Because a genetic population is described as the sum of gene (or allelic) frequencies for all the genes represented by that population, it follows that for evolution of a species to occur the gene frequencies of that population must undergo change. The Hardy-Weinberg Law described a population that exists in genetic equilibrium. Several factors can act to change fitness. Viability and fertility are traits that are associated with fitness and are directly related to the ability of an individual to survive long enough to reproduce. By altering the fitness of an individual, the mating distribution will change. The distribution will change because genotypes in the subsequent generation will not appear in direct relationship to the gene frequencies of that population prior to the change. Consequently the gene frequencies will change and the population will evolve.
The synthetic theory of evolution as described by Sewell Wright attempts to explain evolution in terms of changes in gene frequencies. This theory states that a species evolves when gene frequencies changes and the species moves it to a higher level of adaptation for a specific ecological niche. Several factors such as mutation of alleles and migration of individuals with those new alleles will create variation in the population. Selection will then chose the better adapted individuals, and the population will have evolved.
The classic example which supports this theory is that of the peppered moth in England. The moth can be either dark or light colored. Prior to the industrialization of central England, the light-colored allele was most prevalent. The light-colored moths would hide on the white-barked trees and avoid bird predation. But the pollution generated by the new industries stained the light-colored trees dark. Gradually the light-colored moth was attacked and that allele became much less prevalent. In its place, the dark-colored allele became the most predominant allele because moths that carried that allele could camouflage themselves on the stained trees and avoid being eaten by their bird predators. Clearly the population had evolved to a higher adaptive condition.
Because population changes require changes in gene frequencies, it is important to understand how these frequencies can change. The three primary methods of change are mutation, migration and selection. Each will be considered individually.
Mutations are classified as beneficial, harmful or neutral. Harmful mutations will be lost if they reduce the fitness of the individual. If fitness is improved by a mutation, then frequencies of that allele will increase from generation to generation. The mutation could be a change in one allele to resemble one currently in the population, for example from a dominant to a recessive allele. Alternatively, the mutation could generate an entirely new allele. Most of these mutations though will be detrimental and lost. But if the environment changes, then the new mutant allele may be favored and eventually become the dominant alelle in that population. If the mutation is beneficial to the species as a whole, migration from the population in which it initially arose must occur for it to spread to other populations of the species.
The most basic type of mutation is the change in a single nucleotide in the gene. Mutations are generally deleterious and are selected against. But the genome of a species can undergo another type of change, gene duplication, which actually favors mutational events. If a single gene that is important undergoes a duplication, mutation in the duplicated copy would not necessarily reduce the fitness of the individual because it still would have a functioning copy of the original gene. With this adaptive constraint removed, further changes can occur that generate a new gene that has a similar function in the organism, but may function at a specific time in development, or in a unique location in the individual. This type of evolution generates multigene families. Many important genes such as hemoglobin and muscle genes in humans, and seed storage and photosynthetic genes in plants are organized as multigene families.
One of the assumptions of the Hardy-Weinberg Law is that the population is closed. But for many populations this is not the case. Human populations clearly are not closed. Migration will change gene frequencies by bringing in more copies of an allele already in the population or by bringing in a new allele that has arisen by mutation. Because mutations do not occur in every population, migration will be required for that allele to spread throughout that species. Migration, in a sociological context, implies the movement of individuals into new populations. In a genetic context, though, migration requires that this movement be coupled with the introduction of new alleles into the population. This will only occur after the migrant has successfully mated with an individual in the population. The term that is used to described this introduction of new alleles is gene flow. The two effects of migration are to increase variability within a population and at the same time prevent a population of that species from diverging to the extent that it becomes a new species. The first effect is important because it provides the variability that a population will require to survive if the environment changes drastically. As migration continues over a period of time, the new mutation will be shared between populations. This blending effect helps stabilize the similarities between the population and prevent more isolated populations form evolving reproductive barriers that may lead to speciation.
A natural result of mutation is that new forms develop, and these new forms may or may not add to the fitness of the individual. If the fitness of the individual leads to a reproductive advantage then the alleles present in that individual will be more prevalent in the population. In this manner the alleles of this individual are selected. The process is called selection. In a Darwinian, context this is also called natural selection. The three forces that have been described lead to changes in gene frequencies within a population. But evolution, as defined by Darwin, is driven by natural selection.
Allele frequency is the rate of occurrence of an allele in the population gene pool.
Alleles are "rival" variants for the same gene. For instance, if hair colour is coded for by a single gene, then brown may be one allele for that gene, and blond another.
For humans, the population gene pool would consist of the collection of all copies of all alleles present in the population. Note that humans are diploid, so may carry two copies of an allele, or even rival alleles for the same gene at the same time.
The allele frequency is the number of copies of a particular allele found in that collection.
Evolution is often measured in terms of the changes in allele frequencies in population gene pools.