| The evolutionary importance of genetic constraints has always been recognized by biologists, but very little data is available to quantitatively assess the role of constraints in shaping the biology of organisms. The field of quantitative genetics provides the tools necessary to study evolutionary constraints, mainly through the estimation of the matrix of additive genetic variance and covariance (the G matrix). The main goals of this Ph.D. dissertation were to study the persistence of constraints across environments and across species, to explore the consequences of constraints on species divergence, and to try to understand some morphological and life history characteristics of field crickets in light of genetic variation. Populations of seven wing-dimorphic cricket species from the genera Gryllus and Teleogryllus were sampled from the wild and reared in the laboratory. Using multiple statistical approaches to the comparison of G matrices, results revealed little variation in G matrices across species. Moreover, the relatively small effect of rearing environment and of the two wing morphologies on G were shown to be of the same magnitude as variation between species, therefore confirming the general constancy of genetic constraints through evolutionary time scales. Mean trait values, selection regimes and phylogenetic distances were all shown not to be predictors of G matrix variation. In agreement with the constraint hypothesis of quantitative genetic theory, morphological divergence between species was shown to be predictable from a reconstructed ancestral G matrix. In addition, information on genetic variation was used to explain various patterns relating to size, ovipositor length, wing morphology and diapause occurrence in field crickets. Overall, we suggest that genetic constraints, as described by quantitative genetics, have played a major role in shaping the observed biological diversity of field cricket species, a conclusion that seems to apply to many other types of organisms. |
