for other advantageous traits in a phenomenon known as preadaptation . A famous example of a vestigial structure, the eye of the blind mole-rat , is believed to retain function in photoperiod perception. Speciation requires a degree of reproductive isolation —that is, a reduction in gene flow. However, it
4949-442: The assumption that groups replicate and mutate in an analogous way to genes and individuals. There is an ongoing debate over the degree to which group selection occurs in nature. Finally, selection can be classified according to the resource being competed for. Sexual selection results from competition for mates. Sexual selection typically proceeds via fecundity selection, sometimes at the expense of viability. Ecological selection
5050-450: The best chance of being preserved in the struggle for life; and from the strong principle of inheritance they will tend to produce offspring similarly characterised. This principle of preservation, I have called, for the sake of brevity, Natural Selection. Once he had his theory, Darwin was meticulous about gathering and refining evidence before making his idea public. He was in the process of writing his "big book" to present his research when
5151-601: The botanical descriptions for The Botany of the Voyage of H.M.S. Sulphur . The editor, Richard Brinsley Hinds , had been surgeon on HMS Sulphur 1835-41 while she explored the Pacific coast of the Americas . In 1854 he found the maintenance of a herbarium and library too expensive. He, therefore, offered them to the government on the understanding that they should form the foundation of such necessary aids to research in
SECTION 50
#17327804845275252-476: The concept of the "cost" of natural selection. Sewall Wright elucidated the nature of selection and adaptation. In his book Genetics and the Origin of Species (1937), Theodosius Dobzhansky established the idea that mutation, once seen as a rival to selection, actually supplied the raw material for natural selection by creating genetic diversity. Ernst Mayr recognised the key importance of reproductive isolation for speciation in his Systematics and
5353-434: The different alleles, or variants of the gene that produces the variants of the trait. Selection can be divided into three classes, on the basis of its effect on allele frequencies: directional , stabilizing , and disruptive selection . Directional selection occurs when an allele has a greater fitness than others, so that it increases in frequency, gaining an increasing share in the population. This process can continue until
5454-458: The dog-days, but only if we have it in winter. If then, it is agreed that things are either the result of coincidence or for an end, and these cannot be the result of coincidence or spontaneity, it follows that they must be for an end; and that such things are all due to nature even the champions of the theory which is before us would agree. Therefore action for an end is present in things which come to be and are by nature. The struggle for existence
5555-579: The evolution of particular populations of a species, and populations that evolve to be sufficiently different eventually become different species. Darwin's ideas were inspired by the observations that he had made on the second voyage of HMS Beagle (1831–1836), and by the work of a political economist, Thomas Robert Malthus , who, in An Essay on the Principle of Population (1798), noted that population (if unchecked) increases exponentially , whereas
5656-399: The extravagant plumage of some male birds such as the peacock. An alternate theory proposed by the same Ronald Fisher in 1930 is the sexy son hypothesis , that mothers want promiscuous sons to give them large numbers of grandchildren and so choose promiscuous fathers for their children. Aggression between members of the same sex is sometimes associated with very distinctive features, such as
5757-512: The father of modern genetics, was a contemporary of Darwin's, his work lay in obscurity, only being rediscovered in 1900. With the early 20th-century integration of evolution with Mendel's laws of inheritance, the so-called modern synthesis , scientists generally came to accept natural selection. The synthesis grew from advances in different fields. Ronald Fisher developed the required mathematical language and wrote The Genetical Theory of Natural Selection (1930). J. B. S. Haldane introduced
5858-590: The first flora of any large continental area that had ever been finished. His greatest work was the Genera Plantarum , begun in 1862, and concluded in 1883 in collaboration with Joseph Dalton Hooker . His most famous work, however, was the Handbook of the British flora , begun in 1853 and first published in 1858. This was used by students for over a century, running into many editions. After his death, it
5959-473: The fittest" does not give an "improvement in fitness", it only represents the removal of the less fit variants from a population. A mathematical example of "survival of the fittest" is given by Haldane in his paper "The Cost of Natural Selection". Haldane called this process "substitution" or more commonly in biology, this is called "fixation". This is correctly described by the differential survival and reproduction of individuals due to differences in phenotype. On
6060-472: The food supply grows only arithmetically ; thus, inevitable limitations of resources would have demographic implications, leading to a "struggle for existence". When Darwin read Malthus in 1838 he was already primed by his work as a naturalist to appreciate the "struggle for existence" in nature. It struck him that as population outgrew resources, "favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The result of this would be
6161-421: The formation of new species." Darwin wrote: If during the long course of ages and under varying conditions of life, organic beings vary at all in the several parts of their organisation, and I think this cannot be disputed; if there be, owing to the high geometrical powers of increase of each species, at some age, season, or year, a severe struggle for life, and this certainly cannot be disputed; then, considering
