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151-397: Badass or bad ass may refer to: A quality of personality or behavior inspiring fear , awe and respect ; see masculinity , toughness and macho . Fear In humans and other animals, fear is modulated by the process of cognition and learning. Thus, fear is judged as rational and appropriate, or irrational and inappropriate (or unconscious). An irrational fear is called

302-584: A phobia . Fear is closely related to the emotion anxiety , which occurs as the result of often future threats that are perceived to be uncontrollable or unavoidable. The fear response serves survival by engendering appropriate behavioral responses, so it has been preserved throughout evolution . Sociological and organizational research also suggests that individuals' fears are not solely dependent on their nature but are also shaped by their social relations and culture, which guide their understanding of when and how much fear to feel. Many physiological changes in

453-420: 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

604-457: 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

755-458: A Santa Claus mask with white cotton balls in the beard. Fear can be learned by experiencing or watching a frightening traumatic accident. For example, if a child falls into a well and struggles to get out, he or she may develop a fear of wells, heights ( acrophobia ), enclosed spaces ( claustrophobia ), or water ( aquaphobia ). There are studies looking at areas of the brain that are affected in relation to fear. When looking at these areas (such as

906-527: A chemical smelling of banana, and a component of bee alarm pheromone. The experiment also showed that the bees' fear-induced pain tolerance was mediated by an endorphin . By using the forced swimming test in rats as a model of fear-induction, the first mammalian "alarm substance" was found. In 1991, this "alarm substance" was shown to fulfill criteria for pheromones: well-defined behavioral effect, species specificity, minimal influence of experience and control for nonspecific arousal. Rat activity testing with

1057-434: A child's psyche development or personality. For example, parents tell their children not to talk to strangers in order to protect them. In school, they would be motivated to not show fear in talking with strangers, but to be assertive and also aware of the risks and the environment in which it takes place. Ambiguous and mixed messages like this can affect their self-esteem and self-confidence. Researchers say talking to strangers

1208-519: 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

1359-400: A cold person warmer or a frightened animal look more impressive), sweating, increased blood glucose ( hyperglycemia ), increased serum calcium, increase in white blood cells called neutrophilic leukocytes, alertness leading to sleep disturbance and "butterflies in the stomach" ( dyspepsia ). This primitive mechanism may help an organism survive by either running away or fighting the danger. With

1510-415: A comparably strong emotional response in both females and males, stress-induced sweat from females produced markedly stronger arousal in women than in men. Statistical tests pinpointed this gender-specificity to the right amygdala and strongest in the superficial nuclei. Since no significant differences were found in the olfactory bulb , the response to female fear-induced signals is likely based on processing

1661-418: A creature, species, or situations that should be avoided. SSDRs are an evolutionary adaptation that has been seen in many species throughout the world including rats, chimpanzees , prairie dogs , and even humans , an adaptation created to help individual creatures survive in a hostile world. Fear learning changes across the lifetime due to natural developmental changes in the brain. This includes changes in

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1812-554: 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

1963-424: A fear-provoking situation. This suggests that fear can develop in both conditions, not just simply from personal history. Fear is affected by cultural and historical context. For example, in the early 20th century, many Americans feared polio , a disease that can lead to paralysis. There are consistent cross-cultural differences in how people respond to fear. Display rules affect how likely people are to express

2114-460: A genetic effect that is the result of natural selection . From an evolutionary psychology perspective, different fears may be different adaptations that have been useful in our evolutionary past. They may have developed during different time periods. Some fears, such as fear of heights, may be common to all mammals and developed during the mesozoic period. Other fears, such as fear of snakes, may be common to all simians and developed during

2265-524: A high testosterone level related to unhappiness in response to androstenone in men, and a high estradiol level related to disliking of androstenone in women. A German study from 2006 showed when anxiety-induced versus exercise-induced human sweat from a dozen people was pooled and offered to seven study participants, of five able to olfactorily distinguish exercise-induced sweat from room air, three could also distinguish exercise-induced sweat from anxiety induced sweat. The acoustic startle reflex response to

