ORIGIN AND EVOLUTION OF ETHOLOGICAL ISOLATION IN SUBTERRANEAN MOLE RATS

Reproductive isolation is essential for speciation and for the maintenance of species genetic integrity, yet its origin and evolution, particularly of ethological isolation, is still largely controversial and in most cases unknown (Mayr, 1970). The nature, origin and evolution of reproductive isolation may be highlighted particularly in actively evolving species, and thereby contribute to understanding the process of speciation and the evolutionary future of hybrid zones between parapatric species. There are two major classical theories of the origin of isolating mechanisms (Mayr, 1970; Dobzhansky et al., 1977): (a) the allopatric, or incidental origin theory and (b) the sympatric, or reinforcement theory. According to the former, pre-zygotic isolating mechanisms arise as an incidental by-product of genetic divergence in isolated populations (Darwin, 1859; Muller, 1940, 1942; Mayr, 1942, 1970). The latter theory postulates that isolating mechanisms are perfected through direct selection when two incipient species begin to become sympatric, through progressive elimination of hybrids of lower fitness, in the overlapping (sympatric) zones (Wallace, 1889; Fisher, 1930; Dobzhansky, 1940, 1970). It is widely accepted that post-zygotic isolating mechanisms are the incidental result of evolutionary divergence (Moore, 1957; Littlejohn, 1969; Dobzhansky et al., 1977; but see also alternative views in Grant, 1966; Watson and Martin, 1968; Coyne, 1974). The main controversy between the allopatric and sympatric theories concerns the origins of pre-zygotic isolating mechanisms, and is focused on the question of the extent to which direct selection contributes to this acquisition. The problem may be restated as follows: is the pre-zygotic isolation accumulated and perfected strictly as an incidental by-product of divergence in isolated populations-or, do these mechanisms evolve primarily by direct selection in sympatry (Dobzhansky, 1970; Dobzhansky and Pavlovsky, 1971; Ehrman, 1971, 1979). A third alternative combines the two theories suggesting that the principal genetic basis of this isolation was acquired in separate gene pools and then perfected by direct selection in sympatry. The studies and techniques which support the allopatric theory have been extensively reviewed (Grant, 1966, 1977; Littlejohn, 1969, 1980; Mayr, 1970; Brown, 1975), but only in a small number of cases was there clear experimental evidence for this process (e.g., Nevo, 1969b; Capranica et al., 1973). A much larger number of experiments studied the reinforcement process, and results of these studies are generally interpreted as supporting this theory (see the above reviews and also Dobzhansky, 1970; Blair, 1974; Dobzhansky et al., 1977). Yet critical analysis of these studies reveal that only a few of them provide unequivocal evidence for reinforcement in pre-zygotic isolation (see reviews of different taxa in Thielcke, 1973; Walker, 1974; Fouquette, 1975; Loftus-Hills, 1975). Therefore, critical tests of the origin of pre-zygotic isolation are indispensable in as large of array of taxa as possible. Suitable methodologies for such studies are suggested and discussed by Grant (1966), Littlejohn (1969), and Nevo (1969b). Recently, an additional, chiefly stochastic theory of the origin of pre-zygotic isolating mechanisms has been suggested (Carson, 1975, 1978; Kaneshiro, 1976).