Evolutionary Biology

Neiman, Maurine; Schwander, Tanja

doi: 10.1007/s11692-011-9113-zpmid: N/A

The overwhelming predominance of sexual reproduction in nature is surprising given that sex is expected to confer profound costs in terms of production of males and the breakup of beneficial allele combinations. Recognition of these theoretical costs was the inspiration for a large body of empirical research—typically focused on comparing sexual and asexual organisms, lineages, or genomes—dedicated to identifying the advantages and maintenance of sex in natural populations. Despite these efforts, why sex is so common remains unclear. Here, we argue that we can generate general insights into the advantages of sex by taking advantage of parthenogenetic taxa that differ in such characteristics as meiotic versus mitotic offspring production, ploidy level, and single versus multiple and hybrid versus non-hybrid origin. We begin by evaluating benefits that sex can confer via its effects on genetic linkage, diversity, and heterozygosity and outline how the three classes of benefits make different predictions for which type of parthenogenetic lineage would be favored over others. Next, we describe the type of parthenogenetic model system (if any) suitable for testing whether the hypothesized benefit might contribute to the maintenance of sex in natural populations, and suggest groups of organisms that fit the specifications. We conclude by discussing how empirical estimates of characteristics such as time since derivation and number of independent origins of asexual lineages from sexual ancestors, ploidy levels, and patterns of molecular evolution from representatives of these groups can be used to better understand which mechanisms maintain sex in natural populations.

Dijker, Anton

doi: 10.1007/s11692-011-9119-6pmid: 21654906

The evolution of psychological adaptations for cooperation is still puzzling due to a tendency to frame social interaction in mathematical and game-theoretical terms, without systematically examining its causal structure and underlying mechanisms. Complementarily, empirical approaches to cooperation tend to focus on isolated components of mechanisms without sufficiently indicating how different components are combined into a single mechanism and different mechanisms fit into a single organism. An alternative approach to the evolution of cooperation is proposed, starting from a description of basic physical properties of individuals and their environment, and the limited physical or mechanistic possibilities to generate adaptive responses to those properties. This approach reveals that some forms of symmetrical cooperation do not require mechanisms “specifically designed for” benefiting others, whereas effective helping requires a specific mechanism that relatively unconditionally and persistently responds to the vulnerability of other individuals. Unraveling the causal structure of different types of other-benefiting shows that a mechanism for asymmetrical helping may considerably improve symmetrical cooperation through properties such as tolerance, patience, and the human capacity to experience a wide variety of moral emotions. The proposed mechanistic approach to cooperation provides the mathematical/game-theoretical approach with realistic assumptions about psychological adaptations, and helps to integrate the scattered facts about mechanisms gathered by the empirical approach. It also helps to build bridges between the two approaches by providing a common language for thinking about psychological mechanisms.

Webster, Mark; Zelditch, Miriam

doi: 10.1007/s11692-011-9110-2pmid: N/A

Phenotypic integration can influence evolutionary rate and direction by channeling variation into few dimensions. The extent to which that channeling serves as a constraint over macroevolutionary timescales is determined in part by the evolutionary lability of phenotypic integration. Evolutionary change in patterns of pleiotropy, potentially reducing that constraint, is thought to be more readily achieved when pleiotropy is structured by variation arising in parallel along different developmental pathways rather than by variation arising from direct interactions within and between those pathways. Herein we test two predictions that follow from that hypothesis: (1) that clades undergoing dramatic diversification are characterized by integration that is weakly influenced by direct interactions; and (2) that the structure of integration arising from direct interactions is more conservative than that arising from parallel variation. We examine integration of the cranidium of two Cambrian ptychoparioid trilobites, Crassifimbra walcotti and Eokochaspis nodosa, comparing them to each other and to a previously studied species, C.? metalaspis. Shape variation is decomposed into components representing variation among individuals and variation due to direct interactions. In all three species, variation among individuals was only weakly influenced by direct interactions, suggesting that integration was unlikely to have been a long-term constraint on the Cambrian diversification of ptychoparioids. Phenotypic integration of E. nodosa is no more similar than expected by chance to either Crassifimbra species, but the component due to direct interactions is more similar than expected by chance to that of C.? metalaspis. Conversely, the two Crassifimbra species are generally similar (although not identical) in phenotypic integration, but markedly differ in their structure of direct interactions. Integration arising from direct interactions was therefore not immune to restructuring over even short evolutionary timescales, and was not always more conservative than that arising from parallel variation.

