168 Increase in Number of Species 



such as Tasmania either by being swept there by storm or by 

 fortuitous wandering. Successive colonizations could follow these 

 theoretical steps: 



(1) Initial colonization will be possible only when a male and 

 female reach the island during the same breeding season and at 

 the same locality. When this combination occurs and the pair mate 

 and raise a family, the first colonization begins. 



(2) The genetic composition of the island colony will change 

 independently of the parent colony because the ecological setting 

 will be slightly different, thus exerting slightly different selection 

 pressures. The original genetic make-up of the colony will be 

 different from the parent population because it will contain only 

 the genetic make-up of two parent individuals rather than the 

 entire gene pool of the parent population. Thus a genetic divergence 

 will arise between the parent population and the colony. Vagrants 

 reaching the island during the initial stages of this divergence 

 would presumably interbreed freely with members of the colony. 

 The total number of the latter would be so large in proportion, 

 however, that genetic differences of an occasional vagrant would 

 be absorbed and diluted with rapidity. 



(3) The genetic divergence between the colony and the parent 

 population would reach a point at which only limited genetic com- 

 patibility existed between them. Vagrants reaching the colony 

 would mate with members of the colony, but the offspring from 

 these unions would be chiefly sterile, hence few of them would be 

 able to reproduce normally. Thus if the number of vagrants re- 

 mained constant, their effect on the gene pool of the colony would 

 diminish with the passage of time. 



(4) When the genetic divergence reached such proportions that 

 the colony was completely incompatible genetically with the parent 

 species, vagrants would undoubtedly continue to mate with mem- 

 bers of the colony, even though all progeny of such matings were 

 completely sterile. As previously cited observations in Drosophila 

 and Microhyla indicate, it is a commonplace for allopatric species 

 to cross-mate when members of closely related species accidentaly 

 meet, and further it appears that sexual or behavorial isolation (at 

 least between closely related species ) is a result of contact between 

 the main populations of the species involved. Because our island 

 colony, now a new species, would be in contact with its parent 

 species only through an occasional vagrant, there would be only 

 negligible selection pressure towards the evolution of sexual isolat- 

 ing mechanisms. Also, because vagrants would presumably arrive 



