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Theoretical models identify maternal behavior as critical for the maintenance and evolution of sex ratios in organisms with environmental sex determination (ESD). Maternal choice of nest site is generally thought to respond more rapidly to sex ratio selection than environmental sensitivity of offspring sex (threshold temperatures) in reptiles with temperature‐dependent sex determination (TSD, a form of ESD). However, knowledge of the evolutionary potential for either of these traits in a field setting is limited. I developed a simulation model using local climate data and observed levels of phenotypic variation for nest‐site choice and threshold temperatures in painted turtles (Chrysemys picta) with TSD. Both nest‐site choice and threshold temperatures, and hence sex ratios, evolved slowly to simulated climate change scenarios. In contrast to expectations from previous models, nest‐site choice evolved more slowly than threshold temperatures because of large climatic effects on nest temperatures and indirect selection on maternally expressed traits. A variant of the model, assuming inheritance of nest‐site choice through natal imprinting, demonstrated that natal imprinting inhibited adaptive responses in female nest‐site choice to climate change. These results predict that females have relatively low potential to adaptively adjust sex ratios through nest‐site choice.

Mechanisms maintaining sex ratios in populations with temperature-dependent sex determination (TSD) remain elusive. Although geographic variation in embryonic sex determination (i.e., pivotal temperature) has been widely investigated in reptiles exhibiting TSD, no previous studies have directly addressed geographic variation in maternal behavior affecting nest thermal conditions. I evaluated patterns of nest-site selection and its effects on thermal and hydric nest conditions for a population of painted turtles (Chrysemys picta bellii) exhibiting TSD in New Mexico. These results are compared to data collected from a well-studied, conspecific population experiencing relatively cooler climatic conditions in Illinois. Since canopy vegetation cover reduces nest temperatures in Illinois, I expected females in New Mexico to nest under high amounts of canopy vegetation cover. However, females from New Mexico placed nests under significantly less canopy vegetation cover, but closer to standing water, than did females from Illinois. Experimental nests in New Mexico demonstrated that increased canopy vegetation cover and soil moisture reduced nest temperatures. By nesting close to standing water rather than under canopy vegetation cover, females in New Mexico nested in habitats more closely associated with maximizing moisture around nests rather than reducing nest temperatures through shading. Mean July nest temperatures were similar for both populations. Since nest hydric conditions affect hatching success and hatchling size in C. picta, nesting patterns in New Mexico may primarily reflect selection for microhabitats affecting offspring survivorship or size.

The framework for modern studies of speciation was established as part of the Neo-Darwinian synthesis of the early twentieth century. Here we evaluate this framework in the light of recent empirical and theoretical studies. Evidence from experimental studies of selection, quantitative genetic studies of species' differences, and the molecular evolution of 'isolation' genes, all agree that directional selection is the primary cause of speciation, as initially proposed by Darwin. Likewise, as suggested by Dobzhansky and Mayr, gene flow does hold species together, but probably more by facilitating the spread of beneficial mutants and associated hitchhiking events than by homogenizing neutral loci. Reproductive barriers are important as well in that they preserve adaptations, but as has been stressed by botanists for close to a century, they rarely protect the entire genome from gene flow in recently diverged species. Contrary to early views, it is now clear that speciation can occur in the presence of gene flow. However, recent theory does support the long-held view that population structure and small population size may increase speciation rates, but only under special conditions and not because of the increased efficacy of drift as suggested by earlier authors. Rather, low levels of migration among small populations facilitates the rapid accumulation of beneficial mutations that indirectly cause hybrid incompatibilities.

Males of the dimorphic jumping spider (Maevia inclemens) differ in both their morphologies and courtship displays (i.e. phase I).The tufted morph stilts and waves from an average distance of 9 cm from a female, whereas the grey morph crouches and sidles from an average distance of 3 cm from a female. The objective of this study was to determine the significance of the different courtship displays using computeranimated versions of males performing phase I courtship in a Y-maze where first male movement and then the distance of the stimulus was controlled. Females selected the first male that they orientated to at the close distance of 4 cm and at the far distance of 16cm. However, there was no preference for the first male at the intermediate distance of 8 cm or the furthest distance of 24 cm. In addition, males have morph-specific advantages regarding the time it takes to attract female attention. Grey males attracted female attention in less time than tufted males at 4 and 8 cm. However, tufted males attracted female attention in less time than grey males at 16 cm. These results suggest a mechanism for the evolution of two different courtship displays whereby each morph has an advantage at different distances from the female.

We describe observations of a nesting female T. gaigeae, characteristics of the nest, and overwintering status of hatchlings in Socorro Co., New Mexico.

Many reptiles exhibit temperature-dependent sex determination (TSD), a sex-determining mechanism in which the incubation environment permanently determines offspring sex. This research had two main objectives: (1) to evaluate the roles of two traits (nesting behavior by females and offspring sex ratios in response to thermal incubation conditions) thought to be important for maintaining sex ratios in this system, using the painted turtle ( Chrysemys picta), and (2) evaluate the adaptive significance of TSD in a genotypic sex determining system (GSD). Observations on the nesting behavior of painted turtles suggest that either females in this population do not use soil surface temperature as a cue for selecting nest sites, or select sites with intermediate soil surface temperatures that may be less likely to bias sex ratios. Geographic comparisons from two populations of C. picta inhabiting differing climates (Illinois and New Mexico) demonstrated that the New Mexico population exhibited a significantly higher pivotal temperature (temperature producing a 1:1 sex ratio) than the Illinois population. However, this difference was small compared to differences in climatic conditions experienced by the populations. Chrysemys picta nests from Illinois and New Mexico experienced similar nest temperatures despite a relatively hot year in New Mexico. Nests in New Mexico were not laid in sites most likely to reduce nest temperatures, but instead at sites experiencing high soil moisture, which indirectly reduced nest temperatures. In simulation models, pivotal temperatures evolved more rapidly than did nest-site choice by females in response to perturbed sex ratios. Natal philopatry to nest sites also caused maladaptive nesting behavior in terms of Fisherian sex ratio selection. Simulation models demonstrated that TSD invades populations exhibiting GSD and reaches fixation through several avenues that do not include a widely accepted adaptive function for TSD (the Charnov-Bull model). Results from these studies suggest that the likelihood of TSD being relatively neutral compared to GSD in reptiles deserves more attention. Consequently, selection for female behavior and offspring thermal sensitivity to adaptively adjust sex ratios may be fairly weak. This conclusion is supported by the small observed differences in pivotal temperatures and lack of strong patterns of thermally-based nest-site selection.