Ceballos, Claudia; Valenzuela, Nicole

doi: 10.1007/s11692-011-9117-8pmid: N/A

Sex-specific plasticity, the differential response that the genome of males and females may have to different environments, is a mechanism that can affect the degree of sexual dimorphism. Two adaptive hypotheses have been proposed to explain how sex-specific plasticity affects the evolution of sexual size dimorphism. The adaptive canalization hypothesis states that the larger sex exhibits lesser plasticity compared to the smaller sex due to strong directional selection for a large body size, which penalizes individuals attaining sub-optimal body sizes. The condition-dependence hypothesis states that the larger sex exhibits greater plasticity than the smaller sex due to strong directional selection for a large body size favoring a greater sensitivity as an opportunistic mechanism for growth enhancement under favorable conditions. While the relationship between sex-specific plasticity and sexual dimorphism has been studied mainly in invertebrates, its role in long-lived vertebrates has received little attention. In this study we tested the predictions derived from these two hypotheses by comparing the plastic responses of body size and shape of males and females of the snapping turtle (Chelydra serpentina) raised under common garden conditions. Body size was plastic, sexually dimorphic, and the plasticity was also sex-specific, with males exhibiting greater body size plasticity relative to females. Because snapping turtle males are larger than females, sexual size dimorphism in this species appears to be driven by an increased plasticity of the larger sex over the smaller sex as predicted by the condition-dependent hypothesis. However, male body size was enhanced under relatively limited resources, in contrast to expectations from this model. Body shape was also plastic and sexually dimorphic, however no sex by environment interaction was found in this case. Instead, plasticity of sexual shape dimorphism seems to evolve in parallel for males and females as both sexes responded similarly to different environments.

Fox, Charles; Wagner, James; Cline, Sara; Thomas, Frances; Messina, Frank

doi: 10.1007/s11692-011-9116-9pmid: N/A

Animal lifespans can vary substantially among closely related species and even among conspecific populations, but it is often difficult to identify environmental and genetic factors producing such variation. We used experimental evolution to examine how transfer to a novel environment affects adult lifespan and rates of senescence in a seed-feeding beetle. Three replicate lines of Callosobruchus maculatus (F.) were switched to a new host plant (cowpea), and each evolved shorter adult lifespans compared to a line maintained on the ancestral host (mung bean). However, the evolution of lifespan differed between the sexes; female lifespan was reduced by ~11% in all cowpea replicates, whereas male lifespan decreased by an average of only 5.6% and the magnitude of the reduction varied among replicates. Reduced lifespan in lines switched to cowpea mirrored the shorter lifespan observed in a separate population chronically associated with cowpea. We then performed crosses between the mung bean and cowpea lines to estimate the genetic architecture underlying the rapid evolution of a shorter lifespan on cowpea. Dominance (overdominance) contributed substantially to the difference between the cowpea and mung bean lines for female lifespan but not for male lifespan. However, details of the genetic architecture varied among the three replicate crosses, so that the convergent evolution of shorter female lifespan in the different cowpea lines did not arise from identical allelic substitutions. Our study demonstrates that insect lifespan can be predictably modified by a switch to a novel host plant, that both the magnitude of this response and its underlying genetic architecture can be sex-specific, and that convergent evolution of a complex trait such as lifespan can arise from different genetic mechanisms.

Pincheira-Donoso, Daniel; Tregenza, Tom

doi: 10.1007/s11692-011-9118-7pmid: N/A

Fecundity is a primary component of fitness. Theory predicts that the evolution of fecundity through increased brood size results from fecundity selection favouring larger female size to accommodate more offspring and to store more energy. This is expected to generate asymmetric selection on body size between the sexes, ultimately driving evolution of female-biased sexual size dimorphism. Additionally, it has been predicted that the intensity of fecundity selection increases when the opportunities for reproduction are reduced by the limiting thermal effects of increasing latitude-elevation (i.e. decreasing environmental temperatures) on the length of the reproductive season. This later factor would be particularly strong among ectotherms, where reproduction is heavily temperature-dependent. However, this integrative perspective on reproductive evolution by fecundity selection has rarely been investigated. Here, we employ a comparative approach to investigate these predictions in Liolaemus, a prominent lizard radiation. As expected, Liolaemus reproductive output (i.e. offspring number per reproductive episode) increases predictably with increasing female size. However, contrary to predictions, we found that increased fecundity does not translate into female-biased SSD, and that combined latitude-elevation does not impose a detectable effect on fecundity. Finally, our allometric analyses reveal that SSD scales with body size, which supports the occurrence of Rensch’s rule in these lizards. We discuss the evolutionary implications of our results, and the assumptions of the investigated hypotheses.

