Two concepts of species

We can now consider evidence for the evolution of new species. Most of the evidence so far has been for small-scale change within a species: house sparrows from all over North America are still classified as house sparrows, the same species as was introduced in the 1850s.

The amounts of artificially selected change in pigeons and other domestic animals borders on the species level, but to decide whether the species barrier has been crossed we need a concept of what a biological species is.

What does it mean to say a new species has evolved?

The question lacks a simple answer that would satisfy all biologists. We can take two of the most important species concepts and see for each what the evidence for the evolution of a new species is.

• One important species concept is reproductive, and defines a species as a set of organisms that interbreed among themselves but do not breed with members of other species. Humans (Homo sapiens ) are a separate reproductive species from the common chimpanzee (Pan troglodytes ) like the one pictured opposite: any human can interbreed with any other human (of appropriate sex), but not with a chimp.

• The second important concept uses phenotypic appearance: it defines a species as a set of organisms that are sufficiently similar to one another and sufficiently different from members of other species. Again, humans and common chimpanzees belong to different species, and they are clearly distinct in phenotypic appearance.

Because some biologists reject one or other concept, we should look at the evidence for the evolution of new species according to both concepts.