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When we examine the living world, we see that individual organisms are usually clustered into collections that resemble one another more or less closely and are clearly distinct from other clusters. A close examination of a sibship of Drosophila will show differences in bristle number, eye size, and details of color pattern from fly to fly, but an entomologist has no difficulty whatsoever in distinguishing Drosophila melanogaster from, say, Drosophila pseudo-Obscura. 

One never sees a fly that is halfway between these two kinds. Clearly, in nature at least, there is no effective interbreeding between these two forms. A group of organisms that exchanges genes within the group but cannot do so with other groups is what is meant by a species. Within a species, there may exist local populations that are also easily distinguished from one another by some phenotypic characters, but it is also the case that genes can easily be exchanged between them. For example, no one has any difficulty distinguishing a “typical” Senegalese from a “typical” Swede, but such people are able to mate with each other and produce progeny. 

In fact, there have been many such matings in North America in the past 300 years, creating an immense number of people of every degree of inter- mediacy between these local geographical types. They are not separate species. In general, there is some difference in the frequency of various genes in different geographical populations of any species; so the marking out of a particular population as a distinct race is arbitrary and, as a consequence, the concept of race is no longer much used in biology

MESSAGE A species is a group of organisms that can exchange genes among themselves but are genetically unable to exchange genes in nature with individuals in other such groups. A geographical race is a phenotypically distinguishable local population within a species that is capable of exchanging genes with other races within that species. Because nearly all geographical populations are different from others in the frequencies of some genes, race is a concept that makes no clear biological distinction.

All the species now existing are related to each other, having had a common ancestor at some time in the evolutionary past. That means that each of these species has separated out from a previously existing species and has become genetically distinct and genetically isolated from its ancestral line. In extraordinary circumstances, a single mutation might be enough to found such a genetically isolated group, but the carrier of that mutation would need to be capable of self-fertilization or vegetative reproduction. Moreover, that mutation would have to cause complete mating incompatibility between its carrier and the original species and to allow the new line to compete successfully with the previously established group. Although not impossible, such events must be rare.

More commonly, new species form as a result of geographical isolation. We have already seen how populations that are geographically separated will diverge from one another genetically as a consequence of unique mutations, selection, and genetic drift. Migration between populations will prevent them from diverging too far, however. As shown on page 685, even a single migrant per generation is sufficient to prevent populations from fixing at alternative alleles by genetic drift alone, and even selection toward different adaptive peaks will not succeed in causing complete divergence unless it is extremely strong. 

As a consequence, populations that diverge enough to become new, reproductively isolated species must first be virtually totally isolated from one another by some mechanical barrier. This isolation almost always requires some spatial separation, and the separation must be great enough or the natural barriers to the passage of migrants must be strong enough to prevent any effective migration. Such spatially isolated populations are referred to as allopatric. The isolating barrier might be, for example, the extending tongue of a continental glacier during glacial epochs that forces apart a previously continuously distributed population or the drifting apart of continents that become separated by water, or the infrequent colonization of islands that are far from shore. 

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