14.13. The Biological Species Concept

A key aspect of Darwin’s theory of evolution is his proposal that adaptation (microevolution) leads ultimately to large- scale changes leading to species formation and higher taxonomic groups (macroevolution). The way natural selection leads to the formation of new species has been thoroughly documented by biologists, who have observed the stages of the species-forming process, or speciation, in many different plants, animals, and microorganisms. Speciation usually involves successive change: First, local populations become increasingly specialized; then, if they become different enough, natural selection may act to keep them that way.

Before we can discuss how one species gives rise to another, we need to understand exactly what a species is. The evolutionary biologist Ernst Mayr coined the biological species concept, which defines species as “groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.”

In other words, the biological species concept says that a species is composed of populations whose members mate with each other and produce fertile offspring—or would do so if they came into contact. Conversely, populations whose members do not mate with each other or who cannot produce fertile offspring are said to be reproductively isolated and, thus, members of different species.

What causes reproductive isolation? If organisms cannot interbreed or cannot produce fertile offspring, they clearly belong to different species. However, some populations that are considered to be separate species can interbreed and produce fertile offspring, but they ordinarily do not do so under natural conditions. They are still considered to be reproductively isolated in that genes from one species generally will not be able to enter the gene pool of the other species. Table 14.1 summarizes the steps at which barriers to successful reproduction may occur. Such barriers are termed reproductive isolating mechanisms because they prevent genetic exchange between species. We will first discuss prezygotic isolating mechanisms, those that prevent the formation of zygotes. Then we will examine postzygotic isolating mechanisms, those that prevent the proper functioning of zygotes after they have formed.

Even though the definition of what constitutes a species is of fundamental importance to evolutionary biology, this issue has still not been completely settled and is currently the subject of considerable research and debate. For example, the biological species concept has had a number of problems. Plants of different species cross fertilize and produce fertile hybrids at much higher frequencies than first thought. Hybridization is common enough to cast doubt about whether reproductive isolation is the only force maintaining the integrity of plant species.

TABLE 14.1. ISOLATING MECHANISMS

Key Learning Outcome 14.13. A species is generally defined as a group of similar organisms that does not exchange genes extensively with other groups in nature.