Natural selection operates on the variability that is inherent in all populations. Natural selection can be seen as differences in reproductive rates among the variants within a population. Since the possessors of different genotypes produce differing numbers of offspring, their contribution to the next generation differs, and this brings about changes in the gene pool. Individuals or populations with higher survival or fertility rates are said to be better fitted to the environment in which they live. There are three types of natural selection: directional selection, stabilizing selection, and disruptive selection.

In addition to natural selection, there are two other types of selection: kin selection and sexual selection. In kin selection, an individual contributes to his or her reproductive success by acting in a manner that allows his or her kin to be reproductively successful. One type of sexual selection is intersexual selection, which selects for traits that make males more attractive to females. Intrasexual selection involves males competing with one another with the successful individuals contributing genes to the next generation.

Macroevolution includes those processes responsible for the evolution of species and higher taxa. The local reproductive population is the deme, and the forces of evolution operate to bring about changes in gene frequencies within gene pools of demes. When demes or groups of demes become reproductively isolated, subspecies may develop. This elimination of gene flow between demes - which is usually the result of some type of geographical barrier - allows for the accumulation of different mutations within each deme. These accumulations and gene-frequency changes - generated within and restricted to each deme - ultimately make successful reproduction between the demes impossible. Over time, these populations may become distinct species, called allopatric species.

Sympatric species are closely related species that have come to reside in the same geographical area. Yet gene flow is effectively prevented by one of several reproductive isolating mechanisms: ecological isolation, seasonal isolation, sexual isolation, mechanical isolation, gametic mortality, zygotic mortality, hybrid inviability, and hybrid sterility.

Populations within a species will tend to disperse into new regions where they occupy similar ecological niches, but these new niches can never be identical to the original ones. Certain individuals within the population may possess preadapted variations that increase their adaptation in the new niche. When a population enters an area in which it has no competition, or when a population evolves new anatomical or physiological adaptations, speciation maybe quite rapid. This rapid proliferation of species is an adaptive radiation. However, if populations unable to compete in their original niche do not adapt to new or changing niches, extinction may result.