2416-404: A higher level of fear. Pathogens can suppress amygdala activity. Rats infected with the toxoplasmosis parasite become less fearful of cats, sometimes even seeking out their urine-marked areas. This behavior often leads to them being eaten by cats. The parasite then reproduces within the body of the cat. There is evidence that the parasite concentrates itself in the amygdala of infected rats. In

2567-399: A natural pheromone-containing solution, it has been shown that the impairment was associated with defective detection of related pheromones, and with changes in their inborn preference for pheromones related to sexual and reproductive activities. Lastly, alleviation of an acute fear response because a friendly peer (or in biological language: an affiliative conspecific ) tends and befriends

2718-479: 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

2869-429: 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

3020-542: A rat perceived alarm pheromones, it increased its defensive and risk assessment behavior, and its acoustic startle reflex was enhanced. It was not until 2011 that a link between severe pain, neuroinflammation and alarm pheromones release in rats was found: real time RT-PCR analysis of rat brain tissues indicated that shocking the footpad of a rat increased its production of proinflammatory cytokines in deep brain structures, namely of IL-1β , heteronuclear Corticotropin-releasing hormone and c-fos mRNA expressions in both

3171-441: 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

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3322-430: 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

3473-458: 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

3624-403: A response to the stimulus in the form of fear or aggression commences, the amygdalae may elicit the release of hormones into the body to put the person into a state of alertness, in which they are ready to move, run, fight, etc. This defensive response is generally referred to in physiology as the fight-or-flight response regulated by the hypothalamus, part of the limbic system . Once the person

3775-414: A result of learning. This has been studied in psychology as fear conditioning , beginning with John B. Watson's Little Albert experiment in 1920, which was inspired after observing a child with an irrational fear of dogs. In this study, an 11-month-old boy was conditioned to fear a white rat in the laboratory. The fear became generalized to include other white, furry objects, such as a rabbit, dog, and even

3926-476: A separate experiment, rats with lesions in the amygdala did not express fear or anxiety towards unwanted stimuli. These rats pulled on levers supplying food that sometimes sent out electrical shocks. While they learned to avoid pressing on them, they did not distance themselves from these shock-inducing levers. Several brain structures other than the amygdalae have also been observed to be activated when individuals are presented with fearful vs. neutral faces, namely

4077-435: A sound when sensing anxiety sweat was larger than when sensing exercise-induced sweat, as measured by electromyography analysis of the orbital muscle, which is responsible for the eyeblink component. This showed for the first time that fear chemosignals can modulate the startle reflex in humans without emotional mediation; fear chemosignals primed the recipient's "defensive behavior" prior to the subjects' conscious attention on

4228-552: A source of feedback or even stimulus change. Intrinsic feedback or information coming from within, muscle twitches, increased heart rate, are seen to be more important in SSDRs than extrinsic feedback, stimuli that comes from the external environment. Dr. Bolles found that most creatures have some intrinsic set of fears, to help assure survival of the species. Rats will run away from any shocking event, and pigeons will flap their wings harder when threatened. The wing flapping in pigeons and

4379-454: 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

4530-494: A term coined in analogy to keystone species . Pheromones may determine species compositions and affect rates of energy and material exchange in an ecological community . Thus pheromones generate structure in a food web and play critical roles in maintaining natural systems . Evidence of chemosensory alarm signals in humans has emerged slowly: Although alarm pheromones have not been physically isolated and their chemical structures have not been identified in humans so far, there

4681-480: A variety of translational animal models. These models are particularly important for research that would be too invasive for human studies. Rodents such as mice and rats are common animal models, but other species are used. Certain aspects of fear research still requires more research such as sex, gender, and age differences. Natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype . It

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4832-483: 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

