How do populations get genetic variation

The appearance of new mutations is the most common way to introduce novel genotypic and phenotypic variance. Some mutations are unfavorable or harmful and are quickly eliminated from the population by natural selection. Others are beneficial and will spread through the population. Whether or not a mutation is beneficial or harmful is determined by whether it helps an organism survive to sexual maturity and reproduce. Some mutations have no effect on an organism and can linger, unaffected by natural selection, in the genome while others can have a dramatic effect on a gene and the resulting phenotype.

Mutation in a garden rose : A mutation has caused this garden moss rose to produce flowers of different colors. This mutation has introduce a new allele into the population that increases genetic variation and may be passed on to the next generation. Population structure can be altered by nonrandom mating the preference of certain individuals for mates as well as the environment.

Explain how environmental variance and nonrandom mating can change gene frequencies in a population. If individuals nonrandomly mate with other individuals in the population, i. There are many reasons nonrandom mating occurs. One reason is simple mate choice or sexual selection; for example, female peahens may prefer peacocks with bigger, brighter tails. Traits that lead to more matings for an individual lead to more offspring and through natural selection, eventually lead to a higher frequency of that trait in the population.

Assortative mating in the American Robin : The American Robin may practice assortative mating on plumage color, a melanin based trait, and mate with other robins who have the most similar shade of color. However, there may also be some sexual selection for more vibrant plumage which indicates health and reproductive performance. Another cause of nonrandom mating is physical location. This is especially true in large populations spread over large geographic distances where not all individuals will have equal access to one another.

Some might be miles apart through woods or over rough terrain, while others might live immediately nearby.

Genes are not the only players involved in determining population variation. Phenotypes are also influenced by other factors, such as the environment. A beachgoer is likely to have darker skin than a city dweller, for example, due to regular exposure to the sun, an environmental factor.

Some major characteristics, such as gender, are determined by the environment for some species. For example, some turtles and other reptiles have temperature-dependent sex determination TSD.

TSD means that individuals develop into males if their eggs are incubated within a certain temperature range, or females at a different temperature range. Temperature-dependent sex determination : The sex of the American alligator Alligator mississippiensis is determined by the temperature at which the eggs are incubated.

Eggs incubated at 30 degrees C produce females, and eggs incubated at 33 degrees C produce males. Geographic separation between populations can lead to differences in the phenotypic variation between those populations. Such geographical variation is seen between most populations and can be significant. One type of geographic variation, called a cline, can be seen as populations of a given species vary gradually across an ecological gradient. Geographic variation in moose : This graph shows geographical variation in moose; body mass increase positively with latitude.

This is considered a latitudinal cline. Alternatively, flowering plants tend to bloom at different times depending on where they are along the slope of a mountain, known as an altitudinal cline. If there is gene flow between the populations, the individuals will likely show gradual differences in phenotype along the cline. Restricted gene flow, on the other hand, can lead to abrupt differences, even speciation.

Privacy Policy. Skip to main content. The Evolution of Populations. Search for:. Population Genetics. Genetic Variation Genetic variation is a measure of the variation that exists in the genetic makeup of individuals within population. Learning Objectives Assess the ways in which genetic variance affects the evolution of populations.

Key Takeaways Key Points Genetic variation is an important force in evolution as it allows natural selection to increase or decrease frequency of alleles already in the population.

Genetic variation is advantageous to a population because it enables some individuals to adapt to the environment while maintaining the survival of the population. 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 Drift Genetic drift is the change in allele frequencies of a population due to random chance events, such as natural disasters. In many species, special genetic variations give animals a camouflaged appearance to blend in with their environment, like this Catalpa Sphinx moth Ceratomia catalpae which uses its textured wings to blend in with a tree's bark.

Genetic variation refers to differences among the genomes of members of the same species. A genome is all the hereditary information—all the genes—of an organism. For instance, the human genome contains somewhere between twenty and twenty-five thousand genes. Genes are units of hereditary information, and they carry instructions for building proteins. The genes that are encoded within these proteins are what enable cells to function. Most organisms that reproduce sexually have two copies of each gene , because each parent cell or organism donates a single copy of its genes to its offspring.

Additionally, genes can exist in slightly different forms, called alleles, which further adds to genetic variation. The combination of alleles of a gene that an individual receives from both parents determines what biologists call the genotype for a particular trait, such as hair texture. The genotype that an individual possesses for a trait, in turn, determines the phenotype —the observable characteristics—such as whether that individual actually ends up with straight, wavy, or curly hair.

Genetic variation within a species can result from a few different sources. Mutations, the changes in the sequences of genes in DNA , are one source of genetic variation. Another source is gene flow , or the movement of genes between different groups of organisms. Finally, genetic variation can be a result of sexual reproduction , which leads to the creation of new combinations of genes. Genetic variation in a group of organisms enables some organisms to survive better than others in the environment in which they live.

Organisms of even a small population can differ strikingly in terms of how well suited they are for life in a certain environment. An example would be moths of the same species with different color wings. Moths with wings similar to the color of tree bark are better able to camouflage themselves than moths of a different color. 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. This page appears in the following eBook.

Aa Aa Aa.