Apparent Selection Intensity for the Cytochrome Oxidase Subunit I Gene Varies with Mode of Reproduction in Echinoderms

When most amino acid substitutions in protein-coding genes are slightly deleterious rather than selectively neutral, life history differences can potentially modify the effective population size or the selective regime, resulting in altered ratios of non-synonymous to synonymous substitutions among taxa. We studied substitution patterns for the mitochondrial cytochrome oxidase subunit I (COI) gene in a sea star genus (Leptasterias spp.) with an obligate brood-protecting mode of reproduction and small-scale population genetic subdivision, and compared the results to available COI sequences in nine other genera of echinoderms with pelagic larvae: three sea stars, five sea urchins and one brittle star. We predicted that this life history difference would be associated with differences in the ratio of non-synonymous (dN) to synonymous (dS) substitution rates. Leptasterias had a significantly greater dN/dS ratio (both between species and within species), a significantly smaller transition/transversion rate ratio, and a significantly lower average nucleotide diversity within species, than did the non-brooding genera. Other explanations for the results, such as altered mutation rates or selective sweeps, were not supported by the data analysis. These findings highlight the potential influence of reproductive traits and other life history factors on patterns of nucleotide substitution within and between species.     

Likelihood ratio tests for detecting positive selection and application to primate lysozyme evolution

An excess of nonsynonymous substitutions over synonymous ones is an important indicator of positive selection at the molecular level. A lineage that underwent Darwinian selection may have a nonsynonymous/synonymous rate ratio (dN/dS) that is different from those of other lineages or greater than one. In this paper, several codon-based likelihood models that allow for variable dN/dS ratios among lineages were developed. They were then used to construct likelihood ratio tests to examine whether the dN/dS ratio is variable among evolutionary lineages, whether the ratio for a few lineages of interest is different from the background ratio for other lineages in the phylogeny, and whether the dN/dS ratio for the lineages of interest is greater than one. The tests were applied to the lysozyme genes of 24 primate species. The dN/dS ratios were found to differ significantly among lineages, indicating that the evolution of primate lysozymes is episodic, which is incompatible with the neutral theory. Maximum- likelihood estimates of parameters suggested that about nine nonsynonymous and zero synonymous nucleotide substitutions occurred in the lineage leading to hominoids, and the dN/dS ratio for that lineage is significantly greater than one. The corresponding estimates for the lineage ancestral to colobine monkeys were nine and one, and the dN/dS ratio for the lineage is not significantly greater than one, although it is significantly higher than the background ratio. The likelihood analysis thus confirmed most, but not all, conclusions Messier and Stewart reached using reconstructed ancestral sequences to estimate synonymous and nonsynonymous rates for different lineages.      

A database of vertebrate longevity records and their relation to other life‐history traits

Longevity is a major characteristic of animals that has long fascinated scientists. In this work, we present a comprehensive database of animal longevity records and related life‐history traits entitled AnAge, which we compiled and manually curated from an extensive literature. AnAge started as a collection of longevity records, but has since been expanded to include quantitative data for numerous other life‐history traits, including body masses at different developmental stages, reproductive data such as age at sexual maturity and measurements of reproductive output, and physiological traits related to metabolism. AnAge features over 4000 vertebrate species and is a central resource for applying the comparative method to studies of longevity and life‐history evolution across the tree of life. Moreover, by providing a reference value for longevity and other life‐history traits, AnAge can prove valuable to a broad range of biologists working in evolutionary biology, ecology, zoology, physiology and conservation biology. AnAge is freely available online ( http://genomics.senescence.info/species/ ).     

Lineage‐specific sequence evolution and exon edge conservation partially explain the relationship between evolutionary rate and expression level in A. thaliana

Rapidly evolving proteins can aid the identification of genes underlying phenotypic adaptation across taxa, but functional and structural elements of genes can also affect evolutionary rates. In plants, the ‘edges’ of exons, flanking intron junctions, are known to contain splice enhancers and to have a higher degree of conservation compared to the remainder of the coding region. However, the extent to which these regions may be masking indicators of positive selection or account for the relationship between dN/dS and other genomic parameters is unclear. We investigate the effects of exon edge conservation on the relationship of dN/dS to various sequence characteristics and gene expression parameters in the model plant Arabidopsis thaliana. We also obtain lineage‐specific dN/dS estimates, making use of the recently sequenced genome of Thellungiella parvula, the second closest sequenced relative after the sister species Arabidopsis lyrata. Overall, we find that the effect of exon edge conservation, as well as the use of lineage‐specific substitution estimates, upon dN/dS ratios partly explains the relationship between the rates of protein evolution and expression level. Furthermore, the removal of exon edges shifts dN/dS estimates upwards, increasing the proportion of genes potentially under adaptive selection. We conclude that lineage‐specific substitutions and exon edge conservation have an important effect on dN/dS ratios and should be considered when assessing their relationship with other genomic parameters.     