4983-532: 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

5134-512: 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

5285-405: 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

5436-401: Is called " social buffering ". The term is in analogy to the 1985 "buffering" hypothesis in psychology, where social support has been proven to mitigate the negative health effects of alarm pheromone mediated distress. The role of a "social pheromone" is suggested by the recent discovery that olfactory signals are responsible in mediating the "social buffering" in male rats. "Social buffering"

5587-500: 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

5738-455: 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

5889-478: Is determined by the level of fear as well as the specific context, such as environmental characteristics (escape route present, distance to refuge), the presence of a discrete and localized threat, the distance between threat and subject, threat characteristics (speed, size, directness of approach), the characteristics of the subject under threat (size, physical condition, speed, degree of crypsis , protective morphological structures), social conditions (group size), and

6040-428: Is evidence for their presence. Androstadienone , for example, a steroidal, endogenous odorant, is a pheromone candidate found in human sweat, axillary hair and plasma. The closely related compound androstenone is involved in communicating dominance, aggression or competition; sex hormone influences on androstenone perception in humans showed a high testosterone level related to heightened androstenone sensitivity in men,

6191-432: Is how animals survive in the wild. Humans and animals both share these species-specific defense reactions, such as the flight-or-fight, which also include pseudo-aggression, fake or intimidating aggression and freeze response to threats, which is controlled by the sympathetic nervous system . These SSDRs are learned very quickly through social interactions between others of the same species, other species, and interaction with

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6342-524: Is in safe mode, meaning that there are no longer any potential threats surrounding them, the amygdalae will send this information to the medial prefrontal cortex (mPFC) where it is stored for similar future situations, which is known as memory consolidation . Some of the hormones involved during the state of fight-or-flight include epinephrine , which regulates heart rate and metabolism as well as dilating blood vessels and air passages, norepinephrine increasing heart rate, blood flow to skeletal muscles and

6493-420: Is increased, which activates processes with the expected consequence of increasing consistency and decreasing arousal." In this context, it has been proposed that fear behavior is caused by an inconsistency between a preferred, or expected, situation and the actually perceived situation, and functions to remove the inconsistent stimulus from the perceptual field, for instance by fleeing or hiding, thereby resolving

6644-456: 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

6795-438: 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

6946-663: 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,

7097-483: 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

7248-555: 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

7399-461: 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

7550-451: Is not something to be thwarted but allowed in a parent's presence if required. Developing a sense of equanimity to handle various situations is often advocated as an antidote to irrational fear and as an essential skill by a number of ancient philosophies. Fear of the unknown (FOTU) "may be a, or possibly the, fundamental fear" from early times when there were many threats to life. Although fear behavior varies from species to species, it

7701-489: Is often divided into two main categories; namely, avoidance/flight and immobility. To these, different researchers have added different categories, such as threat display and attack, protective responses (including startle and looming responses), defensive burying, and social responses (including alarm vocalizations and submission). Finally, immobility is often divided into freezing and tonic immobility . The decision as to which particular fear behavior to perform

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7852-410: 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

8003-412: 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

8154-461: 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 ,

8305-405: 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

8456-435: 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

8607-522: Is to defend themselves and at the same time to inform members of the same species of danger and leads to observable behavior change like freezing, defensive behavior, or dispersion depending on circumstances and species. For example, stressed rats release odorant cues that cause other rats to move away from the source of the signal. After the discovery of pheromones in 1959, alarm pheromones were first described in 1968 in ants and earthworms, and four years later also found in mammals, both mice and rats. Over

8758-474: 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

8909-551: 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

9060-459: The amygdala ), it was proposed that a person learns to fear regardless of whether they themselves have experienced trauma, or if they have observed the fear in others. In a study completed by Andreas Olsson, Katherine I. Nearing and Elizabeth A. Phelps, the amygdala were affected both when subjects observed someone else being submitted to an aversive event, knowing that the same treatment awaited themselves, and when subjects were subsequently placed in

