In the Hamadryas baboon, males are substantially larger than females. A troop of baboons is subdivided into a number of ‘one-male groups’, consisting of one adult male and one or more females with their young. The male prevents any of ‘his’ females from moving too far from him. Kummer (1971) performed the following experiment. Two males, A and B, previously unknown to each other, were placed in a large enclosure. Male A was free to move about the enclosure, but male B was shut in a small cage, from which he could observe A but not interfere. A female, unknown to both males, was then placed in the enclosure. Within 20 minutes male A had persuaded the female to accept his ownership. Male B was then released into the open enclosure. Instead of challenging male A , B avoided any contact, accepting A’s ownership.

Evolution and the Theory of Games

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The Genetical Theory of Natural Selection.

Geographic Variation, Speciation, and Clines.

Learning is widespread in nature, occurring in most animal taxa and in several different ecological contexts and, thus, might play a key role in evolutionary processes. Here, we review the accumulating empirical evidence for the involvement of learning in mate choice and the consequences for sexual selection and reproductive isolation. We distinguish two broad categories: learned mate preferences and learned traits under mate selection (such as bird song). We point out that the context of learning, namely how and when learning takes place, often makes a crucial difference to the predicted evolutionary outcome. Factors causing biases in learning and when one should expect the evolution of learning itself are also explored.

The impact of learning on sexual selection and speciation.

The ‘centre–periphery hypothesis’ (CPH) is a long-standing postulate in ecology that states that genetic variation and demographic performance of a species decrease from the centre to the edge of its geographic range. This hypothesis is based on an assumed concordance between geographical peripherality and ecological marginality such that environmental conditions become harsher towards the limits of a species range. In this way, the CPH sets the stage for understanding the causes of distribution limits. To date, no study has examined conjointly the consistency of these postulates. In an extensive literature review we discuss the birth and development of the CPH and provide an assessment of the CPH by reviewing 248 empirical studies in the context of three main themes. First, a decrease in species occurrence towards their range limits was observed in 81% of studies, while only 51% demonstrated reduced abundance of individuals. A decline in genetic variation, increased differentiation among populations and higher rates of inbreeding were demonstrated by roughly one in two studies (47, 45 and 48%, respectively). However, demographic rates, size and population performance less often followed CPH expectations (20–30% of studies). We highlight the impact of important methodological, taxonomic, and biogeographical biases on such validation rates. Second, we found that geographic and ecological marginality gradients are not systematically concordant, which casts doubt on the reliability of a main assumption of the CPH. Finally, we attempt to disentangle the relative contribution of geographical, ecological and historical processes on the spatial distribution of genetic and demographic parameters. While ecological marginality gradients explain variation in species' demographic performance better than geographic gradients, contemporary and historical factors may contribute interactively to spatial patterns of genetic variation. We thereby propose a framework that integrates species' ecological niche characteristics together with current and past range structure to investigate spatial patterns of genetic and demographic variation across species ranges.

Geographic variation in genetic and demographic performance: new insights from an old biogeographical paradigm.

Many Odonata species exhibit female-limited polymorphisms, where one morph is similar to the conspecific male in body color and other traits (andromorph), whereas one or more other morphs differ from the male (gynomorphs). Here we investigated the differentially expressed transcripts (DETs) among males and two female morph groups (gynomorphs and andromorphs) using RNA-seq to identify candidate transcripts encoding female-limited polymorphisms in the damselfly Ischnura senegalensis. Seven DETs that had significantly different expression levels between males and gynomorphs, but not between males and andromorphs, were identified. The expression levels of four of these candidate genes, doublesex (dsx), black, ebony, and chaoptin (chp), were selected for further analysis using qRT-PCR. Sequence analysis of the dsx amplicons revealed that this gene produced at least three transcripts. Two short transcripts were mainly expressed in males and andromorphs, whereas the long transcript was specifically expressed in both morph female groups; that is, the expression pattern of the dsx splice variants in andromorphs was an intermediate between that of males and gynomorphs. Because the dsx gene functions as a transcription factor that regulates the sex-specific expression of multiple genes, its splice variants in I. senegalensis may explain why the andromorph is female but exhibits some masculinized traits. Because we did not detect different coding sequences of the candidate genes among the different morphs, a diallelic genomic region controlling alternative splicing of dsx, thus determining female-limited polymorphism in I. senegalensis most likely lies in a non-coding region of the dsx gene or in a gene upstream of it.

