162 Increase in Number of Species 



If this period of separation and increasing genetic divergence is 

 sufficiently long, the isolated daughter populations will become ge- 

 netically incompatible with each other. In other words, they will 

 evolve into two distinct species, and each of these new species will 

 be at least slightly different ecologically from its sister species. 



(4) The final step in this process of species fission is the de- 

 velopment of sexual isolation. Many distinctive species which evolved 

 in isolation and are still isolated will hybridize freely if brought 

 together artificially, even though the hybrids are sterile or inviable 

 (Stalker, 1953). In much more closely related sympatric species, in- 

 terspecific matings seldom or never occur. In these latter species 

 cross-mating is prevented by a variety of behavior patterns to which 

 one or both sexes of the species respond actively. Distinctive court- 

 ship patterns have been studied in Drosophila (Spieth, 1952), fish 

 (Gordon, 1947), mice (Blair and Howard, 1944), and various birds 

 and crickets. Under normal conditions females reject suitors belong- 

 ing to other sympatric species, and frequently males do not court 

 females of other sympatric species. In these same species the dis- 

 crimination frequently breaks down when the sexes of isolated but 

 closely related species are brought together experimentally. From 

 these data the inference seems clear that distinctive interspecific 

 courtship patterns do not evolve as rapidly between two species in 

 isolation as between species whose ranges have come into contact 

 and overlapped. 



The mechanism leading to these different behavior patterns is 

 natural selection operating through the medium of gametic wastage. 

 If the ranges of two species met and the species hybridized to form 

 unsuccessful progeny, selection pressures would immediately favor 

 those individuals breeding with their own species, for those indi- 

 viduals which cross-bred would leave no progeny. The effectiveness 

 of the selection has been demonstrated experimentally in the genus 

 Drosophila. Koopman (1950) mixed two species of Drosophila 

 which mated almost as readily between the two species as within 

 each species. In each generation he discarded all hybrids so that 

 continuing generations were composed only of the offspring of those 

 individuals which had mated with their own species. In succeeding 

 generations the number of hybrids decreased rapidly; hence this 

 rigid selection against cross-breeding actually increased the intensity 

 of sexual isolation between the populations of the two species. 



For such a rapid response to selection pressure, it is necessary to 

 suppose that pertinent differences in genetic factors affecting be- 

 havior already existed before the experiments were made and that 