9211-417: The cenozoic time period (the still-ongoing geological era encompassing the last 66 million of history). Still others, such as fear of mice and insects, may be unique to humans and developed during the paleolithic and neolithic time periods (when mice and insects become important carriers of infectious diseases and harmful for crops and stored foods). Nonhuman animals and humans innovate specific fears as

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9362-524: 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

9513-434: 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

9664-424: 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,

9815-407: The paraventricular nucleus and the bed nucleus of the stria terminalis , and it increased stress hormone levels in plasma ( corticosterone ). The neurocircuit for how rats perceive alarm pheromones was shown to be related to the hypothalamus , brainstem , and amygdalae , all of which are evolutionary ancient structures deep inside or in the case of the brainstem underneath the brain away from

9966-615: The prefrontal cortex and the amygdala . The visual exploration of an emotional face does not follow a fixed pattern but modulated by the emotional content of the face. Scheller et al. found that participants paid more attention to the eyes when recognising fearful or neutral faces, while the mouth was fixated on when happy faces are presented, irrespective of task demands and spatial locations of face stimuli. These findings were replicated when fearful eyes are presented and when canonical face configurations are distorted for fearful, neutral and happy expressions. The brain structures that are

10117-468: 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

10268-402: The "recipient" rat (the rat perceiving the pheromone) depending which body region they were released from: Pheromone production from the face modified behavior in the recipient rat, e.g. caused sniffing or movement, whereas pheromone secreted from the rat's anal area induced autonomic nervous system stress responses, like an increase in core body temperature. Further experiments showed that when

10419-447: The "unknown". The irrational fear can branch out to many areas such as the hereafter, the next ten years or even tomorrow. Chronic irrational fear has deleterious effects since the elicitor stimulus is commonly absent or perceived from delusions. Such fear can create comorbidity with the anxiety disorder umbrella. Being scared may cause people to experience anticipatory fear of what may lie ahead rather than planning and evaluating for

10570-496: 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

10721-433: 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

10872-608: The acoustic startle reflex level. In analogy to the social buffering of rats and honeybees in response to chemosignals, induction of empathy by "smelling anxiety" of another person has been found in humans. A study from 2013 provided brain imaging evidence that human responses to fear chemosignals may be gender-specific . Researchers collected alarm-induced sweat and exercise-induced sweat from donors extracted it, pooled it and presented it to 16 unrelated people undergoing functional brain MRI . While stress-induced sweat from males produced

11023-441: 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

11174-446: The alarm pheromone, and their preference/avoidance for odors from cylinders containing the pheromone, showed that the pheromone had very low volatility . In 1993 a connection between alarm chemosignals in mice and their immune response was found. Pheromone production in mice was found to be associated with or mediated by the pituitary gland in 1994. In 2004, it was demonstrated that rats' alarm pheromones had different effects on

11325-551: 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

11476-427: 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,

11627-447: The amount of experience with the type of the threat. Often laboratory studies with rats are conducted to examine the acquisition and extinction of conditioned fear responses. In 2004, researchers conditioned rats ( Rattus norvegicus ) to fear a certain stimulus, through electric shock. The researchers were able to then cause an extinction of this conditioned fear, to a point that no medications or drugs were able to further aid in

11778-487: The amygdala are generated by activation of the neurons in the region. Experimental data supports the notion that synaptic plasticity of the neurons leading to the lateral amygdalae occurs with fear conditioning. In some cases, this forms permanent fear responses such as post-traumatic stress disorder (PTSD) or a phobia . MRI and fMRI scans have shown that the amygdalae in individuals diagnosed with such disorders including bipolar or panic disorder are larger and wired for

11929-544: 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

12080-404: 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

12231-442: 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

12382-450: 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

12533-457: The body are associated with fear, summarized as the fight-or-flight response . An innate response for coping with danger, it works by accelerating the breathing rate ( hyperventilation ), heart rate, vasoconstriction of the peripheral blood vessels leading to blood pooling, dilating the pupils, increasing muscle tension including the muscles attached to each hair follicle to contract and causing "goosebumps", or more clinically, piloerection (making