Predictors of incubation costs in seabirds: an evolutionary perspective

Energy costs during breeding play an important role in the evolution of life history traits. Seabirds show substantial variation in both incubation shift length (ISL) and metabolic rates. However, it is still unclear how variation in life history traits relates to incubation metabolic rates (IMR). Here, we examine the relationship between IMR and life history traits, including ISL, fledging strategy (precocial to altricial), incubation period, nest location (surface vs. underground) and clutch mass relative to adult body mass for 30 species of seabirds collated from the literature. Using both conventional non‐phylogenetic and phylogenetic generalized least‐squares approaches, we show that IMR is negatively associated with ISL, relative clutch mass and with underground nesting, while fledging strategy and incubation period have no impact on IMR once phylogeny is accounted for. Maximum likelihood reconstructions further suggest than ancestral seabirds had average ISL and relative clutch mass, and were surface nesters. We conclude that lower metabolic rates during incubation are associated with both an increased incubation shift length that allows animals to travel farther, as well as the evolutionary emergence of underground nesting that requires less social interaction.     

Preponderance of slightly deleterious polymorphism in mitochondrial DNA: nonsynonymous/synonymous rate ratio is much higher within species than between species

We estimated synonymous (dN) and nonsynonymous (dS) substitution rates for protein-coding genes of the mitochondrial genome from two individuals each of the species human, chimpanzee, and gorilla. The genes were analyzed both separately and in a combined data set. Pairwise sequence comparisons suggest that the dN/dS rate ratios are about 5-10 times higher in within-species comparisons than in between-species comparisons. This result is confirmed by a more rigorous likelihood ratio test, which rejected the null hypothesis that the dN/dS rate ratios are identical within and between species. The likelihood models account for the genetic code structure, transition/transversion rate ratio, and codon usage bias and are expected to produce more reliable results than the commonly used contingency test. Separate analyses of different genes show that the dN/dS rate ratios are higher within species than between species for all 13 mitochondrial genes, with the difference being statistically significant for all except three small or slowly evolving genes. Furthermore, in conserved genes, nonsynonymous rates within species tend to be higher than the between-species rates by a greater proportion than in fast-changing genes. Our findings confirm and extend earlier results obtained from smaller data sets and suggest the operation of slightly deleterious mutations throughout the mitochondrial genome in the hominoids. Implications of the results for evolutionary studies and, in particular, for studies of the origin of modern humans, are discussed.     

LOSS AND RE‐EVOLUTION OF COMPLEX LIFE CYCLES IN MARSUPIAL FROGS: DOES ANCESTRAL TRAIT RECONSTRUCTION MISLEAD?

Using phylogeny-based methods to identify evolutionary transitions has become an integral part of evolutionary biology. Here, we demonstrate the potential for these methods to give statistically well-supported but misleading inferences about character evolution. We also show how inferences of character evolution can be informed using GIS-based methods to reconstruct ancestral environmental regimes. We reconstruct a phylogeny for marsupial frogs (Hemiphractidae) using nuclear and mitochondrial DNA sequences and estimate patterns of life-history evolution across the resulting tree. We find that Gastrotheca species with complex life cycles (i.e., egg, tadpole, and adult stages) are phylogenetically nested among species and genera with direct development (i.e., egg and adult stages only). Assuming a single rate for gains and losses in likelihood reconstructions, there is strong statistical support for the hypothesis that the tadpole stage was lost early in the phylogeny but reappeared within Gastrotheca. Assuming different rates of gain and loss, the model with significantly higher statistical support, the tadpole stage seems to have been lost multiple times but never regained. Given that both hypotheses cannot be correct, at least one reconstruction model must be giving well-supported but misleading results. Several lines of evidence (including GIS-based reconstructions of the ancestral climatic regime) suggest that the former hypothesis is correct, and that the tadpole stage has evolved from direct development within Gastrotheca, the only known case of such a reversal in frogs.     

