Genetic variance is a measure of the genetic differences, or genetic variation, that exist within a population. Genetic variations are the differences in DNA segments or genes between individuals and each variation of a gene is called an allele. For example, a population with many different alleles at a single chromosome locus has a high amount of genetic variation. Genetic variation is essential for natural selection to operate since natural selection can only increase or decrease frequency of alleles that already exist in the population. Genetic variation drives evolution by supplying raw material for evolutionary change. Often, the genetically-determined components of phenotypic variation can have evolutionary effects.
Causes of Genetic Variation
Genetic variation is caused by:
- 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, known as phenotypic variation.
Evolution and Adaptation to the Environment
Variation allows some individuals within a population to adapt to the changing environment. Because natural selection acts directly only on phenotypes, more genetic variance within a population usually enables more phenotypic variance (Figure 1). Some new alleles allow for greater survivability and, therefore, are selected for, while others are detrimental so those individuals die out. Since neutral alleles are neither selected for nor against, they usually remain in the population. Genetic variation is advantageous because it enables some individuals and, therefore, a population, to survive despite a changing environment. Genes and alleles are mixed to contribute to variation through breeding.
Species display geographic variation as well as variation within a population. Geographic variation, or the distinctions in the genetic makeup of different populations, often occurs when populations are geographically separated by environmental barriers or when they are under selection pressures from a different environment. One example of geographic variation are clines: graded changes in a character down a geographic axis.
Sources of Genetic Variation
Gene duplication, mutation, or other processes can produce new genes and alleles and increase genetic variation. New genetic variants can be created within generations in a population, so a population with rapid reproduction rates will probably have high genetic variance. However, existing genes can be arranged in new ways from chromosomal crossing over and recombination in sexual reproduction. Overall, the main sources of genetic variation are the formation of new alleles, the altering of gene number or position, rapid reproduction, and sexual reproduction.