Microevolution refers to evolution on a small scale, as in a single population. A population in this context would be represented by a group of organisms that interbreed with one another. An example would be all the individuals of one ant species living in a particular area of the desert. Populations are quite literally the units of evolution. A review of several key processes helps to provide a better understanding of what occurs within populations at the microevolution level.

The four forces of evolution have widely been recognized as mutation, gene flow, genetic drift, and natural selection. Each of these processes is a mechanism that has a role in evolutionary change. A mutation is a random, permanent genetic change that can create traits that can be advantageous, deleterious or neutral for the organism. Mutations are an essential component to evolution, for without them evolution cannot occur. Gene flow, or migration, is gene transfer from one population to another and a primary source of genetic variation. Genetic drift is a random change in the gene pool that occurs in smaller populations. As a result, a genetic trait can either be lost or become more widespread. In general, genetic drift decreases variation within a population. Natural selection is a non random process that occurs gradually which allows for traits to become more or less common. Organisms then have a greater chance of survival and reproduction due to features that are suitable to the environment. As a result, the population evolves.

These mechanisms can have an effect on the gene frequencies in populations, and result in evolutionary change. Genetic variation is the force responsible for natural selection and genetic drift. As an example, if a population of ants were 100% red, selection and drift would not have an effect since their genetic composition could not change. The most common sources for genetic variation include mutations, gene flow and sex.

Speciation is most easily described as a group of individuals that interbreed in nature. In this sense, a species is the largest gene pool possible under what would be considered natural conditions. Speciation can also be the result of isolated mechanisms. In the broadest sense, isolation mechanisms are divided into two categories, geographical (geographic isolation between populations) and reproductive (gene exchange between populations). The concept of isolation and its influence on evolution is widely recognized. Isolation maintains variation produced in populations through the process of natural selection. The idea is that when populations are geographically separate, they will differ from one another. These changes could be attributed to natural selection or random chance (i.e., genetic drift), but in both instances result in reproductive isolation.

Natural selection continues to remain a central concept in the explanation of evolution. Given sufficient time it can result in speciation. Although important, it is not the only process responsible for evolution within a population. The other basic evolutionary forces of mutation, migration and genetic drift can all produce significant changes over time. Presently, life as we know it consists of an accumulation of mutations that have passed through the filter of natural selection for centuries. These processes are responsible for creating events that have shaped evolution’s history. Macroevolution occurs at or above the species level. This implies that populations and species are evolving. Ultimately, macroevolution occurs because it involves the microevolutionary changes that occur over time.