CONVERGENT AND CORRELATED EVOLUTION OF MAJOR LIFE‐HISTORY TRAITS IN THE ANGIOSPERM GENUS LEUCADENDRON (PROTEACEAE)

Natural selection is expected to cause convergence of life histories among taxa as well as correlated evolution of different life‐history traits. Here, we quantify the extent of convergence of five key life‐history traits (adult fire survival, seed storage, degree of sexual dimorphism, pollination mode, and seed‐dispersal mode) and test hypotheses about their correlated evolution in the genus Leucadendron (Proteaceae) from the fire‐prone South African fynbos. We reconstructed a new molecular phylogeny of this highly diverse genus that involves more taxa and molecular markers than previously. This reconstruction identifies new clades that were not detected by previous molecular study and morphological classifications. Using this new phylogeny and robust methods that account for phylogenetic uncertainty, we show that the five life‐history traits studied were labile during the evolutionary history of the genus. This diversity allowed us to tackle major questions about the correlated evolution of life‐history strategies. We found that species with longer seed‐dispersal distances tended to evolve lower pollen‐dispersal distance, that insect‐pollinated species evolved decreased sexual dimorphism, and that species with a persistent soil seed‐bank evolved toward reduced fire‐survival ability of adults.     

Phylogenetic relationships of the Cobitoidea (Teleostei: Cypriniformes) inferred from mitochondrial and nuclear genes with analyses of gene evolution

The superfamily Cobitoidea of the order Cypriniformes is a diverse group of fishes, inhabiting freshwater ecosystems across Eurasia and North Africa. The phylogenetic relationships of this well-corroborated natural group and diverse clade are critical to not only informing scientific communities of the phylogeny of the order Cypriniformes, the world's largest freshwater fish order, but are key to every area of comparative biology examining the evolution of traits, functional structures, and breeding behaviors to their biogeographic histories, speciation, anagenetic divergence, and divergence time estimates. In the present study, two mitochondrial gene sequences (COI, ND4+5) and four single-copy nuclear gene segments (RH1, RAG1, EGR2B, IRBP) were used to infer the phylogenetic relationships of the Cobitoidea as reconstructed from maximum likelihood (ML) and partitioned Bayesian Analysis (BA). Analyses of the combined mitochondrial/nuclear gene datasets revealed five strongly supported monophyletic Cobitoidea families and their sister-group relationships: Botiidae+(Vaillantellidae+(Cobitidae+(Nemacheilidae+Balitoridae))). These recovered relationships are in agreement with previous systematic studies on the order Cypriniformes and/or those focusing on the superfamily Cobitoidea. Using these relationships, our analyses revealed pattern lineage- or ecological-group-specific evolution of these genes for the Cobitoidea. These observations and results corroborate the hypothesis that these group-specific-ancestral ecological characters have contributed in the diversification and/or adaptations within these groups. Positive selections were detected in RH1 of nemacheilids and in RAG1 of nemacheilids and genus Vaillantella, which indicated that evolution of RH1 (related to eye's optic sense) and RAG1 (related to immunity) genes appeared to be important for the diversification of these groups. The balitorid lineage (those species inhabiting fast-flowing riverine habitats) had, as compared with other cobitoid lineages, significantly different dN/dS, dN and dS values for ND4 and IRBP genes. These significant differences are usually indicative of weaker selection pressure, and lineage-specific evolution on genes along the balitorid lineage. Furthermore, within Cobitoidea, excluding balitorids, species living in subtropics had significantly higher dN/dS values in RAG1 and IRBP genes than those living in temperate and tropical zones. Among tropical cobitoids, genes COI, ND5, EGR2B, IRBP and RH1, had a significantly higher mean dS value than those species in subtropical and temperate groups. These findings suggest that the evolution of these genes could also be ecological-group-specific and may have played an important role in the adaptive evolution and diversification of these groups. Thus, we hypothesize that the genes included in the present study were actively involved in lineage- and/or ecological-group-specific evolutionary processes of the highly diverse Cobitoidea. These two evolutionary patterns, both subject to further testing, are hypothesized as integral in the diversification with this major clade of the world's most diverse group of freshwater fishes.     

