The Genetic Variation in a Population Is Caused by Multiple Factors

When a population interbreeds, nonrandom mating can sometimes occur because one organism chooses to mate with another based on certain traits. In this case, individuals in the population make specific behavioral choices, and these choices shape the genetic combinations that appear in successive generations. When this happens, the mating patterns of that population are no longer random.

Nonrandom mating can occur in two forms, with different consequences. One form of nonrandom mating is inbreeding, which occurs when individuals with similar genotypes are more likely to mate with each other rather than with individuals with different genotypes. The second form of nonrandom mating is called outbreeding, wherein there is an increased probability that individuals with a particular genotype will mate with individuals of another particular genotype. Whereas inbreeding can lead to a reduction in genetic variation, outbreeding can lead to an increase.

Sometimes, there can be random fluctuations in the numbers of alleles in a population. These changes in relative allele frequency, called genetic drift, can either increase or decrease by chance over time.

Typically, genetic drift occurs in small populations, where infrequently-occurring alleles face a greater chance of being lost. Once it begins, genetic drift will continue until the involved allele is either lost by a population or is the only allele present at a particular gene locus within a population. Both possibilities decrease the genetic diversity of a population.

Genetic drift is common after a population experiences a population bottleneck. A population bottleneck arises when a significant number of individuals in a population die or are otherwise prevented from breeding, resulting in a drastic decrease in the size of the population. Genetic drift can result in the loss of rare alleles, and can decrease the size of the gene pool. Genetic drift can also cause a new population to be genetically distinct from its original population, which has led to the hypothesis that genetic drift plays a role in the evolution of new species.

How does the physical distribution of individuals affect a population? A species with a broad distribution rarely has the same genetic makeup over its entire range. For example, individuals in a population living at one end of the range may live at a higher altitude and encounter different climatic conditions than others living at the opposite end at a lower altitude. What effect does this have? At this more extreme boundary, the relative allele frequency may differ dramatically from those at the opposite boundary. Distribution is one way that genetic variation can be preserved in large populations over wide physical ranges, as different forces will shift relative allele frequencies in different ways at either end.

Migration is the movement of organisms from one location to another. Although it can occur in cyclical patterns (as it does in birds), migration when used in a population genetics context often refers to the movement of individuals into or out of a defined population. What effect does migration have on relative allele frequencies? If the migrating individuals stay and mate with the destination individuals, they can provide a sudden influx of alleles. After mating is established between the migrating and destination individuals, the migrating individuals will contribute gametes carrying alleles that can alter the existing proportion of alleles in the destination population.

How do populations respond to all these forces? As relative allele frequencies change, relative genotype frequencies may also change. Each genotype in the population usually has a different fitness for that particular environment. In other words, some genotypes will be favored, and individuals with those genotypes will continue to reproduce. Other genotypes will not be favored: individuals with those genotypes will be less likely to reproduce. What type of genotype would be unfavorable? Unfavorable genotypes take many forms, such as increased risk of predation, decreased access to mates, or decreased access to resources that maintain health. Overall, the forces that cause relative allele frequencies to change at the population level can also influence the selection forces that shape them over successive generations.

For example, if moths with genotype aa migrate into a population composed of AA and Aa individuals, they will increase the relative allele frequency of a. However, if the aa genotype has a clear disadvantage to survival (e.g. vulnerability to predation), eventually the changes brought about by the initial migration will be reversed.