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Candidate genes associated with color morphs of female-limited polymorphisms of the damselfly Ischnura senegalensis

Females of coenagrionid damselflies exhibit color dimorphism, consisting of an andromorph and a gynomorph. This study compared reproductive traits between the female morphs in both field-captured and laboratory-reared females of the female-dimorphic damselfly Ischnura senegalensis. No difference was found in the onset of egg development between the morphs. The andromorphs developed significantly smaller mature eggs and had significantly more immature eggs than the gynomorphs. These results suggest that the andromorphs are r-strategists (high fecundity with small eggs), whereas the gynomorphs are K-strategists (low fecundity with large eggs). Fecundity and egg size might determine the quantity and quality of the offspring, respectively, indicating that morph-specific reproductive traits would contribute to the overall fitness of each female morph, and consequently be key factors affecting morph frequency in a population.

/pdf/morph-specific-fecundity-and-egg-size-in-the-female-2ht5r55rkz.pdf

Morph-specific fecundity and egg size in the female-dimorphic damselfly Ischnura senegalensis.

Understanding the relative importance of selection and stochastic factors in population divergence of adaptive traits is a classical topic in evolutionary biology. However, it is difficult to separate these factors and detect the effects of selection when two or more contrasting selective factors are simultaneously acting on a single locus. In the damselfly Ischnura senegalensis, females exhibit color dimorphism and morph frequencies change geographically. We here evaluated the role of selection and stochastic factors in population divergence of morph frequencies by comparing the divergences in color locus and neutral loci. Comparisons between population pairwise FST for neutral loci and for the color locus did not detect any stochastic factors affecting color locus. Although comparison between population divergence in color and neutral loci using all populations detected only divergent selection, we detected two antagonistic selective factors acting on the color locus, that is, balancing and divergent selection, when considering geographical distance between populations. Our results suggest that a combination of two antagonistic selective factors, rather than stochastic factors, establishes the geographic cline in morph frequency in this system.

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Antagonistic selection factors induce a continuous population divergence in a polymorphism

Although genetic diversity within a population is suggested to improve population-level fitness and productivity, the existence of these effects is controversial because empirical evidence for an ecological effect of genetic diversity and the underlying mechanisms is scarce and incomplete. Here, we show that the natural single-gene behavioural polymorphism (Rover and sitter) in Drosophila melanogaster has a positive effect on population fitness. Our simple numerical model predicted that the fitness of a polymorphic population would be higher than that expected with two monomorphic populations, but only under balancing selection. Moreover, this positive diversity effect of genetic polymorphism was attributable to a complementarity effect, rather than to a selection effect. Our empirical tests using the behavioural polymorphism in D. melanogaster clearly supported the model predictions. These results provide direct evidence for an ecological effect of genetic diversity on population fitness and its condition dependence.

https://royalsocietypublishing.org/doi/pdf/10.1098/rspb.2017.2045

Balanced genetic diversity improves population fitness.

Negative frequency-dependent selection derived from positive frequency-dependent foraging is the best-known selection force maintaining genetic polymorphism within a population. However, in flowering plants, positive frequency-dependent foraging by pollinators is expected to accelerate the loss of low-frequency morphs by conferring a fitness advantage to the common morph, leading to monomorphism. In Japan, a non-native species, Sisyrinchium sp., exhibits conspicuous flower color polymorphism within a population comprising both purple morphs (homozygous recessive) and white morphs (heterozygous or homozygous dominant). Here we quantified genotype-specific reproductive success in order to reveal the contribution of overdominant selection on the maintenance of flower color polymorphism in this species. In artificial pollination experiments using individuals with identified genotypes, female reproductive success was higher in the heterozygote than in either homozygote. The frequency of purple morphs in natural populations (ca. 31 %) is similar to the frequency predicted by overdominant selection (25 %). Our results suggest that overdominant selection contributes to the maintenance of color morphs in the natural population of this species.

Yuma Takahashi

Papers

Candidate genes associated with color morphs of female-limited polymorphisms of the damselfly Ischnura senegalensis

Morph-specific fecundity and egg size in the female-dimorphic damselfly Ischnura senegalensis.

Antagonistic selection factors induce a continuous population divergence in a polymorphism

Balanced genetic diversity improves population fitness.

Flower color polymorphism maintained by overdominant selection in Sisyrinchium sp.