MUTATIONAL COLLAPSE OF FITNESS IN MARGINAL HABITATS AND THE EVOLUTIONOF ECOLOGICAL SPECIALIZATION

In spatially heterogeneous environments, natural selection for mainten ance of adaptation to habitats that contribute little to the populatio n's reproduction is weak. In this paper we model a mechanism that can result in loss of fitness in such marginal habitats, and thus lead to specialisation on the main habitat. It involves accumulation of mutati ons that are deleterious in the marginal habitat but neutral or nearly so in the main habitat (mutations deleterious in the main habitat and neutral in the marginal habitat have a negligible influence). If the contribution of the marginal habitat to total reproduction in the abse nce of the mutations is less than a threshold value, selection is too weak to counter accumulation of such mutations. A positive feedback th en results in loss of fitness in the marginal habitat. This mechanism does not require antagonistic pleiotropy in adaptation to different ha bitats, although antagonistic pleiotropy facilitates the mutational co llapse of fitness in the marginal habitat. We suggest that deleterious mutations with habitat-specific expression may play a role in the evo lution of ecological specialisation and promote evolutionary conservat ism of ecological niches.