The scientific definition of a species is a discrete group of organisms that can only breed within its own confines. In other words, the members of one species cannot interbreed with the members of another species. Each species is said to experience reproductive isolation. If you think about evolution in terms of genetics, this definition of species makes a great deal of sense: if species could interbreed, they could share gene flow, and their evolution would not be separate. But since species cannot interbreed, each species exists on its own individual path.

 

    As populations change, new species evolve. This process is known as speciation. Through speciation, the earliest simple organisms were able to branch out and populate the world with millions of different species. Speciation is also called divergent evolution, since when a new species develops, it diverges from a previous form. All homologous traits are produced by divergent evolution. Whales and humans share a distant common ancestor. Through speciation, that ancestor underwent divergent evolution and gave rise to new species, which in turn gave rise to new species, which over the course of millions of years resulted in whales and humans. The original ancestor had a limb structure that, over millions of years and successive occurrences of divergent evolution, evolved into the fin of the whale and the arm of the human.

 

    Speciation occurs when two populations become reproductively isolated. Once reproductive isolation occurs for a new species, it will begin to evolve independently. There are two main ways in which speciation might occur. Allopatric speciation occurs when populations of a species become geographically isolated so that they cannot interbreed. Over time, the populations may become genetically different in response to the unique selection pressures operating in their different environments. Eventually the genetic differences between the two populations will become so extreme that the two populations would be unable to interbreed even if the geographic barrier disappeared.

A second, more common form of speciation is adaptive radiation, which is the creation of several new species from a single parent species. Think of a population of a given species, which we’ll imaginatively name population 1. The population moves into a new habitat and establishes itself in a niche, or role, in the habitat (we discuss niches in more detail in the chapter on Ecology). In so doing, it adapts to its new environment and becomes different from the parent species. If a new population of the parent species, population 2, moves into the area, it too will try to occupy the same niche as population 1. Competition between population 1 and population 2 ensues, placing pressure on both groups to adapt to separate niches, further distinguishing them from each other and the parent species. As this happens many times in a given habitat, several new species may be formed from a single parent species in a relatively short time. The immense diversity of finches that Darwin observed on the Galápagos Islands is an excellent example of the products of adaptive radiation.