Sex‐specific mitonuclear epistasis and the evolution of mitochondrial bioenergetics,ageing, and life history in seed beetles

The role of mitochondrial DNA for the evolution of life‐history traits remains debated. We examined mitonuclear effects on the activity of the multisubunit complex of the electron transport chain (ETC) involved in oxidative phosphorylation (OXPHOS) across lines of the seed beetle Acanthoscelides obtectus selected for a short (E) or a long (L) life for more than >160 generations. We constructed and phenotyped mitonuclear introgression lines, which allowed us to assess the independent effects of the evolutionary history of the nuclear and the mitochondrial genome. The nuclear genome was responsible for the largest share of divergence seen in ageing. However, the mitochondrial genome also had sizeable effects, which were sex‐specific and expressed primarily as epistatic interactions with the nuclear genome. The effects of mitonuclear disruption were largely consistent with mitonuclear coadaptation. Variation in ETC activity explained a large proportion of variance in ageing and life‐history traits and this multivariate relationship differed somewhat between the sexes. In conclusion, mitonuclear epistasis has played an important role in the laboratory evolution of ETC complex activity, ageing, and life histories and these are closely associated. The mitonuclear architecture of evolved differences in life‐history traits and mitochondrial bioenergetics was sex‐specific.     

Dynamic colonization exchanges between continents and islands drive diversification in paradise‐flycatchers (Terpsiphone,Monarchidae)

Aim We use parametric biogeographical reconstruction based on an extensive DNA sequence dataset to characterize the spatio‐temporal pattern of colonization of the Old World monarch flycatchers (Monarchidae). We then use this framework to examine the role of dispersal and colonization in their evolutionary diversification and to compare plumages between island and continental Terpsiphone species. Location Africa, Asia and the Indian Ocean. Methods We generate a DNA sequence dataset of 2300 bp comprising one nuclear and three mitochondrial markers for 89% (17/19) of the Old World Monarchidae species and 70% of the Terpsiphone subspecies. By applying maximum likelihood and Bayesian phylogenetic methods and implementing a Bayesian molecular clock to provide a temporal framework, we reveal the evolutionary history of the group. Furthermore, we employ both Lagrange and Bayes‐ Lagrange analyses to assess ancestral areas at each node of the phylogeny. By combining the ancestral area reconstruction with information on plumage traits we are able to compare patterns of plumage evolution on islands and continents. Results We provide the first comprehensive molecular phylogenetic reconstruction for the Old World Monarchidae. Our phylogenetic results reveal a relatively recent diversification associated with several dispersal events within this group. Moreover, ancestral area analyses reveal an Asian origin of the Indian Ocean and African clades. Ancestral state reconstruction analyses of plumage characters provide an interpretation of the plumage differentiation on islands and continents. Ancestral plumage traits are inferred to be close to those of the Asian paradise‐flycatcher (Terpsiphone paradisi), and island species display a high degree of plumage autapomorphy compared with continental species. Main conclusions Terpsiphone paradisi is polyphyletic and comprises populations that have retained the ancestral plumage of the widespread Terpsiphone genus. The genus appears to have colonized south‐west Asia, the Indian Ocean and Africa from eastern Asia. The phylogeny and divergence time estimates indicate multiple simultaneous colonizations of the western Old World by Terpsiphone. These results reinforce a hypothesis of range expansions of a Terpsiphone paradisi‐like ancestor into eastern Asia and the western Old World.     

Something for nothing? Reconstruction of ancestral character states in asterinid sea star development

SUMMARY Traits from early development mapped onto phylogenetic trees can potentially offer insight into the evolutionary history of development by inferring the states of those characters among ancestors at nodes in the phylogeny. A key and often-overlooked aspect of such mapping is the underlying model of character evolution. Without a well-supported and realistic model ("nothing"), character mapping of ancestral traits onto phylogenetic trees might often return results ("something") that lack a sound basis. Here we reconsider a challenging case study in this area of evolutionary developmental biology: the inference of ancestral states for ecological and morphological characters in the reproduction and larval development of asterinid sea stars. We apply improved analytical methods to an expanded set of asterinid phylogenetic data and developmental character states. This analysis shows that the new methods might generally offer some independent insight into choice of a model of character evolution, but that in the specific case of asterinid sea stars the quantitative features of the model (especially the relative probabilities of different directions of change) have an important effect on the results. We suggest caution in applying ancestral state reconstructions in the absence of an independently corroborated model of character evolution, and highlight the need for such modeling in evolutionary developmental biology.     