12684-520: The brain, there are various regions of the brain involved in deciphering fear in humans and other nonhuman species. The amygdala communicates both directions between the prefrontal cortex , hypothalamus , the sensory cortex , the hippocampus , thalamus , septum , and the brainstem . The amygdala plays an important role in SSDR, such as the ventral amygdalofugal, which is essential for associative learning , and SSDRs are learned through interaction with

12835-414: The capacity to fear is part of human nature . Many studies have found that certain fears (e.g. animals, heights) are much more common than others (e.g. flowers, clouds). These fears are also easier to induce in the laboratory. This phenomenon is known as preparedness . Because early humans that were quick to fear dangerous situations were more likely to survive and reproduce; preparedness is theorized to be

12986-418: The center of most neurobiological events associated with fear are the two amygdalae , located behind the pituitary gland. Each amygdala is part of a circuitry of fear learning. They are essential for proper adaptation to stress and specific modulation of emotional learning memory. In the presence of a threatening stimulus, the amygdalae generate the secretion of hormones that influence fear and aggression. Once

13137-476: 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

13288-449: The cortex, and involved in the fight-or-flight response , as is the case in humans. Alarm pheromone-induced anxiety in rats has been used to evaluate the degree to which anxiolytics can alleviate anxiety in humans. For this, the change in the acoustic startle reflex of rats with alarm pheromone-induced anxiety (i.e. reduction of defensiveness) has been measured. Pretreatment of rats with one of five anxiolytics used in clinical medicine

13439-559: The development of the olfactory bulb and odor discrimination and it is highly expressed in developing rat brains, but absent in most regions of adult rat brains. Conditional deletion of the MAPK7gene in mouse neural stem cells impairs several pheromone-mediated behaviors, including aggression and mating in male mice. These behavior impairments were not caused by a reduction in the level of testosterone, by physical immobility, by heightened fear or anxiety or by depression. Using mouse urine as

13590-434: 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

13741-458: 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

13892-454: The environment and others of the same species. An emotional response is created only after the signals have been relayed between the different regions of the brain, and activating the sympathetic nervous systems; which controls the flight, fight, freeze, fright, and faint response . Often a damaged amygdala can cause impairment in the recognition of fear (like the human case of patient S.M. ). This impairment can cause different species to lack

14043-416: The environment. These acquired sets of reactions or responses are not easily forgotten. The animal that survives is the animal that already knows what to fear and how to avoid this threat. An example in humans is the reaction to the sight of a snake, many jump backwards before cognitively realizing what they are jumping away from, and in some cases, it is a stick rather than a snake. As with many functions of

14194-579: 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

14345-442: The extinction process. The rats showed signs of avoidance learning, not fear, but simply avoiding the area that brought pain to the test rats. The avoidance learning of rats is seen as a conditioned response , and therefore the behavior can be unconditioned, as supported by the earlier research. Species-specific defense reactions (SSDRs) or avoidance learning in nature is the specific tendency to avoid certain threats or stimuli, it

14496-399: 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

14647-559: The facial expression of fear and other emotions. Fear of victimization is a function of perceived risk and seriousness of potential harm. According to surveys, some of the most common fears are of demons and ghosts , the existence of evil powers, cockroaches , spiders , snakes , heights , water , enclosed spaces , tunnels , bridges , needles , social rejection , failure , examinations , and public speaking . Regionally some may more so fear terrorist attacks , death , war , criminal or gang violence , being alone ,

14798-512: 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

14949-473: 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

15100-472: 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

15251-421: 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

15402-453: 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

15553-453: The future, nuclear war , flying , clowns , intimacy , people , and driving . Fear of the unknown or irrational fear is caused by negative thinking ( worry ) which arises from anxiety accompanied by a subjective sense of apprehension or dread. Irrational fear shares a common neural pathway with other fears, a pathway that engages the nervous system to mobilize bodily resources in the face of danger or threat. Many people are scared of

