Chromosomal rearrangements maintain a polymorphic supergene controlling butterfly mimicry. Kettlewell HBD Selection experiments on industrial melanism in the Lepidoptera. Heredity 9 : — A resume of investigations on the evolution of melanism in the Lepidoptera. A survey of the frequencies of Biston betularia L. Heredity 12 : 51— Geographical melanism in the Lepidoptera of Shetland.
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Trends Genet 19 : — Flight periodicity and the vertical distribution of high-altitude moth migration over southern Britain. Bull Ent Res 99 : — Wright S Am Nat 63 : — The low line, on the other hand, has linearly decreased in size for approximately 25 generations, but then further decrease was no longer feasible. Even though selection was on the lightest chickens every generation, the next generation was not getting any lighter anymore.
It is not clear why this is the case. Selection results are always expressed at phenotypic level. It could be that the genetically smallest birds showed the same phenotype as the genetically larger birds, so directional selection was no longer possible.
In that case this selection limit represents a physiological limit, rather than limited genetic variation. Which of the two was the case could be tested by selecting the light birds upwards again.
If that is still possible then the genetic variation is still present. Another reason for the reached selection limit could be that the smallest birds were no longer capable of reproduction. That would be a typical example of natural selection working in the opposite direction of artificial selection.
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The theory of natural selection was explored by 19th-century naturalist Charles Darwin. Natural selection explains how genetic traits of a species may change over time. This may lead to speciation, the formation of a distinct new species.
Select from these resources to teach your classroom about this subfield of evolutionary biology. Artificial selection is the identification by humans of desirable traits in plants and animals, and the steps taken to enhance and perpetuate those traits in future generations. Artificial selection works the same way as natural selection, except that with natural selection it is nature, not human interference, that makes these decisions.
Charles Darwin and Alfred Wallace developed the idea of evolution through natural selection. But this idea was not accepted by scientists until more evidence came along. Use this infographic to explore how Darwinism and genetics came together to explain what we know today about evolution.
Join our community of educators and receive the latest information on National Geographic's resources for you and your students. 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.
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