Falconer (1989) made the statement that traits closely related to fitness generally experience greater inbreeding depression than traits less closely related to fitness. However, he did not provide statistical support for this statement. In a review of the literature I provide statistical support for Falconer's (1989) statement by showing that life history traits (closely related to fitness) display more severe inbreeding depression than morphological traits (less closely related to fitness). Because inbreeding depression is greater in traits closely related to fitness, it may impact the success of colonization. In an investigation of the influence of inbreeding in a colonizing wing dimorphic species of cricket, Gryllus firmus, I have provided further empirical support for Falconer's (1989) statement, in addition to showing that inbreeding does not affect the equilibrium morph frequencies between two the morphs of G. firmus.

No description available.

The amount of heritable variation and the pattern of genetic covariation in traits are the chief determinants in the evolution of a population, at least over the short term. A preponderance of heritability (h$ sp2)$ and genetic (r$ rm sb g)$ experiments are performed under laboratory conditions which are characterized by low levels of environmental variability. This thesis describes the investigation of the effects of environmental heterogeneity on phenotypic components of variance and covariance. Full-sib families of the field cricket, Gryllus pennsylvanicus, were split between a homogeneous laboratory environment and a more variable field environment. The values of h$ sp2$ were, on average, lower in the variable field environment, with a mean reduction of 19%. Development time was an exception, exhibiting high levels of additive variance in the field. Substantial decreases in generic components of variance contributed to the lower field h$ sp2$ estimates, as well as the expected increases in the components of environmental variance. Within-environment r$ rm sb g$ among morphological traits were not found to differ between the two environments. Laboratory measures of r$ rm sb g$ involving life history traits, though, were not found to reflect the same correlations measured in the heterogeneous environment. A significant negative genetic correlation between fecundity and development time was found in the field environment, yet was not observed in the laboratory. Male-female genetic correlations between morphological traits were high (all r$ rm sb g>0.73),$ and were consistently higher in the field environment than in the laboratory. The genetic correlations between the sexes in development time followed the same trend, but gonad weight showed no similarity between the sexes in either environment. Across-environment correlations within each trait are also discussed.

No description available.

The species-specific calling songs of male crickets are used by females for species recognition and mate choice. Heritabilities of variation of morphological structures involved in song production, components of the calling song, and body size were estimated for G.firmus. All morphological structures were shown to possess significant additive genetic variation (h$ sp2 sb{ rm S+D} > 0.42)$. One of the five song components examined, pulse rate, was shown to have a significant heritability (h$ sp2 sb{ rm S+D}$ = 0.35). Due to the low correlation between body size and song components, it is unlikely that female G.firmus could use the calling song to assess male body size or wing morph (micropterous or macropterous).

The influence of photoperiod and genotype on thermal preference of juvenile Gryllus firmus was explored. Both rearing condition and genotype affect temperature preference. The following results were obtained: (1) the wing morph of juveniles raised at 28$ sp circ$C 16hL:8hD is determined before 20 days post hatching. (2) Crickets distribute themselves on the apparatus differently in the presence of a thermal gradient than in its absence: such controls for positioning preference are rare in the literature. (3) Juveniles younger than 20 days old tend to choose higher surface temperatures and achieve higher body temperatures if either rearing photoperiod or selection predisposes them to microptery and choose lower temperatures if they are predisposed towards macroptery. (4) The differences in thermal preference between presumptive macropters and micropters remains significant after differences in body size are taken into account. The differences in thermal preference could act as a novel mechanism for lowering the heritability of wing morph.

No description available.

No description available.