Why is variation essential for evolution

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Mind Read. Eyes on Environment. For example, a big, powerful male gorilla will mate with more females than a small, weak male and therefore more of his genes will be passed on to the next generation. His offspring may continue to dominate the troop and pass on their genes as well.

Over time, the selection pressure will cause the allele frequencies in the gorilla population to shift toward large, strong males. Unlike natural selection, genetic drift describes the effect of chance on populations in the absence of positive or negative selection pressure.

Through random sampling, or the survival or and reproduction of a random sample of individuals within a population, allele frequencies within a population may change. Rather than a male gorilla producing more offspring because he is stronger, he may be the only male available when a female is ready to mate.

His genes are passed on to future generation because of chance, not because he was the biggest or the strongest. Genetic drift is the shift of alleles within a population due to chance events that cause random samples of the population to reproduce or not.

Effect of genetic drift : Genetic drift in a population can lead to the elimination of an allele from that population by chance. In this example, the brown coat color allele B is dominant over the white coat color allele b. In the first generation, the two alleles occur with equal frequency in the population, resulting in p and q values of.

Only half of the individuals reproduce, resulting in a second generation with p and q values of. Only two individuals in the second generation reproduce and, by chance, these individuals are homozygous dominant for brown coat color.

As a result, in the third generation the recessive b allele is lost. Small populations are more susceptible to the forces of genetic drift. Large populations, on the other hand, are buffered against the effects of chance. Genetic drift can also be magnified by natural events, such as a natural disaster that kills a large portion of the population at random.

The bottleneck effect occurs when only a few individuals survive and reduces variation in the gene pool of a population. The genetic structure of the survivors becomes the genetic structure of the entire population, which may be very different from the pre-disaster population. Effect of a bottleneck on a population : A chance event or catastrophe can reduce the genetic variability within a population. Another scenario in which populations might experience a strong influence of genetic drift is if some portion of the population leaves to start a new population in a new location or if a population gets divided by a physical barrier of some kind.

In this situation, it is improbable that those individuals are representative of the entire population, which results in the founder effect. The Founder Effect : The founder effect occurs when a portion of the population i. The founder effect is believed to have been a key factor in the genetic history of the Afrikaner population of Dutch settlers in South Africa, as evidenced by mutations that are common in Afrikaners, but rare in most other populations.

This was probably due to the fact that a higher-than-normal proportion of the founding colonists carried these mutations. The Hardy—Weinberg principle states that within sufficiently large populations, the allele frequencies remain constant from one generation to the next unless the equilibrium is disturbed by migration, genetic mutation, or selection.

Because the random sampling can remove, but not replace, an allele, and because random declines or increases in allele frequency influence expected allele distributions for the next generation, genetic drift drives a population towards genetic uniformity over time. Once an allele becomes fixed, genetic drift for that allele comes to a halt, and the allele frequency cannot change unless a new allele is introduced in the population via mutation or gene flow.

Thus even while genetic drift is a random, directionless process, it acts to eliminate genetic variation over time. Genetic drift over time : Ten simulations of random genetic drift of a single given allele with an initial frequency distribution 0. In these simulations, alleles drift to loss or fixation frequency of 0.

An important evolutionary force is gene flow: the flow of alleles in and out of a population due to the migration of individuals or gametes.

While some populations are fairly stable, others experience more movement and fluctuation. Many plants, for example, send their pollen by wind, insects, or birds to pollinate other populations of the same species some distance away. Key Terms genetic diversity : the level of biodiversity, refers to the total number of genetic characteristics in the genetic makeup of a species crossing over : the exchange of genetic material between homologous chromosomes that results in recombinant chromosomes phenotypic variation : variation due to underlying heritable genetic variation ; a fundamental prerequisite for evolution by natural selection genetic variation : variation in alleles of genes that occurs both within and among populations.

Genetic Variation Genetic variation is a measure of the genetic differences that exist within a population. Genetic variation is caused by: mutation random mating between organisms random fertilization crossing over or recombination between chromatids of homologous chromosomes during meiosis The last three of these factors reshuffle alleles within a population, giving offspring combinations which differ from their parents and from others.

This phenotypic variation is due at least partly to genetic variation within the coquina population. Evolution and Adaptation to the Environment Variation allows some individuals within a population to adapt to the changing environment.

Because wild cheetahs are threatened, their species has a very low genetic diversity. Geographic Variation Some species display geographic variation as well as variation within a population. Random forces lead to genetic drift. If the individuals at either end of the range reconnect and continue mating, the resulting genetic intermixing can contribute to more genetic variation overall. However, if the range becomes wide enough that interbreeding between opposite ends becomes less and less likely, and the different forces acting at either end become more and more pronounced, and the individuals at each end of the population range may eventually become genetically distinct from one another.

Here is an example of migration affecting relative allele frequency:. The overall effect. Here is an example of how a specific genotype is less favorable than another genotype:. Genetic variation in a population is derived from a wide assortment of genes and alleles. The persistence of populations over time through changing environments depends on their capacity to adapt to shifting external conditions.

Sometimes the addition of a new allele to a population makes it more able to survive; sometimes the addition of a new allele to a population makes it less able. Still other times, the addition of a new allele to a population has no effect at all, yet the new allele will persist over generations because its contribution to survival is neutral.

Key Questions How can genetic variation influence evolution? What is an example of genetic drift? Topic rooms within Genetics Close. No topic rooms are there. Browse Visually. Other Topic Rooms Genetics. Student Voices. Creature Cast. Simply Science. Green Screen. Green Science.