15704-448: The gene stathmin show no avoidance learning, or a lack of fear, and will often walk directly up to cats and be eaten. Animals use these SSDRs to continue living, to help increase their chance of fitness , by surviving long enough to procreate. Humans and animals alike have created fear to know what should be avoided, and this fear can be learned through association with others in the community, or learned through personal experience with

15855-487: 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 ,

16006-434: The inconsistency. This approach puts fear in a broader perspective, also involving aggression and curiosity . When the inconsistency between perception and expectancy is small, learning as a result of curiosity reduces inconsistency by updating expectancy to match perception. If the inconsistency is larger, fear or aggressive behavior may be employed to alter the perception in order to make it match expectancy, depending on

16157-502: 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

16308-444: 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

16459-403: 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

16610-477: The meaning, i.e. on the emotional level, rather than the strength of chemosensory cues from each gender, i.e. the perceptual level. An approach-avoidance task was set up where volunteers seeing either an angry or a happy cartoon face on a computer screen pushed away or pulled toward them a joystick as fast as possible. Volunteers smelling androstadienone, masked with clove oil scent responded faster, especially to angry faces than those smelling clove oil only, which

16761-406: 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

16912-457: The natural world. He theorized the species-specific defense reaction (SSDR). There are three forms of SSDRs: flight, fight (pseudo-aggression), or freeze. Even domesticated animals have SSDRs, and in those moments it is seen that animals revert to atavistic standards and become "wild" again. Dr. Bolles states that responses are often dependent on the reinforcement of a safety signal, and not the aversive conditioned stimuli. This safety signal can be

17063-546: 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

17214-437: 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

17365-530: The next two decades, identification and characterization of these pheromones proceeded in all manner of insects and sea animals, including fish, but it was not until 1990 that more insight into mammalian alarm pheromones was gleaned. In 1985, a link between odors released by stressed rats and pain perception was discovered: unstressed rats exposed to these odors developed opioid-mediated analgesia. In 1997, researchers found that bees became less responsive to pain after they had been stimulated with isoamyl acetate ,

17516-545: The occipito cerebellar regions including the fusiform gyrus and the inferior parietal / superior temporal gyri. Fearful eyes, brows and mouth seem to separately reproduce these brain responses. Scientists from Zurich studies show that the hormone oxytocin related to stress and sex reduces activity in your brain fear center. In threatening situations, insects, aquatic organisms, birds, reptiles, and mammals emit odorant substances, initially called alarm substances, which are chemical signals now called alarm pheromones . This

17667-410: 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

17818-412: 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

17969-405: 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

18120-404: 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

18271-511: 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

18422-441: 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

18573-688: 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;

18724-406: 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,

18875-563: 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

19026-527: 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

19177-442: The release of glucose from energy stores, and cortisol which increases blood sugar, increases circulating neutrophilic leukocytes, calcium amongst other things. After a situation which incites fear occurs, the amygdalae and hippocampus record the event through synaptic plasticity . The stimulation to the hippocampus will cause the individual to remember many details surrounding the situation. Plasticity and memory formation in

19328-421: 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,

19479-572: The same. For example, "continuation of scholarly education" is perceived by many educators as a risk that may cause them fear and stress, and they would rather teach things they've been taught than go and do research. The ambiguity of situations that tend to be uncertain and unpredictable can cause anxiety in addition to other psychological and physical problems in some populations; especially those who engage it constantly, for example, in war-ridden places or in places of conflict, terrorism, abuse, etc. Poor parenting that instills fear can also debilitate

19630-645: The scattered running of rats are considered species-specific defense reactions or behaviors. Bolles believed that SSDRs are conditioned through Pavlovian conditioning, and not operant conditioning; SSDRs arise from the association between the environmental stimuli and adverse events. Michael S. Fanselow conducted an experiment, to test some specific defense reactions, he observed that rats in two different shock situations responded differently, based on instinct or defensive topography, rather than contextual information. Species-specific defense responses are created out of fear, and are essential for survival. Rats that lack

