Quantitative genetics has traditionally been used to examine selection responses in domesticated organisms, but has recently been applied by evolutionary biologists to natural populations. Evolutionary biologists use quantitative genetics to model variation in traits related to fitness and, ultimately, to make predictions about the impact of natural selection on populations. However, there are few cases in which quantitative genetics has been used to predict changes in natural populations, and only two cases in which it has been used to predict a correlated response to natural selection. / I use a quantitative genetic model to predict the correlated response to natural selection in geographically distinct wild populations of the wing dimorphic sand cricket, Gryllus firmus. Three populations of G. firmus that naturally vary in proportion macroptery are used to qualitatively assess, for the first time, predictions from a quantitative genetic model postulating that an increase in proportion macroptery is correlated with a decrease in fecundity. Variation observed among the populations when raised in a common garden supports predictions and demonstrates that a difference in mean fecundity is the result of a response in fecundity within wing morphs in addition to a change in the proportion of macropterous females. My results indicate that the complexity behind evolutionary changes in traits genetically correlated with proportion macroptery can be modeled through quantitative genetics.

Successional changes in a habitat may result in bottlenecks where few individuals in a population survive. During such events, changes in the genetic architecture of traits are predicted to occur as is subsequent inbreeding depression. In two literature reviews, I document that, (1) dominance variance increases in traits that are subject to strong selection and, (2) inbreeding depression is substantially higher in the wild as compared to captive populations. In addition to these changes, successional pressures may also result in the evolution of morphologies that allow organisms to avoid unfavourable conditions. A common dimorphism in insects is wing dimorphism, in which the macropterous morph is long-winged (LW), has functional flight muscles and is flight-capable while the micropterous morph (SW) has reduced wings and cannot fly. Due to the energy required to maintain the flight apparatus, macropterous individuals are predicted to have less energy available for reproduction. Trade-offs to macroptery have been documented in female insects. Gryllus firmus is a wing-dimorphic cricket of the southeastern USA. Although there are well established trade-offs between macroptery and reproduction in female crickets, no trade-offs have been demonstrated in male crickets. The prediction is that LW males, because they have to expend energy to maintain the flight apparatus, will call less and therefore attract fewer females than SW males. To be evolutionarily important, the traits involved in the trade-off; call duration, wing morph, wing muscle condition and lipid weight should have significant heritabilities and be genetically correlated. I found that SW males attracted significantly more females than LW males (mean % = 70% (SW) 30% (LW)). A significant difference in time spent calling was found between SW and LW males and as the difference in calling time between males increased, the likelihood of a female choosing the longer-calling male also increased. All the traits ha

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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 tha