Bayesian inference of character evolution

Much recent progress in evolutionary biology is based on the inference of ancestral states and past transformations in important traits on phylogenetic trees. These exercises often assume that the tree is known without error and that ancestral states and character change can be mapped onto it exactly. In reality, there is often considerable uncertainty about both the tree and the character mapping. Recently introduced Bayesian statistical methods enable the study of character evolution while simultaneously accounting for both phylogenetic and mapping uncertainty, adding much needed credibility to the reconstruction of evolutionary history.     

Cytochrome b evolution in birds and mammals: an evaluation of the avian constraint hypothesis.

Patterns of molecular evolution in birds have long been considered anomalous. Compared with other vertebrates, birds have reduced levels of genetic divergence between groups of similar taxonomic ranks for a variety of nuclear and mitochondrial markers. This observation led to the avian constraint hypothesis, which identifies increased functional constraint on avian proteins as the cause for the reduction in genetic divergence. Subsequent investigations provided additional support for the avian constraint hypothesis when rates of molecular evolution were found to be slower in birds than in mammals in a variety of independent calibrations. It is possible to test the avian constraint hypothesis as an explanation for this avian slowdown by comparing DNA sequence data from protein-coding regions in birds and homologous regions in mammals. The increased selective constraints should lead to a reduction in the proportion of amino acid replacement substitutions. To test for such a decrease, we calculated the numbers of amino acid replacement substitutions per replacement site (dN) and silent substitutions per silent site (dS) for the complete mitochondrial cytochrome b gene using 38 avian and 43 mammalian comparisons that were phylogenetically independent. We find that dN/dS is significantly smaller in birds than in mammals. This difference cannot be explained by differences in codon bias affecting dS values. We suggest that the avian slowdown can be explained, at least in part, by a decreased tolerance for amino acid substitutions in avian species relative to mammalian species.     

Reverse evolution of fitness in Drosophila melanogaster

Abstract The evolution of fitness is central to evolutionary theory, yet few experimental systems allow us to track its evolution in genetically and environmentally relevant contexts. Reverse evolution experiments allow the study of the evolutionary return to ancestral phenotypic states, including fitness. This in turn permits well‐defined tests for the dependence of adaptation on evolutionary history and environmental conditions. In the experiments described here, 20 populations of heterogeneous evolutionary histories were returned to their common ancestral environment for 50 generations, and were then compared with both their immediate differentiated ancestors and populations which had remained in the ancestral environment. One measure of fitness returned to ancestral levels to a greater extent than other characters did. The phenotypic effects of reverse evolution were also contingent on previous selective history. Moreover, convergence to the ancestral state was highly sensitive to environmental conditions. The phenotypic plasticity of fecundity, a character directly selected for, evolved during the experimental time frame. Reverse evolution appears to force multiple, diverged populations to converge on a common fitness state through different life‐history and genetic changes.     

Phylogenetic patterns of trait and trait plasticity evolution: Insights from amphibian embryos

Environmental variation favors the evolution of phenotypic plasticity. For many species, we understand the costs and benefits of different phenotypes, but we lack a broad understanding of how plastic traits evolve across large clades. Using identical experiments conducted across North America, we examined prey responses to predator cues. We quantified five life‐history traits and the magnitude of their plasticity for 23 amphibian species/populations (spanning three families and five genera) when exposed to no cues, crushed‐egg cues, and predatory crayfish cues. Embryonic responses varied considerably among species and phylogenetic signal was common among the traits, whereas phylogenetic signal was rare for trait plasticities. Among trait‐evolution models, the Ornstein–Uhlenbeck (OU) model provided the best fit or was essentially tied with Brownian motion. Using the best fitting model, evolutionary rates for plasticities were higher than traits for three life‐history traits and lower for two. These data suggest that the evolution of life‐history traits in amphibian embryos is more constrained by a species’ position in the phylogeny than is the evolution of life history plasticities. The fact that an OU model of trait evolution was often a good fit to patterns of trait variation may indicate adaptive optima for traits and their plasticities.     

