In a population that is not at Hardy-Weinberg equilibrium, the frequency of alleles changes over time. Therefore, any deviations from the five conditions of Hardy-Weinberg equilibrium can alter the genetic variation of a given population. Conditions that change the genetic variability of a population include mutations, natural selection, non-random mating, gene flow, and genetic drift (small population size).
The original sources of genetic variation are mutations, which are changes in the nucleotide sequence of DNA. Mutations create new alleles and increase genetic variability. Most mutations do not cause significant changes to the health or functioning of an organism. However, if a mutation reduces the chances of survival, the organism may die before reproducing. Therefore, such harmful mutations are likely to be eliminated by natural selection.
Individuals in natural populations may also select their mates based on certain characteristics, and thus do not reproduce randomly. In this case, alleles for the traits that are selected against will become less frequent in the population.
Furthermore, populations can experience gene flow, the transfer of alleles into and out of gene pools, due to migration. A classic example of gene flow is observed in most baboon species. Female baboons mate most frequently with dominant males in a troop. Juvenile male baboons almost always leave their birth troops, likely to avoid inbreeding, and join a new troop, where they may pass their genes to offspring.
In genetic drift, chance events alter the allele frequencies of a population. A major disturbance, such as a natural disaster, may drastically reduce population size and thereby diminish genetic variation. The resulting composition of the gene pool was selected randomly (i.e., surviving the disturbance was not determined by the genetic make-up of the individual). Such a reduction of genetic diversity is called a genetic bottleneck.
Sometimes, a population may become fragmented into smaller populations due to urban development or other events. A new population is started by a small group of members of the original population and by chance, a previously rare allele may be relatively frequent. This effect on gene frequencies is known as the founder effect.
Genetic variation is the basis for evolution. Natural selection can occur only if multiple forms of genes (alleles) are present in the population—favoring alleles that confer a fitness advantage under current conditions. On the other hand, loss of genetic variation can have detrimental effects on populations. If the gene pool does not contain gene variants that promote survival and reproduction when the environment changes, the population cannot adapt and may vanish. These negative effects are more pronounced in smaller populations, as the gene pool is smaller to begin with. Smaller populations are hence more vulnerable to stochastic events. Conservation efforts often focus on increasing genetic variability via selective breeding of individuals in small and endangered populations.
