How is directional selection related to evolution

Sexual selection, the selection pressure on males and females to obtain matings, can result in traits designed to maximize sexual success.

The selection pressures on males and females to obtain matings is known as sexual selection. The limiting sex is the sex which has the higher parental investment, which therefore faces the most pressure to make a good mate decision. Sexual selection in elk : This male elk has large antlers to compete with rival males for available females intrasexual competition. Tn addition, the many points on his antlers represent health and longevity, and therefore he may be more desirable to females intersexual selection.

Males and females of certain species are often quite different from one another in ways beyond the reproductive organs. These differences are called sexual dimorphisms and arise from the variation in male reproductive success. Females almost always mate, while mating is not guaranteed for males.

The bigger, stronger, or more decorated males usually obtain the vast majority of the total matings, while other males receive none. This can occur because the males are better at fighting off other males, or because females will choose to mate with the bigger or more decorated males. In either case, this variation in reproductive success generates a strong selection pressure among males to obtain those matings, resulting in the evolution of bigger body size and elaborate ornaments in order to increase their chances of mating.

Females, on the other hand, tend to get a handful of selected matings; therefore, they are more likely to select more desirable males. Sexual dimorphism : Morphological differences between males and females of the same species is known as sexual dimorphism.

These differences can be observed in a peacocks and peahens, b Argiope appensa spiders the female spider is the large one , and c wood ducks. Sexual dimorphism varies widely among species; some species are even sex-role reversed. In such cases, females tend to have a greater variation in their reproductive success than males and are, correspondingly, selected for the bigger body size and elaborate traits usually characteristic of males.

In addition to being more visible to predators, it makes the males slower in their attempted escapes. There is some evidence that this risk, in fact, is why females like the big tails in the first place.

Because large tails carry risk, only the best males survive that risk and therefore the bigger the tail, the more fit the male. This idea is known as the handicap principle. A male bird of paradise : This male bird of paradise carries an extremely long tail as the result of sexual selection. This may be an example of the handicap principle. The good genes hypothesis states that males develop these impressive ornaments to show off their efficient metabolism or their ability to fight disease.

Females then choose males with the most impressive traits because it signals their genetic superiority, which they will then pass on to their offspring. Though it might be argued that females should not be so selective because it will likely reduce their number of offspring, if better males father more fit offspring, it may be beneficial.

Fewer, healthier offspring may increase the chances of survival more than many, weaker offspring. This is an example of the extreme behaviors that arise from intense sexual selection pressure.

Natural selection cannot create novel, perfect species because it only selects on existing variations in a population. Natural selection is a driving force in evolution and can generate populations that are adapted to survive and successfully reproduce in their environments. However, natural selection cannot produce the perfect organism. Natural selection can only select on existing variation in the population; it cannot create anything from scratch. Natural selection is also limited because it acts on the phenotypes of individuals, not alleles.

Some alleles may be more likely to be passed on with alleles that confer a beneficial phenotype because of their physical proximity on the chromosomes. Alleles that are carried together are in linkage disequilibrium. When a neutral allele is linked to beneficial allele, consequently meaning that it has a selective advantage, the allele frequency can increase in the population through genetic hitchhiking also called genetic draft.

Any given individual may carry some beneficial alleles and some unfavorable alleles. Natural selection acts on the net effect of these alleles and corresponding fitness of the phenotype. As a result, good alleles can be lost if they are carried by individuals that also have several overwhelmingly bad alleles; similarly, bad alleles can be kept if they are carried by individuals that have enough good alleles to result in an overall fitness benefit.

Contains introductory material on directional selection. Mitton, J. Selection in natural populations. A review and synthesis of studies of selection on genetic variation and protein polymorphisms in natural populations, and why this comes about. The approaches described have been outdated by the modern march to use genomic methods, but the book documents many classic and easily understood examples. Williams, G. Adaptation and natural selection: A critique of some current Evolutionary thought.

An enormously influential book. 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.

Users without a subscription are not able to see the full content on this page. Please subscribe or login. Oxford Bibliographies Online is available by subscription and perpetual access to institutions. For more information or to contact an Oxford Sales Representative click here. Not a member?

Sign up for My OBO. Already a member? Publications Pages Publications Pages. Subscriber sign in You could not be signed in, please check and try again. Username Please enter your Username.

We can think about this issue in terms of multiple replicate populations, each of which represents a deme subpopulation within a metapopulation collection of demes. 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.

Our results are in full agreement with previous results for two-trait systems and further extend them to include scenarios of greater complexity. Finally, we discuss the evolutionary consequences of modular patterns being molded by directional selection.