Interspecific competition affects evolutionary links between cavity nesting,migration and clutch size in Old World flycatchers (Muscicapdae)

The ecology of cavity nesting in passerine birds has been studied extensively, yet there are no phylogenetic comparative studies that quantify differences in life history traits between cavity‐ and open‐nesting birds within a passerine family. We test existing hypotheses regarding the evolutionary significance of cavity nesting in the Old World flycatchers (Muscicapidae). We used a multi‐locus phylogeny of 252 species to reconstruct the evolutionary history of cavity nesting and to quantify correlations between nest types and life history traits. Within a phylogenetic generalized linear model framework, we found that cavity‐nesting species are larger than open‐nesting species and that maximum clutch sizes are larger in cavity‐nesting lineages. In addition to differences in life history traits between nest types, species that breed at higher latitudes have larger average and maximum clutch sizes and begin to breed later in the year. Gains and losses of migratory behaviour have occurred far more often in cavity‐nesting lineages than in open‐nesting taxa, suggesting that cavity nesting may have played a crucial role in the evolution of migratory behaviour. These findings identify important macro‐evolutionary links between the evolution of cavity nesting, clutch size, interspecific competition and migratory behaviour in a large clade of Old World songbirds.     

Unequal rates of Y chromosome gene divergence during speciation of the family Ursidae

Evolution of the bear family Ursidae is well investigated in terms of morphological, paleontological, and genetic features. However, several phylogenetic ambiguities occur within the subfamily Ursinae (the family Ursidae excluding the giant panda and spectacled bear), which may correlate with behavioral traits of female philopatry and male-biased dispersal which form the basis of the observed matriarchal population structure in these species. In the process of bear evolution, we investigate the premise that such behavioral traits may be reflected in patterns of variation among genes with different modes of inheritance: matrilineal mitochondrial DNA (mtDNA), patrilineal Y chromosome, biparentally inherited autosomes, and the X chromosome. In the present study, we sequenced 3 Y-linked genes (3,453 bp) and 4 X-linked genes (4,960 bp) and reanalyzed previously published sequences from autosome genes (2,347 bp) in ursid species to investigate differences in evolutionary rates associated with patterns of inheritance. The results describe topological incongruence between sex-linked genes and autosome genes and between nuclear DNA and mtDNA. In more ancestral branches within the bear phylogeny, Y-linked genes evolved faster than autosome and X-linked genes, consistent with expectations based on male-driven evolution. However, this pattern changes among branches leading to each species within the lineage of Ursinae whereby the evolutionary rates of Y-linked genes have fewer than expected substitutions. This inconsistency between more recent nodes of the bear phylogeny with more ancestral nodes may reflect the influences of sex-biased dispersal as well as molecular evolutionary characteristics of the Y chromosome, and stochastic events in species natural history, and phylogeography unique to ursine bears.     

CSR ecological strategies and plant mating systems: outcrossing increases with competitiveness but stress‐tolerance is related to mixed mating

A number of plant traits influence the success of fertilization and reproduction in plants. Collectively these traits represent ecological syndromes that are of evolutionary significance. However, while an association between mating system and colonizing ability has been proposed, the existence of a broader relationship between mating system and a species’ position in ecological succession has not been extensively investigated. Grime's CSR theory stresses that an ecological succession can involve changes from colonizing to either competitive or stress‐tolerant strategies. How distinct dimensions of competitiveness and stress tolerance covary with mating systems has still not been considered. We designed a comparative approach to evaluate the link between mating system, life form and CSR strategies for 1996 herbaceous and woody species. We found that CSR strategies are significantly related to mating systems. Ruderal species – colonizers in early succession – were mostly selfers while more competitive species were more often outcrossers. On the other hand, greater physiological stress tolerance was associated with mixed mating systems. Outcrossing is classically expected to be advantageous for most life history strategies other than colonizers, but we suggest that reproductive assurance can counterbalance this effect in stressful environments where populations are sparse and pollinators are rare. Therefore, our results emphasize that competition and abiotic stresses are not equivalent selective pressures on the evolution of mating systems. Finally, we found plant life span to convey additional information on mating system variation, supporting its role for mating system evolution. These findings encourage further investigation of the evolutionary role of ecological strategies as syndromes of traits and suggest that the emergence of large databases of plant traits will help address the major evolutionary hypotheses on such syndromes.