Reacting to the group selectionist and teleological thinking that was common at the time of its writing, Williams was the first clearly and explicitly to advocate, in an accessible style, that the explanation for evolutionary adaptations should be sought mainly in the simple operation of natural selection at the level of the individual and the gene.
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Related Articles Expand or collapse the "related articles" section about About Related Articles close popup. Your current browser may not support copying via this button. Introduction Directional selection occurs when individuals with traits on one side of the mean in their population survive better or reproduce more than those on the other. General Overviews Most undergraduate textbooks on evolution, such as Futuyma and Barton, et al.
How to Subscribe Oxford Bibliographies Online is available by subscription and perpetual access to institutions. Jump to Other Articles:. Evolution and Development of Individual Behavioral Variati Oxford University Press. All rights reserved. Diversifying selection can also occur when environmental changes favor individuals on either end of the phenotypic spectrum.
Imagine a population of mice living at the beach where there is light-colored sand interspersed with patches of tall grass. In this scenario, light-colored mice that blend in with the sand would be favored, as well as dark-colored mice that can hide in the grass. Medium-colored mice, on the other hand, would not blend in with either the grass or the sand and, thus, would more probably be eaten by predators.
The result of this type of selection is increased genetic variance as the population becomes more diverse. Learning Objectives Contrast stabilizing selection, directional selection, and diversifying selection. Diversifying or disruptive selection increases genetic variance when natural selection selects for two or more extreme phenotypes that each have specific advantages.
In diversifying or disruptive selection, average or intermediate phenotypes are often less fit than either extreme phenotype and are unlikely to feature prominently in a population.
Key Terms directional selection : a mode of natural selection in which a single phenotype is favored, causing the allele frequency to continuously shift in one direction disruptive selection : or diversifying selection a mode of natural selection in which extreme values for a trait are favored over intermediate values stabilizing selection : a type of natural selection in which genetic diversity decreases as the population stabilizes on a particular trait value.
Stabilizing Selection If natural selection favors an average phenotype by selecting against extreme variation, the population will undergo stabilizing selection. Directional Selection When the environment changes, populations will often undergo directional selection, which selects for phenotypes at one end of the spectrum of existing variation. Furthermore, natural selection can be constrained by the relationships between different polymorphisms.
One morph may confer a higher fitness than another, but may not increase in frequency because the intermediate morph is detrimental. Polymorphism in the grove snail : Color and pattern morphs of the grove snail, Cepaea nemoralis. The polymorphism, when two or more different genotypes exist within a given species, in grove snails seems to have several causes, including predation by thrushes. For example, consider a hypothetical population of mice that live in the desert.
Some are light-colored and blend in with the sand, while others are dark and blend in with the patches of black rock. The dark-colored mice may be more fit than the light-colored mice, and according to the principles of natural selection the frequency of light-colored mice is expected to decrease over time.
However, the intermediate phenotype of a medium-colored coat is very bad for the mice: these cannot blend in with either the sand or the rock and will more vulnerable to predators. As a result, the frequency of a dark-colored mice would not increase because the intermediate morphs are less fit than either light-colored or dark-colored mice.
This a common example of disruptive selection. Finally, it is important to understand that not all evolution is adaptive. Evolution has no purpose. It is not changing a population into a preconceived ideal. It is simply the sum of various forces and their influence on the genetic and phenotypic variance of a population.
Privacy Policy. Skip to main content. The Evolution of Populations. Search for:. Adaptive Evolution. Natural Selection and Adaptive Evolution Natural selection drives adaptive evolution by selecting for and increasing the occurrence of beneficial traits in a population. Learning Objectives Explain how natural selection leads to adaptive evolution. Key Takeaways Key Points Natural selection increases or decreases biological traits within a population, thereby selecting for individuals with greater evolutionary fitness.