SECTION 60
#17327804845276262-453: The function of many other genes. Most, but not all, mutations in regulatory genes result in non-viable embryos. Some nonlethal regulatory mutations occur in HOX genes in humans, which can result in a cervical rib or polydactyly , an increase in the number of fingers or toes. When such mutations result in a higher fitness, natural selection favours these phenotypes and the novel trait spreads in
6363-487: The idea that tiny, virtually imperceptible changes in successive generations could produce consequences on the scale of differences between species. The early 19th-century zoologist Jean-Baptiste Lamarck suggested the inheritance of acquired characteristics as a mechanism for evolutionary change; adaptive traits acquired by an organism during its lifetime could be inherited by that organism's progeny, eventually causing transmutation of species . This theory, Lamarckism ,
6464-502: The infinite complexity of the relations of all organic beings to each other and to their conditions of existence, causing an infinite diversity in structure, constitution, and habits, to be advantageous to them, I think it would be a most extraordinary fact if no variation ever had occurred useful to each being's own welfare, in the same way as so many variations have occurred useful to man. But if variations useful to any organic being do occur, assuredly individuals thus characterised will have
6565-444: The level or unit of selection . Individual selection acts on the individual, in the sense that adaptations are "for" the benefit of the individual, and result from selection among individuals. Gene selection acts directly at the level of the gene. In kin selection and intragenomic conflict , gene-level selection provides a more apt explanation of the underlying process. Group selection , if it occurs, acts on groups of organisms, on
6666-403: The maintenance of the status quo by eliminating less fit variants. Selection can be classified in several different ways, such as by its effect on a trait, on genetic diversity, by the life cycle stage where it acts, by the unit of selection, or by the resource being competed for. Selection has different effects on traits. Stabilizing selection acts to hold a trait at a stable optimum, and in
6767-406: The mutations that caused the maladaptation. At the same time, new mutations occur, resulting in a mutation–selection balance . The exact outcome of the two processes depends both on the rate at which new mutations occur and on the strength of the natural selection, which is a function of how unfavourable the mutation proves to be. Genetic linkage occurs when the loci of two alleles are close on
6868-546: The naturalist Alfred Russel Wallace independently conceived of the principle and described it in an essay he sent to Darwin to forward to Charles Lyell . Lyell and Joseph Dalton Hooker decided to present his essay together with unpublished writings that Darwin had sent to fellow naturalists, and On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection
6969-481: The new line of thought was complete, and included a change from typology in taxonomy to an appreciation that "We cannot form an idea of a species from a single individual, nor of a genus from a single one of its species. We can no more set up a typical species than a typical individual." Bentham was awarded the Royal Medal of the Royal Society in 1859 and elected a Fellow in 1862. He served as president of
7070-437: The next generation. These traits can also become more common within a population if the environment that favours these traits remains fixed. If new traits become more favored due to changes in a specific niche , microevolution occurs. If new traits become more favored due to changes in the broader environment, macroevolution occurs. Sometimes, new species can arise especially if these new traits are radically different from
7171-410: The opposite of over-dominance), and can result in a bimodal distribution of trait values. Finally, balancing selection can occur through frequency-dependent selection, where the fitness of one particular phenotype depends on the distribution of other phenotypes in the population. The principles of game theory have been applied to understand the fitness distributions in these situations, particularly in
7272-412: The other hand, "improvement in fitness" can occur in an environment where "survival of the fittest" is also acting. Richard Lenski 's classic E. coli long-term evolution experiment is an example of adaptation in a competitive environment, ("improvement in fitness" during "survival of the fittest"). The probability of a beneficial mutation occurring on some member of the lineage to give improved fitness
7373-405: The other hand, "improvement in fitness" is not dependent on the differential survival and reproduction of individuals due to differences in phenotype, it is dependent on the absolute survival of the particular variant. The probability of a beneficial mutation occurring on some member of a population depends on the total number of replications of that variant. The mathematics of "improvement in fitness
7474-404: The phenotype. Some traits are governed by only a single gene, but most traits are influenced by the interactions of many genes. A variation in one of the many genes that contributes to a trait may have only a small effect on the phenotype; together, these genes can produce a continuum of possible phenotypic values. When some component of a trait is heritable, selection alters the frequencies of
7575-511: The phrase survival of the fittest , which became a popular summary of the theory. The fifth edition of On the Origin of Species published in 1869 included Spencer's phrase as an alternative to natural selection, with credit given: "But the expression often used by Mr. Herbert Spencer of the Survival of the Fittest is more accurate, and is sometimes equally convenient." Although the phrase
