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. Learning Objectives Distinguish between selection and genetic drift. Key Takeaways Key Points Genetic drift is the change in the frequency of an allele in a population due to random sampling and the random events that influence the survival and reproduction of those individuals. Harvey, M. Protecting the innocent: studying short-range endemic taxa enhances conservation outcomes.
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Evolution in biodiversity policy: current gaps and future needs. Schlick-Steiner, B. Integrative taxonomy: a multisource approach to exploring biodiversity. Genetic variation is the raw material of evolution. Without genetic variation, a population cannot evolve in response to changing environmental variables and, as a result, may face an increased risk of extinction.
For example, if a population is exposed to a new disease, selection will act on genes for resistance to the disease if they exist in the population. But if they do not exist — if the right genetic variation is not present — the population will not evolve and could be wiped out by the disease.
Mating patterns are important. 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.
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