Modes of Animal Speciation

The study of speciation is an ad hoc science. No one has yet observed the develop­ ment from beginning to end of a new plant or animal species in nature. The fossil record, so useful in painting a broad picture of long-term evolutionary trends, lacks the temporal and biological sensitivity to dissect the multitude of processes involved in speciation. Population genetics, the extreme opposite of paleontology in its ability to measure and model gene frequency changes and stability, has also failed to provide much insight into the genetics of speciation (110). At the molecular level, proteins used in the current flush of allozyme studies (110, 158, 174) have almost no direct bearing on speciation. Estimates of genetic differen­ tiation based on allozyme data alone (i.e. 147, 189), therefore, provide only crude indexes of genetic divergence between species and so-called semispecies, most of which could easily be regarded as species (178). Moreover, animals used in these studies have often long since passed through the critical "point of no return," at which hybridization will no longer result in fusion of two races (24, 29, 110). They tell us little about how many or what kind of genes are directly implicated in speciation itself. Thus we are still forced to reconstruct events surrounding specia­ tion from observations on extant species, a task that has fallen almost exclusively to the naturalist (see 44, 69, 72, 124 for historical aspects). Speciation is ultimately an adaptive process that involves establishment of intrin­ sic barriers to gene flow between closely related populations by development of reproductive isolating mechanisms. A study of speciation is, to a considerable extent, a study of the genetics and evolution of reproductive isolating mechanisms. Most evolutionists therefore generally accept as a working definition the biological species concept: species represent groups of interbreeding natural populations repro­ ductively isolated from other such groups (125). Although it has its weak points (44), alternatives (e.g. 65, 164) impose more problems than they solve. Selection for reproductive isolation between closely related populations is funda­ mentally different from the process involved in local adaptation (28, 109, 181). The latter entails only minor genetic adjustments, whereas speciation frequently involves a reorganization of some crucial component in the genetic system that results in a