Changes within a gene pool occurring from generation to generation is called microevolution. Allele frequencies in a population may change due to gene flow, genetic drift, natural selection and mutation. These are referred to as the four fundamental forces of evolution. Note that only mutation can create new genetic variation. The other three forces simply rearrange this variation within and among populations.
The main factors change frequencies of alleles for a single genetic locus and therefore cause genetic variation on a small scale. The following causes of microevolution allow violations of the Hardy Weinberg assumption
- Genetic Drift
- Natural Selection
- Gene Flow
In genetic drift there is a genetic variation among allele frequencies of that population due to random chance. Therefore a small population would allow for higher chance of sampling error and a change in allele frequencies as a result would lead to a misrepresentation of the parent generations gene pool. The misrepresentation of allele frequencies causing a change in the population is an evolutionary process.
The bottleneck effect results in a drastic change of allele frequencies of a gene pool causing genetic drift. This dramatic change in allele's occurs as a result of natural disasters such as earthquakes or floods. The portion of the population that survives such a circumstance will then be overrepresented in the gene pool while causing a reduced population size. Genetic variation is reduced due to the smaller population size and over representation of certain allele frequencies.
Another dramatic mechanism for genetic drift is due to the a small number of individuals from an originally larger population inhabiting a new isolated geographic region such as an island, lake, or some other new habitat. This is not to be confused with immigration because in immigration a population already inhabits the region when new individuals to the region begin to inhabit it. These new individuals make up the gene pool of their original population and now represent it in the new habitat but in a smaller proportion. This affects the allele frequencies of that population.
Inbreeding refers to non-random mating among closely related individuals. These situations tend to increase the chance of the homozygous condition, thus leading to lower fitness and survival rates.
Inbreeding and homozygous conditions lead to higher-possibilities in expressing recessive or negative traits.
A population can evolve by natural selection where the traits characterizing a population can change over time when its individuals differ in heritable traits that are responsible for differences in survival and reproduction.
Gene flow refers to changes in allele frequency that result from migration of individuals between populations. In the absence of gene flow, populations can become genetically distinct from one another through genetic drift, or due to differing natural selection pressures in the different populations. Gene flow has the effect of minimizing the genetic differences between populations. Surprisingly little gene flow is needed to keep populations from diverging. Sewall Wright demonstrated that only one migrant per generation is necessary to prevent two populations from diverging. Natural selection can cause microevolution (change in allele frequencies), with fitness-increasing alleles becoming more common in the population. Fitness is a measure of reproductive success (how many offspring an organism leaves in the next generation, relative to others in the group).
Mutation is the ultimate source of all new genetic variation. Mutations are random, heritable changes in DNA that can alter gene expression. However, they occur rarely. Most new mutations are expected to be neutral to mildly deleterious, with relatively fewer highly advantageous or highly deleterious mutations. How quickly a mutation spreads through a population depends on its effect of fitness (advantageous vs. deleterious), and also whether it is dominant, co-dominant, or recessive. In general recessive mutations should spread slowly, since they initially will show up in heterozygotes, so the effect of the new mutation is not seen until two of these heterozygotes mate to produce offspring which are homozygous for the mutation (and therefore the effect of the mutation can be seen). In contrast, dominant mutations may spread quickly, since their effects are seen immediately when they are present in heterozygous form. Mutations are the ONLY source of new alleles.