Causes of Microevolution

Natural Selection

You can look to Charles Darwin's seminal theory of natural selection as the main mechanism for microevolution. Alleles that produce favorable adaptations get passed to future generations because those desirable traits make it more likely that the individuals possessing them live long enough to reproduce. As a result, unfavorable adaptations eventually get bred out of the population and those alleles disappear from the gene pool. Over time, changes in allele frequency become more apparent when compared to previous generations. 

Migration

Migration, or the movement of individuals into or out of a population, can shift the genetic traits present in that population at any time. Just as northern birds migrate south in the winter, other organisms change their locations seasonally or in response to unexpected environmental pressures. Immigration, or the movement of an individual into a population, introduces different alleles into the new host population. Those alleles can spread among the new population via breeding. Emigration, or the shift of individuals out of a population, results in the loss of alleles, which in turn decreases the available genes in the originating gene pool.

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Mating Choices

Asexual reproduction essentially clones a parent by copying its alleles without any sort of mating between individuals. In some species that use sexual reproduction, individuals choose a partner with no concern for specific traits or characteristics, randomly passing alleles from one generation to the next.

However, many animals, including humans, choose their mates selectively. Individuals seek out particular traits in a potential sexual partner that could translate to an advantage for their offspring. Without the random passing of alleles from one generation to the next, selective mating leads to a reduction of undesirable traits in a population and a smaller overall gene pool, resulting in identifiable microevolution.

Mutations

Mutations shift the occurrence of alleles by changing the actual DNA of an organism. Several types of mutations can occur with varying degrees of change accompanying them. The frequency of alleles may not necessarily increase or decrease with a small change in DNA, such as a point mutation, but mutations can lead to lethal changes for organisms, such as a frameshift mutation. If a change in DNA occurs in gametes, it can be passed to the next generation. This either creates new alleles or removes existing traits from the population. However, cells come equipped with a system of checkpoints to prevent mutations or correct them when they do occur, so mutations within populations rarely change the gene pool.

Genetic Drift

Significant microevolution-related differences between generations occur more frequently in smaller populations. Environmental and other factors of everyday life can cause a random change in a population called genetic drift. Most frequently caused by a chance event that affects the survival of individuals and reproduction success within a population, genetic drift can change the frequency with which some alleles occur in future generations of the affected population.

Genetic drift differs from mutation, even though results can seem similar. While some environmental factors cause mutations in DNA, genetic drift typically results from behavior that occurs in response to an external factor, such as a change in selective breeding standards to compensate for a sudden population reduction following a natural disaster or overcoming geographic obstacles for smaller organisms.