Evolution - the change in genotype of a population over generations - is a foundational concept in all of biology. And while many people understand, or are at least somewhat familiar with, the underpinning mechanism of natural selection - there are a variety of mechanisms and processes that can lead to evolution.

These mechanisms of microevolution are the processes that lead to that change in allele frequency that characterizes the process. The three requirements of evolution are variation, heritability, and differential reproductive success. And these following mechanisms are what affect or contribute to those.

Natural selection is the process by which individuals with traits that increase their fitness in a given environment are more likely to survive and reproduce. Over time, these advantageous traits become more common in the population. This is the only mechanism of evolution that leads to adaptation.

This mechanism is discussed more in depth here.

Nonrandom mating occurs when individuals in a population do not pair up randomly. Instead, mate choice is influenced by specific traits, behaviors, or preferences. 

One common form is assortative mating, where individuals are more likely to mate with others who are similar to themselves in certain traits. This could involve things like size, coloration, or behavior. For example, taller individuals may prefer other tall individuals. Assortative mating tends to increase the number of homozygous individuals in the population and can reduce genetic variation for the trait being selected.

In contrast, disassortative mating is when individuals are more likely to mate with others who are different from themselves. This increases the number of heterozygotes and can help maintain genetic diversity. An example might be individuals preferring mates with different immune system genes, which may improve disease resistance in offspring. You might be questioning how you would know there is significant diversity for genes related to your immune system? Well, this is actually one that impacts humans - we are more attracted to the scent of people who are genetically different from us in the MHC region of chromosome six, which codes for highly linked adaptive immunity proteins.

Other types of nonrandom mating include sexual selection, where traits that improve mating success are favored, and artificial selection, where humans select which individuals reproduce based on specific traits. Both of these are discussed in more detail on the "Selection" page.

Mutations are changes in the DNA sequence. They can be caused by errors in replication, environmental damage, or spontaneous chemical changes. Mutations are the source of genetic variation, providing the raw material that other mechanisms of evolution act upon. 

This mechanism is discussed more in depth here.

Genetic drift is a non-adaptive mechanism of evolution in which chance/random events results in a change in allele or genotypic frequencies in a population. Imagine having a bowl of skittles, where each color skittle represented a different genotype. If you were to reach into the bowl and take out a handful of skittles, it's entirely possible, and depending on how many skittles you take out, even likely, that the proportion of colors in your hand is different than the proportion in the bowl; that is essentially how genetic drift works. This process has a stronger effect on small populations than it does on large populations.

There are two major types of genetic drift.

Gene flow is the movement of alleles from one population to another, usually through the migration of individuals. When organisms immigrate into a new population and reproduce, they introduce new alleles into that gene pool. When individuals emigrate from a population, they take their alleles with them, potentially changing the genetic makeup of both the source and receiving populations.

This exchange of genetic material increases genetic diversity within populations and can reduce genetic differences between them. Gene flow can counteract the effects of natural selection or genetic drift by reintroducing alleles that might otherwise be lost. It’s especially important in maintaining variation in small or isolated populations and helps prevent populations from becoming genetically distinct from one another.