Résumé
Understanding how divergent selection generates adaptive phenotypic and
population diversification provides a mechanistic explanation of
speciation in recently separated species pairs. Toward this goal, we
sought ecological gradients of divergence between the cryptic malaria
vectors Anopheles coluzzii and An. gambiae, and then looked for a
physiological trait that may underlie such divergence. Using a large set
of occurrence records and eco-geographical information, we built a
distribution model to predict the predominance of the two species across
their range of sympatry. Our model predicts two novel gradients along
which the species segregate: distance from the coastline and altitude.
Anopheles coluzzii showed a ‘bimodal’ distribution, predominating in xeric
West African savannas and along the western coastal fringe of Africa. To
test whether differences in salinity tolerance underlie this habitat
segregation, we assessed the acute dose-mortality response to salinity of
thirty-two larval populations from Central Africa. In agreement with its
coastal predominance, Anophels coluzzii was overall more tolerant than An.
gambiae. Salinity tolerance of both species, however, converged in urban
localities, presumably reflecting an adaptive response to osmotic stress
from anthropogenic pollutants. When comparing degree of tolerance in
conjunction with levels of syntopy, we found evidence of character
displacement in this trait.