Dufresnes, Christophe; Luquet, Emilien; Plenet, Sandrine; Stöck, Matthias; Perrin, Nicolas

doi: 10.1007/s11692-011-9114-ypmid: N/A

Nascent sex chromosomes offer a unique opportunity to investigate the evolutionary fate of genes recently trapped in non-recombining segments. A house-keeping gene (MED15) was recently shown to lie on the nascent sex-chromosomes of the European tree frog (Hyla arborea), with different alleles fixed on the X and the Y chromosomes. Here we document a polymorphism (glutamine deletion) in the X copy of the gene, and use population surveys and experimental crosses to test whether this polymorphism is neutral or maintained by sex-antagonistic selection. Tadpoles from parents of known genotypes revealed significant discrepancies from Mendelian inheritance, suggesting possible sex-antagonistic effects under laboratory conditions. Quantitatively, however, these effects did not meet the conditions for polymorphism maintenance. Furthermore, field estimates of female genotypic frequencies did not differ from Hardy–Weinberg equilibrium and allelic frequencies on the X chromosome did not differ between sexes. In conclusion, although sex-antagonistic effects cannot be excluded given the laboratory conditions, the X-linked polymorphism under study appears neutral in the wild. Alternatively, sex-antagonistic selection might still account for the fixation of a male-specific allele on the Y chromosome.

Hahn, D.; Reisen, William

doi: 10.1007/s11692-011-9112-0pmid: N/A

Immunologists and evolutionary biologists are interested in how the immune system evolves to fit an ecological niche. We studied the relationship between exposure to parasites and strength of immunity by investigating the response of two species of New World cowbirds (genus Molothrus, Icteridae), obligate brood parasites with contrasting life history strategies, to experimental arboviral infection. The South American shiny cowbird (M. bonariensis) is an extreme host-generalist that lays its eggs in the nests of >225 different avian species. The Central American bronzed cowbird (M. aeneus) is a relative host-specialist that lays its eggs preferentially in the nests of approximately 12 orioles in a single sister genus. West Nile virus provided a strong challenge and delineated immune differences between these species. The extreme host-generalist shiny cowbird, like the North American host-generalist, the brown-headed cowbird, showed significantly lower viremia to three arboviruses than related icterid species that were not brood parasites. The bronzed cowbird showed intermediate viremia. These findings support the interpretation that repeated exposure to a high diversity of parasites favors the evolution of enhanced immunity in brood parasitic cowbirds and makes them useful models for future studies of innate immunity.

Marroig, Gabriel; Melo, Diogo; Porto, Arthur; Sebastião, Harley; Garcia, Guilherme

doi: 10.1007/s11692-010-9107-2pmid: N/A

Genetic and phenotypic variance/covariance matrices are a fundamental measure of the amount of variation and the pattern of association among traits for current investigations in evolutionary biology. Still, few methods have been developed to accomplish the goal of pinpointing in which traits two matrices differ most, hampering further works on the field. We here described a novel method for dissecting matrix comparisons. This method is called Selection Response Decomposition and is an extension of the random skewers in the sense that evolutionary responses produced by known simulated selection vectors are unfolded and then compared in terms of the direct and indirect responses to selection for any trait. We also applied the method in diverse case studies, illustrating its potential. Both theoretical matrices and empirical biological data were used in the comparisons made. In the theoretical ones, the method was able to determine exactly which traits were responsible for the known a priori differences between the matrices, as well as where matrices remained similar to each other. Similar support could be observed in comparisons carried on between matrices produced from empirical biological data, since reasonable and detailed interpretations could be made regarding matrix comparisons. SRD represents an excellent tool for matrix comparisons and should provide quantitative evolutionary biology with a new method for analyzing and comparing variance/covariance patterns.