The evolution of genetic discontinuity: Computer models of the selection of barriers to interbreeding between subspecies

Abstract

1. Computer simulation techniques were used to investigate the population genetics of the zone of contact between interbreeding plant subspecies, in particular to test the hypothesis that where interbreeding leads to hybrids of low fertility (which involves wastage of the gametes concerned in hybridisation), then any genes tending to prevent interbreeding should have a selective advantage and should increase in frequency with a consequent increase in genetic isolation of the subspecies and a decrease in gene flow between them. 2. Two basic experiments were devised, using flowering period as the potential isolating mechanism. In a situation in which hybrids had a fertility of only 25 per cent, of that of the parent subspecies, both of the latter were given equal possibility of genetic variability in date of flowering and length of the flowering period. 3. In a preliminary experiment, the two subspecies were thoroughly intermingled and pollination took place entirely at random through the whole population. At the beginning, the two subspecies flowered simultaneously identity. But within about 60 generations one subspecies had become lateflowering and the other early-flowering, and both had shortened the length of their flowering periods; there was very little overlap in flowering and consequently there was very little hybridisation; both subspecies had largely regained their identity, and were effectively isolated genetically from one another.4. In a more complex experiment, the subspecies were in contact but not intermingled, and there was a restriction on the distance of pollen and seed movement. Initially, flowering was not quite simultaneous. 5. In the early stages, hybridisation occurred in the region of contact and furthest in the number of hybrids in spite of this much greater intermingling, and there was only slight interpenetration of the two subspecies; hybridisation is a powerful factor in hindering interpenetration. Evolution of flowering time difference occurred as it had done in the first experiment, but much more slowly Until such a difference began clearly to appear, the zone of interpenetration of the subspecies (which never lost their identity) remained narrow, but as the difference developed (most strongly in the plants furthest into the territory of the other subspecies) this zone increased in size and eventually there was very considerable intermingling of the subspecies which became virtually sympatric At the same time there was no increase in the number of hybrids in spite of this much greater intermingling, and this was clearly due to the fact that as a result of selection there had come to be little overlap in flowering period between the subspecies within the zone of interpenetration, and they were now good genetically isolated species. 6. Change in flowering period was not confined to the zone of interpenetration, but the alleles involved flowed outwards into areas well away from it. This was confirmed in an experiment which kept the zone narrow by introducing ecological preferences; at the ends of the population, where the subspecies were widely separated from each other and hybridisation was geographically impossible, there came to be a difference of five weeks between their times of maximum flowering. © 1970 The Genetical Society of Great Britain.