19781-469: 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

19932-417: The sensation of fear, and often can become overly confident, confronting larger peers, or walking up to predatory creatures. Robert C. Bolles (1970), a researcher at University of Washington, wanted to understand species-specific defense reactions and avoidance learning among animals, but found that the theories of avoidance learning and the tools that were used to measure this tendency were out of touch with

20083-470: The series of physiological changes, the consciousness realizes an emotion of fear. There are observable physical reactions in individuals who experience fear. An individual might experience a feeling of dizziness, lightheaded, like they are being choked, sweating, shortness of breath, vomiting or nausea, numbness or shaking and any other like symptoms. These bodily reactions informs the individual that they are afraid and should proceed to remove or get away from

20234-444: 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

20385-435: The size of the inconsistency as well as the specific context. Aggressive behavior is assumed to alter perception by forcefully manipulating it into matching the expected situation, while in some cases thwarted escape may also trigger aggressive behavior in an attempt to remove the thwarting stimulus. In order to improve our understanding of the neural and behavioral mechanisms of adaptive and maladaptive fear, investigators use

20536-586: The stimulus that is causing that fear. An influential categorization of stimuli causing fear was proposed by psychologist Jeffrey Alan Gray ; namely, intensity , novelty , special evolutionary dangers, stimuli arising during social interaction, and conditioned stimuli. Another categorization was proposed by Archer, who, besides conditioned fear stimuli, categorized fear-evoking (as well as aggression -evoking) stimuli into three groups; namely, pain , novelty, and frustration , although he also described " looming ", which refers to an object rapidly moving towards

20687-494: 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

20838-407: 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

20989-446: 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

21140-518: 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

21291-831: The visual sensors of a subject, and can be categorized as "intensity". Russell described a more functional categorization of fear-evoking stimuli, in which for instance novelty is a variable affecting more than one category: 1) Predator stimuli (including movement, suddenness, proximity, but also learned and innate predator stimuli); 2) Physical environmental dangers (including intensity and heights); 3) Stimuli associated with increased risk of predation and other dangers (including novelty, openness, illumination, and being alone); 4) Stimuli stemming from conspecifics (including novelty, movement, and spacing behavior); 5) Species-predictable fear stimuli and experience (special evolutionary dangers); and 6) Fear stimuli that are not species predictable (conditioned fear stimuli). Although many fears are learned,

21442-532: Was able to reduce their anxiety: namely midazolam , phenelzine (a nonselective monoamine oxidase (MAO) inhibitor), propranolol , a nonselective beta blocker , clonidine , an alpha 2 adrenergic agonist or CP-154,526 , a corticotropin-releasing hormone antagonist . Faulty development of odor discrimination impairs the perception of pheromones and pheromone-related behavior, like aggressive behavior and mating in male rats: The enzyme Mitogen-activated protein kinase 7 (MAPK7) has been implicated in regulating

21593-421: 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

21744-508: Was also observed to mitigate the conditioned fear responses of honeybees. A bee colony exposed to an environment of high threat of predation did not show increased aggression and aggressive-like gene expression patterns in individual bees, but decreased aggression. That the bees did not simply habituate to threats is suggested by the fact that the disturbed colonies also decreased their foraging. Biologists have proposed in 2012 that fear pheromones evolved as molecules of "keystone significance",

21895-611: 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

22046-482: 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

22197-434: Was interpreted as androstadienone-related activation of the fear system. A potential mechanism of action is, that androstadienone alters the "emotional face processing". Androstadienone is known to influence the activity of the fusiform gyrus which is relevant for face recognition . Cognitive-consistency theories assume that "when two or more simultaneously active cognitive structures are logically inconsistent, arousal

22348-493: 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

22499-429: 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

22650-797: 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

22801-487: 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

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