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Evolutionary agentsComments

Evolutionary agents

An evolutionary agent causes changes in the allele and genotype frequencies in a population. These are observed as a deviations from the Hardy–Weinberg equilibrium.

One condition for Hardy–Weinberg equilibrium is that there is no mutation. Although this condition is never met, the rate at which mutations arise at single loci is usually so low that mutations result in only very small deviations from Hardy–Weinberg expectations. If large deviations are found, it is appropriate to dismiss mutation as the cause and look for evidence of other evolutionary agents.

The Four Known Evolutionary Agents

MutationA mutation is any random change in the genetic material of an organism. It is a cause of evolution. It is a primary source of variation for natural selection. Only the mutation in gametes is considered as an evolution events. Since mutation occurs randomly and at a very low frequency, by itself, mutation can not account for the speed of evolution. Other agents, such as genetic drift and natural selection, play greater roles in the evolutionary changes. A mutation changes a single allele, random genetic drift and natural selection can take that single change and make it into the prevalent allele type in a population (conversely, they can also extinguish the most common alleles).
Random Genetic Drift

Random genetic drift describes non-directed changes (unlike natural selection, which is directed change) in the frequencies of the alleles found in a population. Small population sizes provide the most likely environment for random genetic drift to occur. To conceptualize it, think about the coin flipping experiment. One common situation is known as the founder effect, in which a small group migrates to a new habitat. The genetic complement of the subsequent population is based upon the genetic composition of those few individuals, and not on the average allele frequencies of the original population. The founder effect is thought to be responsible for the origin of many new species.

Genetic drift is the random loss of individuals and the alleles they possess. In very small populations, genetic drift may be strong enough to influence the direction of change of allele frequencies even when other evolutionary agents are pushing the frequencies in a different direction. Organisms that normally have large populations may pass through occasional periods when only a small number of individuals survive (a population bottleneck).

During a population bottleneck, genetic variation can be reduced by genetic drift. Populations in nature pass through bottlenecks for numerous reasons; for example, predation and habitat destruction may reduce the population to a very small size, resulting in low genetic variation.

When a few pioneering individuals colonize a new region, the resulting population will not have all the alleles found among members of the source population. The resulting pattern of genetic variation is called a founder effect.

Migration

Migration typically counteracts random genetic drift. Large populations that fragment into smaller ones can maintain relative stability of allele frequencies if individuals continually migrate between these populations and exchange alleles. Comigration of initially distinct populations, such as various human races arriving in North America and interbreeding, can create allele combinations never before present in a species' history.

Gene flow results when individuals migrate to another population and breed in their new location. Immigrants may add new alleles to the gene pool of a population, or they may change the frequencies of alleles already present if they come from a population with different allele frequencies. No immigration is allowed for a population to be in Hardy–Weinberg equilibrium.

Natural Selection

Natural selection is an interaction between organisms and their environment that causes differential reproductive success. Three necessary and sufficient conditions are:

  1. Variation in a trait among the individuals in a population;
  2. The trait must be inheritable;
  3. The variation of trait must lead to differential reproductive success among the individuals in a population.
Darwinian fitness refers to how successful an organism is in passing on its alleles to future generations. Artificial selection simply refers to a subset of natural selection in which humans are the agents determining the differential reproductive success. Three common modes of natural selection are:
  1. Stabilizing selection: decreases the frequency of traits on the extreme ends of their range of variation. It serves to increase the frequency of individuals with the average value of a given trait.
  2. Directional selection: changes the average value of a trait over time. The result is that a new value, either higher or lower than the original one, becomes the average for the trait in that population.
  3. Disruptive selection: occurs when the individuals possessing the most common value of a trait are selected against.