SYMPATRIC SPECIATION VIA HABITAT SPECIALIZATION DRIVEN BY DELETERIOUS MUTATIONS.

Theoretical studies have suggested that the evolution of habitat (host) races, regarded as a prelude to sympatric speciation, requires strong trade-offs in adaptation to different habitats: alleles that improve fitness in some habitats and have deleterious effects of similar magnitude in other habitats must be segregating in the population. I argue that such trade-offs are not necessary; the evolution of habitat races can also be driven by genetic variation due to loci that affect fitness in one habitat and are neutral or nearly so in others, that is, when performance in different habitats is genetically independent. One source of such genetic variation are deleterious mutations with habitat-specific fitness effects. I use deterministic two-locus and multilocus models to show that the presence of such mutations in the gene pool results in indirect selection favoring habitat fidelity or habitat preference over acceptance of both suitable habitats. This leads to the evolution of largely genetically isolated populations that use different habitats, from a single panmictic population of individuals accepting both habitats. This study suggests that the conditions favoring habitat race formation, and thus possibly sympatric speciation, are much less stringent than previously thought.