How does natural selection occur

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Ryan Gregory. Reprints and Permissions. Gregory, T. Evo Edu Outreach 2, — Download citation. Received : 14 March Accepted : 16 March Published : 09 April Issue Date : June Anyone you share the following link with will be able to read this content:. Sorry, a shareable link is not currently available for this article.

Provided by the Springer Nature SharedIt content-sharing initiative. Skip to main content. Search all BMC articles Search. Download PDF. Volume 2 Supplement 2. Abstract Natural selection is one of the central mechanisms of evolutionary change and is the process responsible for the evolution of adaptive features. Introduction Natural selection is a non-random difference in reproductive output among replicating entities, often due indirectly to differences in survival in a particular environment, leading to an increase in the proportion of beneficial, heritable characteristics within a population from one generation to the next.

The Basis and Basics of Natural Selection Though rudimentary forms of the idea had been presented earlier e. Full size image. Darwinian Fitness The Meaning of Fitness in Evolutionary Biology In order to study the operation and effects of natural selection, it is important to have a means of describing and quantifying the relationships between genotype gene complement , phenotype physical and behavioral features , survival, and reproduction in particular environments.

Which Traits Are the Most Fit? Natural Selection and Adaptive Evolution Natural Selection and the Evolution of Populations Though each has been tested and shown to be accurate, none of the observations and inferences that underlies natural selection is sufficient individually to provide a mechanism for evolutionary change Footnote 6.

Several important points can be drawn from even such an oversimplified rendition: 1. Natural Selection Is Elegant, Logical, and Notoriously Difficult to Grasp The Extent of the Problem In its most basic form, natural selection is an elegant theory that effectively explains the obviously good fit of living things to their environments. A Catalog of Common Misconceptions Whereas the causes of cognitive barriers to understanding remain to be determined, their consequences are well documented.

Table 3 Major concepts relating to adaptive evolution by natural selection, summarizing both correct and intuitive incorrect interpretations see also Fig.

Concluding Remarks Surveys of students at all levels paint a bleak picture regarding the level of understanding of natural selection. Notes 1. References Alters B. Google Scholar Attenborough D. Google Scholar Bardapurkar A. Google Scholar Bartov H. Google Scholar Beardsley PM. Google Scholar Bell G. Google Scholar Brumby M. Google Scholar Brumby MN. Google Scholar Burkhardt RW. Google Scholar Corsi P. Google Scholar Coyne JA. Google Scholar Curry A. Google Scholar Darwin, C.

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Google Scholar Packard AS. Google Scholar Palumbi SR. Google Scholar Pennock RT. Fitness is quantified relative to the average individual in the population; individuals that produce more viable progeny progeny that can live and reproduce themselves than average have greater fitness. A trait that is heritable and increases the survival and reproduction odds for those that carry that trait is called an adaptation. Evolution by natural selection occurs when the environment exerts a pressure on a population so that only some phenotypes survive and reproduce successfully.

The stronger the selective pressure or the selection event the fewer individuals make it through the sieve of natural selection. Those phenotypes that survive a strong selection event, such as a drought, are a better fit for an environment that suffers drought. Another way to say this is that they have higher Darwinian fitness. The small finches on the island of Daphna Major have strong beaks to feed on seeds. Smaller beaked birds can only crack open the smallest seeds, while birds with larger beaks prefer larger seeds.

In , drought reduced the number of small seeds, so many small-beaked finches starved to death. In the finch example above, the average phenotype has shifted so most individuals have larger beaks, which is a genetically controlled-trait in the finches.

The larger beak size is an adaptation to the seed sizes available during drought conditions. A result of this shift is that small beak phenotypes have become rare or disappeared, so there is reduced phenotypic and therefore reduced genetic diversity in the finch population after selection.

When a population displays a normal distribution for a particular trait, natural selection can drive change in populations in different directions depending on the type of selection. Given 10 finite demes of equal N e , each with a starting frequency of the A allele of 0. Our observations are likely to deviate from those expectations to some extent because we are considering a finite number of demes Figure 2.

Genetic drift thus removes genetic variation within demes but leads to differentiation among demes, completely through random changes in allele frequencies. In this last population, A would eventually reach fixation or loss. Gene flow is the movement of genes into or out of a population. Such movement may be due to migration of individual organisms that reproduce in their new populations, or to the movement of gametes e.

In the absence of natural selection and genetic drift, gene flow leads to genetic homogeneity among demes within a metapopulation, such that, for a given locus, allele frequencies will reach equilibrium values equal to the average frequencies across the metapopulation. In contrast, restricted gene flow promotes population divergence via selection and drift, which, if persistent, can lead to speciation.

Natural selection, genetic drift and gene flow do not act in isolation, so we must consider how the interplay among these mechanisms influences evolutionary trajectories in natural populations. This issue is crucially important to conservation geneticists, who grapple with the implications of these evolutionary processes as they design reserves and model the population dynamics of threatened species in fragmented habitats.

All real populations are finite, and thus subject to the effects of genetic drift. Loss of genetic variation due to drift is of particular concern in small, threatened populations, in which fixation of deleterious alleles can reduce population viability and raise the risk of extinction.

Even if conservation efforts boost population growth, low heterozygosity is likely to persist, since bottlenecks periods of reduced population size have a more pronounced influence on Ne than periods of larger population size. We have already seen that genetic drift leads to differentiation among demes within a metapopulation.

If we assume a simple model in which individuals have equal probabilities of dispersing among all demes each of effective size N e within a metapopulation, then the migration rate m is the fraction of gene copies within a deme introduced via immigration per generation. Natural selection can produce genetic variation among demes within a metapopulation if different selective pressures prevail in different demes. If N e is large enough to discount the effects of genetic drift, then we expect directional selection to fix the favored allele within a given focal deme.

However, the continual introduction, via gene flow, of alleles that are advantageous in other demes but deleterious in the focal deme, can counteract the effects of selection. In this scenario, the deleterious allele will remain at an intermediate equilibrium frequency that reflects the balance between gene flow and natural selection.

The common conception of evolution focuses on change due to natural selection. Natural selection is certainly an important mechanism of allele-frequency change, and it is the only mechanism that generates adaptation of organisms to their environments. Other mechanisms, however, can also change allele frequencies, often in ways that oppose the influence of selection.

A nuanced understanding of evolution demands that we consider such mechanisms as genetic drift and gene flow, and that we recognize the error in assuming that selection will always drive populations toward the most well adapted state. Carroll, S. Conservation Biology: Evolution in Action. Darwin, C. London, England: John Murray, Gillespie, J.

Population Genetics: A Concise Guide , 2nd ed. Haldane, J. A mathematical theory of natural and artificial selection, Part I. Transactions of the Cambridge Philosophical Society 23 , 19—41 Hedrick, P. Genetics of Populations, 3rd ed. Tyson Brown, National Geographic Society. National Geographic Society. For information on user permissions, please read our Terms of Service. If you have questions about how to cite anything on our website in your project or classroom presentation, please contact your teacher.

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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.