7676-441: The phrase "either invariably or normally come about", not the origin of species: ... Yet it is impossible that this should be the true view. For teeth and all other natural things either invariably or normally come about in a given way; but of not one of the results of chance or spontaneity is this true. We do not ascribe to chance or mere coincidence the frequency of rain in winter, but frequent rain in summer we do; nor heat in
7777-688: The point that the methicillin-resistant Staphylococcus aureus (MRSA) has been described as a "superbug" because of the threat it poses to health and its relative invulnerability to existing drugs. Response strategies typically include the use of different, stronger antibiotics; however, new strains of MRSA have recently emerged that are resistant even to these drugs. This is an evolutionary arms race , in which bacteria develop strains less susceptible to antibiotics, while medical researchers attempt to develop new antibiotics that can kill them. A similar situation occurs with pesticide resistance in plants and insects. Arms races are not necessarily induced by man;
7878-406: The population. Established traits are not immutable; traits that have high fitness in one environmental context may be much less fit if environmental conditions change. In the absence of natural selection to preserve such a trait, it becomes more variable and deteriorate over time, possibly resulting in a vestigial manifestation of the trait, also called evolutionary baggage . In many circumstances,
7979-563: The population. In this way the natural environment of an organism "selects for" traits that confer a reproductive advantage, causing evolutionary change, as Darwin described. This gives the appearance of purpose, but in natural selection there is no intentional choice. Artificial selection is purposive where natural selection is not, though biologists often use teleological language to describe it. The peppered moth exists in both light and dark colours in Great Britain, but during
8080-527: The rate of reproduction, given survival. Others split the life cycle into further components of selection. Thus viability and survival selection may be defined separately and respectively as acting to improve the probability of survival before and after reproductive age is reached, while fecundity selection may be split into additional sub-components including sexual selection, gametic selection, acting on gamete survival, and compatibility selection, acting on zygote formation. Selection can also be classified by
8181-542: The result of a careful exploration of the Pyrenees in company with G. A. Walker Arnott (1799–1868), afterwards professor of botany in the University of Glasgow . In the catalogue Bentham adopted the principle from which he never deviated, of citing nothing at second-hand. This was followed by articles on various legal subjects: on codification , in which he disagreed with his uncle, on the laws affecting larceny and on
8282-421: The same sex, that is male–male competition, or intersexual , where one gender chooses mates , most often with males displaying and females choosing. However, in some species, mate choice is primarily by males, as in some fishes of the family Syngnathidae . Phenotypic traits can be displayed in one sex and desired in the other sex, causing a positive feedback loop called a Fisherian runaway , for example,
8383-469: The selection of individuals that reproduce", without regard to whether the basis of the selection is heritable. Traits that cause greater reproductive success of an organism are said to be selected for , while those that reduce success are selected against . Natural variation occurs among the individuals of any population of organisms. Some differences may improve an individual's chances of surviving and reproducing such that its lifetime reproductive rate
8484-444: The simplest case all deviations from this optimum are selectively disadvantageous. Directional selection favours extreme values of a trait. The uncommon disruptive selection also acts during transition periods when the current mode is sub-optimal, but alters the trait in more than one direction. In particular, if the trait is quantitative and univariate then both higher and lower trait levels are favoured. Disruptive selection can be
8585-494: The study of kin selection and the evolution of reciprocal altruism . A portion of all genetic variation is functionally neutral, producing no phenotypic effect or significant difference in fitness. Motoo Kimura 's neutral theory of molecular evolution by genetic drift proposes that this variation accounts for a large fraction of observed genetic diversity. Neutral events can radically reduce genetic variation through population bottlenecks . which among other things can cause
8686-407: The term "Natural Selection", preferring the term "Natural Preservation". For Darwin and his contemporaries, natural selection was in essence synonymous with evolution by natural selection. After the publication of On the Origin of Species , educated people generally accepted that evolution had occurred in some form. However, natural selection remained controversial as a mechanism, partly because it
8787-489: The time of Aristotle . Before sixty copies had been sold the publisher became bankrupt and the stock went for wastepaper. The book passed into oblivion, and it was not until 1873 that Bentham's claims to priority were finally vindicated against those of Sir William Hamilton by Herbert Spencer . In 1836 he published his Labiatarum genera et species . In preparing this work he visited, between 1830 and 1834, every European herbarium, several more than once. The following winter
8888-446: The traits possessed by their predecessors. The likelihood of these traits being 'selected' and passed down are determined by many factors. Some are likely to be passed down because they adapt well to their environments. Others are passed down because these traits are actively preferred by mating partners, which is known as sexual selection . Female bodies also prefer traits that confer the lowest cost to their reproductive health, which
