The spatial sorting of ecological species: Ghost of competition or of hybridization past?

Congeneric species that have evolved on oceanic islands rarely grow with one another. This spatial sorting is thought to be the result of niche pre-emption, where the first species to occupy a given habitat would exclude its relatives through competition. The evidence that competition shapes local species distributions is scant. I propose that hybridization thwarts the invasion of congeners. We know that crossing barriers between products of recent adaptive radiations typically are weak. Colonization of occupied patches would be sporadic, because invaders would be moving from habitats where they are adapted to those where they are not. Nearly all progeny of the scant invaders would be ill-adapted hybrids, because the pollen pool would consist almost exclusively of indigenous pollen. Only if aliens were predominantly inbreeders or apomicts would they have a chance of escaping the deluge of local pollen and becoming established in the occupied patch. The spatial reach of hybridization is much greater than that of competition.