Sex determination is a fundamental biological process, yet its mechanisms are remarkably diverse. In vertebrates, sex can be determined by inherited genetic factors or by the temperature experienced during embryonic development. However, the evolutionary causes of this diversity remain unknown. Here we show that live-bearing lizards at different climatic extremes of the species' distribution differ in their sex-determining mechanisms, with temperature-dependent sex determination in lowlands and genotypic sex determination in highlands. A theoretical model parameterized with field data accurately predicts this divergence in sex-determining systems and the consequence thereof for variation in cohort sex ratios among years. Furthermore, we show that divergent natural selection on sex determination across altitudes is caused by climatic effects on lizard life history and variation in the magnitude of between-year temperature fluctuations. Our results establish an adaptive explanation for intra-specific divergence in sex-determining systems driven by phenotypic plasticity and ecological selection, thereby providing a unifying framework for integrating the developmental, ecological and evolutionary basis for variation in vertebrate sex determination.

When female fecundity is relatively independent of male abundance, while male reproduction is proportional to female abundance, females have a larger effect on population dynamics than males (i.e. female demographic dominance). This population dynamic phenomenon might not appear to influence evolution, because male and female genomes still contribute equally much to the next generation. However, here we examine two evolutionary scenarios to provide a proof of principle that spatial structure can make female demographic dominance matter. Our two simulation models combine dispersal evolution with local adaptation subjected to intralocus sexual conflict and environmentally driven sex ratio biases, respectively. Both models have equilibria where one environment (without being intrinsically poorer) has so few reproductive females that trait evolution becomes disproportionately determined by those environments where females survive better (intralocus sexual conflict model), or where daughters are overproduced (environmental sex determination model). Surprisingly, however, the two facts that selection favours alleles that benefit females, and population growth is improved when female fitness is high, together do not imply that all measures of population performance are improved. The sex-specificity of the source -sink dynamics predicts that populations can evolve to fail to persist in habitats where alleles do poorly when expressed in females.

Plastic responses to temperature during embryonic development are common in ectotherms, but their evolutionary relevance is poorly understood. Using a combination of field and laboratory approaches, we demonstrate altitudinal divergence in the strength of effects of maternal thermal opportunity on offspring birth date and body mass in a live-bearing lizard (Niveoscincus ocellatus).Poor thermal opportunity decreased birth weight at low altitudes where selection on body mass was negligible. In contrast, there was no effect of maternal thermal opportunity on body mass at high altitudes where natural selection favored heavy offspring.The weaker effect of poor maternal thermal opportunity on offspring development at high altitude was accompanied by a more active thermoregulation and higher body temperature in highland females. This may suggest that passive effects of temperature on embryonic development have resulted in evolution of adaptive behavioral compensation for poor thermal opportunity at high altitudes, but that direct effects of maternal thermal environment are maintained at low altitudes because they are not selected against. More generally, we suggest that phenotypic effects of maternal thermal opportunity or incubation temperature in reptiles will most commonly reflect weak selection for canalization or selection on maternal strategies rather than adaptive plasticity to match postnatal environments. K E Y W O R D S :Life-history evolution, maternal effect, phenotypic plasticity, selection-natural.

Paternity protection and the acquisition of multiple mates select for different traits. The consensus from theoretical work is that mate-guarding intensifies with an increasing male bias in the adult sex ratio (ASR). A male bias can thus lead to male monogamyif guarding takes up the entire male time budget. Given that either female-or male-biased ASRs are possible, why is promiscuity clearly much more common than male monogamy? We address this question with two models, differing in whether males can assess temporal cues of female fertility. Our results confirm the importance of the ASR: guarding durations increase with decreasing female availability and increasing number of male competitors. However, several factors prevent the mating system from switching to male monogamy as soon as the ASR becomes male biased. Inefficient guarding, incomplete last male sperm precedence, any mechanism that allows sperm to fertilize eggs after the male's departure, and (in some cases) the unfeasibility of precopulatory guarding all help explain cases where promiscuity exists on its own or alongside temporally limited mate-guarding. Shortening the window of fertilization shifts guarding time budgets from the postcopulatory to the precopulatory stage. K E Y W O R D S :Male mating strategy, mate-guarding, monogamy, paternity protection, sex ratio.A simplified view of mating systems and sex differences in reproductive strategies often emphasizes sex differences in the optimal rate of mating, typically higher for males than for females (Gavrilets 2000;Arnqvist and Rowe 2005;Maklakov et al. 2005;Gavrilets and Hayashi 2006). Such a difference cannot, however, explain those monogamous situations that are based on male strategies of paternity protection, which can take the form of extensive mate-guarding (Beecher and Beecher 1979;Birkhead 1979) or mating plugs (Baer et al. 2001;Foellmer 2008;Fromhage 2012). In these cases, it appears to be in the best interest of a male to achieve high paternity with one or few females, rather than attempt maximally many matings.Clearly, paternity protection and the acquisition of multiple mates can select for different traits. In extreme cases of terminal investment, paternity protection can mean that a male foregoes all chances of finding another female (Fromhage et al. 2005). Mateguarding, which we focus on here (defined as the close association between a male and female prior to and/or after copulation for paternity assurance), does not always have to be that extreme. Still, there is often a direct trade-off involving time: guarding one female often effectively prevents a male from searching for more of them (Birkhead and Møller 1992;Dickinson 1995;Simmons and Siva-Jothy 1998;Fryer et al. 1999). It follows that if mateguarding becomes sufficiently extended over time, the mating system becomes socially monogamous (Mathews 2002; note that a socially monogamous system can be genetically monogamous too, if guarding is efficient enough).Past research has outlined a few principles of whether males should s...