Résumé
Quantifying links between ecological processes and adaptation dynamics in
natura remains a crucial challenge. Many studies have documented the
strength, form and direction of selection, and its variations in space and
time, but only a few managed to link these variations to their proximal
causes. This step is, however, crucial, if we are to understand how the
variation in selective pressure affects adaptive allele dynamics in
natural settings. We used data from a long-term survey (about 30 years)
monitoring the adaptation to insecticides of Culex pipiens mosquitoes in
Montpellier area (France), focusing on three resistance alleles of the
Ester locus. We used a population genetics model taking temporal and
spatial variations in selective pressure into account, to assess the
quantitative relationships between variations in the proximal agent of
selection (amounts of insecticide sprayed) and the fitness of resistance
alleles. The response to variations in selective pressure was fast, and
the alleles displayed different fitness-to-environment relationships: the
analyses revealed that even slight changes in insecticide doses could
induce changes in the strength and direction of selection, thus changing
the fitness ranking of the adaptive alleles. They also revealed that
selective pressures other than the insecticides used for mosquito control
affected the resistance allele dynamics. These fitness-to-environment
relationships, fast responses and continuous evolution limit our ability
to predict the outcome of adaptive allele dynamics in a changing
environment, but they clearly contribute to the maintenance of
polymorphism in natural populations. Our study also emphasizes the
necessity of long-term surveys in evolutionary ecology.