An individual with a high evolutionary fitness will provide more beneficial contributions to the gene pool of the next generation. Stabilizing selection, directional selection, diversifying selection, frequency -dependent selection, and sexual selection all contribute to the way natural selection can affect variation within a population.
Key Terms natural selection : a process in which individual organisms or phenotypes that possess favorable traits are more likely to survive and reproduce fecundity : number, rate, or capacity of offspring production Darwinian fitness : the average contribution to the gene pool of the next generation that is made by an average individual of the specified genotype or phenotype. Stabilizing, Directional, and Diversifying Selection Stabilizing, directional, and diversifying selection either decrease, shift, or increase the genetic variance of a population.
Learning Objectives Contrast stabilizing selection, directional selection, and diversifying selection. Diversifying or disruptive selection increases genetic variance when natural selection selects for two or more extreme phenotypes that each have specific advantages. In diversifying or disruptive selection, average or intermediate phenotypes are often less fit than either extreme phenotype and are unlikely to feature prominently in a population.
Key Terms directional selection : a mode of natural selection in which a single phenotype is favored, causing the allele frequency to continuously shift in one direction disruptive selection : or diversifying selection a mode of natural selection in which extreme values for a trait are favored over intermediate values stabilizing selection : a type of natural selection in which genetic diversity decreases as the population stabilizes on a particular trait value.
Frequency-Dependent Selection In frequency-dependent selection, phenotypes that are either common or rare are favored through natural selection. Learning Objectives Describe frequency-dependent selection. Positive frequency-dependent selection selects for common phenotypes in a population and decreases genetic variance. In the example of male side-blotched lizards, populations of each color pattern increase or decrease at various stages depending on their frequency; this ensures that both common and rare phenotypes continue to be cyclically present.
Infectious agents such as microbes can exhibit negative frequency-dependent selection; as a host population becomes immune to a common strain of the microbe, less common strains of the microbe are automatically favored. Variation in color pattern mimicry by the scarlet kingsnake is dependent on the prevalence of the eastern coral snake, the model for this mimicry, in a particular geographical region. Key Terms frequency-dependent selection : the term given to an evolutionary process where the fitness of a phenotype is dependent on its frequency relative to other phenotypes in a given population polygynous : having more than one female as mate.
Sexual Selection Sexual selection, the selection pressure on males and females to obtain matings, can result in traits designed to maximize sexual success. Learning Objectives Discuss the effects of sexual dimorphism on the reproductive potential of an organism. The handicap principle states that only the best males survive the risks from traits that may actually be detrimental to a species; therefore, they are more fit as mating partners.
In the good genes hypothesis, females will choose males that show off impressive traits to ensure they pass on genetic superiority to their offspring. Sexual dimorphisms, obvious morphological differences between the sexes of a species, arise when there is more variance in the reproductive success of either males or females. Key Terms sexual dimorphism : a physical difference between male and female individuals of the same species sexual selection : a type of natural selection, where members of the sexes acquire distinct forms because members choose mates with particular features or because competition for mates with certain traits succeed handicap principle : a theory that suggests that animals of greater biological fitness signal this status through a behavior or morphology that effectively lowers their chances of survival.
No Perfect Organism Natural selection cannot create novel, perfect species because it only selects on existing variations in a population. Learning Objectives Explain the limitations encountered in natural selection. Natural selection is limited through linkage disequilibrium, where alleles that are physically proximate on the chromosome are passed on together at greater frequencies. In a polymorphic population, two phenotypes may be maintained in the population despite the higher fitness of one morph if the intermediate phenotype is detrimental.
Evolution is not purposefully adaptive; it is the result of various selection forces working together to influence genetic and phenotypical variances within a population. Key Terms linkage disequilibrium : a non-random association of two or more alleles at two or more loci; normally caused by an interaction between genes genetic hitchhiking : changes in the frequency of an allele because of linkage with a positively or negatively selected allele at another locus polymorphism : the regular existence of two or more different genotypes within a given species or population.
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