SKEPTICISM TOWARDS SANTA ROSALIA, OR WHY ARE THERE SO FEW KINDS OF ANIMALS?

In a classic paper, Hutchinson (1959) set the tone for much of the ecological work done during the past 20 years by suggesting that ecologists try to explain the numbers of species of animals. Hutchinson's approach was ecological, explaining diversity by explaining how the species could coexist. There can be little question that competitive exclusion sets an upper limit on species diversity, but it is not obvious that this upper limit will be achieved. There may be additional constraints on the process of speciation, constraints set by genetics rather than ecology. It has been usual for evolutionists to reject the possibility of sympatric speciation, and this amounts to asserting the existence of such genetic constraints. Even if the ecological opportunity for coexistence is present, under the conventional view lack of geographic isolation can prevent speciation. The attempt by Rosenzweig (1975) to give a comprehensive explanation of continental species diversity takes as its starting point the assumption that geographic isolation is a necessary prerequisite to species formation. A number of workers have made and analyzed detailed population genetic models of sympatric or parapatric speciation, in particular Maynard Smith (1966), Dickinson and Antonovics (1973), and Caisse and Antonovics (1978). Balkau and Feldman (1973) made a model of migration modification which can also be regarded as a model of parapatric speciation. The upshot of these models is that it is not difficult for sympatric speciation to occur. While these authors have largely been concerned with showing that sym-