8989-518: The useful forms survived: So what hinders the different parts [of the body] from having this merely accidental relation in nature? as the teeth, for example, grow by necessity, the front ones sharp, adapted for dividing, and the grinders flat, and serviceable for masticating the food; since they were not made for the sake of this, but it was the result of accident. And in like manner as to the other parts in which there appears to exist an adaptation to an end. Wheresoever, therefore, all things together (that
9090-712: Was called to the bar and in 1832 held his first and only legal brief . However, his interest in botany never flagged and he became secretary of the Horticultural Society of London from 1829 to 1840. In 1832, he inherited the property of his uncle, Jeremy Bentham . Having inherited his father's estate the previous year, he was now sufficiently well off to do whatever he wanted, which was botany , jurisprudence and logic . Bentham married Sarah Jones (1798–1881), daughter of Sir Harford Jones Brydges , on 11 April 1833; they did not have children. Bentham died at his London home on 10 September 1884, aged 83. He
9191-421: Was achieved for a purpose, citing the regularity of heredity in species as proof. Nevertheless, he accepted in his biology that new types of animals, monstrosities (τερας), can occur in very rare instances ( Generation of Animals , Book IV). As quoted in Darwin's 1872 edition of The Origin of Species , Aristotle considered whether different forms (e.g., of teeth) might have appeared accidentally, but only
9292-520: Was an English botanist , described by the weed botanist Duane Isely as "the premier systematic botanist of the nineteenth century". Born into a distinguished family, he initially studied law, but had a fascination with botany from an early age, which he soon pursued, becoming president of the Linnaean Society in 1861, and a fellow of the Royal Society in 1862. He was the author of a number of important botanical works, particularly flora. He
9393-611: Was an influence on the Soviet biologist Trofim Lysenko 's ill-fated antagonism to mainstream genetic theory as late as the mid-20th century. Between 1835 and 1837, the zoologist Edward Blyth worked on the area of variation, artificial selection, and how a similar process occurs in nature. Darwin acknowledged Blyth's ideas in the first chapter on variation of On the Origin of Species . In 1859, Charles Darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. He defined natural selection as
9494-482: Was described by Kleinman. An empirical example of "improvement in fitness" is given by the Kishony Mega-plate experiment. In this experiment, "improvement in fitness" depends on the number of replications of the particular variant for a new variant to appear that is capable of growing in the next higher drug concentration region. Fixation or substitution is not required for this "improvement in fitness". On
9595-440: Was edited by Hooker, and was known simply as Bentham & Hooker . He is most famous for his extensive and excellent classification of plants, especially angiosperms, along with Hooker, forming the " Bentham & Hooker system ", which was published in three volumes as Genera Plantarum between 1862 and 1883. The following plants have been named in his honour: Theory of evolution by natural selection Natural selection
9696-541: Was interred in Brompton Cemetery . Bentham's life spanned the Darwinian revolution, and his young colleague Joseph Dalton Hooker was Darwin 's closest friend and one of the first to accept Darwin's ideas. Until then, Bentham unquestioningly believed that species were fixed. In 1874 he wrote that "Fifteen years have sufficed to establish a theory of evolution by natural selection ". Bentham's conversion to
9797-493: Was later described by the Islamic writer Al-Jahiz in the 9th century, particularly in the context of top-down population regulation, but not in reference to individual variation or natural selection. At the turn of the 16th century Leonardo da Vinci collected a set of fossils of ammonites as well as other biological material. He extensively reasoned in his writings that the shapes of animals are not given once and forever by
9898-729: Was passed in Vienna , where he produced his Commentationes de Leguminosarum generibus , published in the annals of the Vienna Museum. In 1842 he moved to Pontrilas in Herefordshire. His chief occupation for the next few years was his contributions to the Prodromus Systematis Naturalis Regni Vegetabilis , which was being carried on by his friend, A. P. de Candolle . In all these dealt with some 4,730 species. In 1844, he provided
9999-429: Was perceived to be too weak to explain the range of observed characteristics of living organisms, and partly because even supporters of evolution balked at its "unguided" and non- progressive nature, a response that has been characterised as the single most significant impediment to the idea's acceptance. However, some thinkers enthusiastically embraced natural selection; after reading Darwin, Herbert Spencer introduced
10100-733: Was read to the Linnean Society of London announcing co-discovery of the principle in July 1858. Darwin published a detailed account of his evidence and conclusions in On the Origin of Species in 1859. In the 3rd edition of 1861 Darwin acknowledged that others—like William Charles Wells in 1813, and Patrick Matthew in 1831—had proposed similar ideas, but had neither developed them nor presented them in notable scientific publications. Darwin thought of natural selection by analogy to how farmers select crops or livestock for breeding, which he called " artificial selection "; in his early manuscripts he referred to
10201-487: Was reversed by the effect of the Clean Air Act 1956 , and the dark moths became rare again, demonstrating the influence of natural selection on peppered moth evolution . A recent study, using image analysis and avian vision models, shows that pale individuals more closely match lichen backgrounds than dark morphs and for the first time quantifies the camouflage of moths to predation risk. The concept of fitness
#526473