Scott, T. J.; Queller, D. C.

Abiotic and biotic environments can change in different ways that affect how populations adapt, diverge, and speciate. However, many models of speciation only examine adaptation to constant environments. We used Fishers geometric model to study adaptation and speciation in different types of changing abiotic and biotic environments. We hypothesized that, when the magnitude of environmental change is equal, fitness would be lowered more by consistently directional abiotic change and by consistently antagonistic biotic conflict. As a result, directional changes and biotic conflict would cause a greater evolutionary response through fixation of adaptive mutations, greater divergence between populations, and lower migrant or hybrid fitnesses (higher speciation potential). Increasing the directionality of abiotic change did indeed have all these effects. However, populations evolving with conflict, despite showing the expected arms-race elevation of deleterious change and evolutionary response, did not result in decreased fitness of migrants and hybrids. Two factors seem to be involved. In tug-of-war conflict, high total phenotypic change does not lead to high phenotypic divergence because the phenotypic change churns around in the same general area of trait space. With victim-exploiter (trait matching) conflict, hybrids and migrants were frequently more fit than those with other kinds of change because of escape from locally adapted antagonists. In this case, foreign genes sometimes allowed populations in conflict to generate useful variation. These results show that different kinds of ecological change can affect adaptation and the formation of new species in distinct ways and that local divergence and incompatibility do not always go hand in hand.