There is an obstetrical dilemma: Misconceptions about the evolution of human childbirth and pelvic form

Compared to other primates, modern humans face high rates of maternal and neonatal morbidity and mortality during childbirth. Since the early 20th century, this "difficulty" of human parturition has prompted numerous evolutionary explanations, typically assuming antagonistic selective forces acting on maternal and fetal traits, which has been termed the "obstetrical dilemma." Recently, there has been a growing tendency among some anthropologists to question the difficulty of human childbirth and its evolutionary origin in an antagonistic selective regime. Partly, this stems from the motivation to combat increasing pathologization and overmedicalization of childbirth in industrialized countries. Some authors have argued that there is no obstetrical dilemma at all, and that the difficulty of childbirth mainly results from modern lifestyles and inappropriate and patriarchal obstetric practices. The failure of some studies to identify biomechanical and metabolic constraints on pelvic dimensions is sometimes interpreted as empirical support for discarding an obstetrical dilemma. Here we explain why these points are important but do not invalidate evolutionary explanations of human childbirth. We present robust empirical evidence and solid evolutionary theory supporting an obstetrical dilemma, yet one that is much more complex than originally conceived in the 20th century. We argue that evolutionary research does not hinder appropriate midwifery and obstetric care, nor does it promote negative views of female bodies. Understanding the evolutionary entanglement of biological and sociocultural factors underlying human childbirth can help us to understand individual variation in the risk factors of obstructed labor, and thus can contribute to more individualized maternal care.

A potential role for restricted intertactical heritability in preventing intralocus conflict

Intralocus sexual conflict, which arises when the same trait has different fitness optima in males and females, reduces population growth rates. Recently, evolutionary biologists have recognized that intralocus conflict can occur between morphs or reproductive tactics within a sex and that intralocus tactical conflict might constrain tactical dimorphism and population growth rates just as intralocus sexual conflict constrains sexual dimorphism and population growth rates. However, research has only recently focused on sexual and tactical intralocus conflict simultaneously, and there is no formal theory connecting the two. We present a graphical model of how tactical and sexual conflict over the same trait could constrain both sexual and tactical dimorphisms. We then use Coho salmon (Oncorhynchus kisutch), an important species currently protected under the Endangered Species Act, to investigate the possibility of simultaneous sexual and tactical conflict. Larger Coho males gain access to females through fighting while smaller males are favored through sneaking tactics, and female reproductive success is positively correlated with length. We tested for antagonistic selection on length at maturity among sexes and tactics and then used parent-offspring regression to calculate sex- and tactic-specific heritabilities to determine whether and where intralocus conflict exists. Selection on length varied in intensity and form among tactics and years. Length was heritable between dams and daughters (h 2 ± 95% CI = 0.361 ± 0.252) and between fighter males and their fighter sons (0.867 ± 0.312), but no other heritabilities differed significantly from zero. The lack of intertactical heritabilities in this system, combined with similar selection on length among tactics, suggests the absence of intralocus conflict between sexes and among tactics, allowing for the evolution of sexual and tactical dimorphisms. Our results suggest that Coho salmon populations are unlikely to be constrained by intralocus conflict or artificial selection on male tactic.

The obstetrical dilemma hypothesis: there's life in the old dog yet

The term 'obstetrical dilemma' was coined by Washburn in 1960 to describe the trade-off between selection for a larger birth canal, permitting successful passage of a big-brained human neonate, and the smaller pelvic dimensions required for bipedal locomotion. His suggested solution to these antagonistic pressures was to give birth prematurely, explaining the unusual degree of neurological and physical immaturity, or secondary altriciality, observed in human infants. This proposed trade-off has traditionally been offered as the predominant evolutionary explanation for why human childbirth is so challenging, and inherently risky, compared to that of other primates. This perceived difficulty is likely due to the tight fit of fetal to maternal pelvic dimensions along with the convoluted shape of the birth canal and a comparatively low degree of ligamentous flexibility. Although the ideas combined under the obstetrical dilemma hypothesis originated almost a century ago, they have received renewed attention and empirical scrutiny in the last decade, with some researchers advocating complete rejection of the hypothesis and its assumptions. However, the hypothesis is complex because it presently captures several, mutually non-exclusive ideas: (i) there is an evolutionary trade-off resulting from opposing selection pressures on the pelvis; (ii) selection favouring a narrow pelvis specifically derives from bipedalism; (iii) human neonates are secondarily altricial because they are born relatively immature to ensure that they fit through the maternal bony pelvis; (iv) as a corollary to the asymmetric selection pressure for a spacious birth canal in females, humans evolved pronounced sexual dimorphism of pelvic shape. Recently, the hypothesis has been challenged on both empirical and theoretical grounds. Here, we appraise the original ideas captured under the 'obstetrical dilemma' and their subsequent evolution. We also evaluate complementary and alternative explanations for a tight fetopelvic fit and obstructed labour, including ecological factors related to nutrition and thermoregulation, constraints imposed by the stability of the pelvic floor or by maternal and fetal metabolism, the energetics of bipedalism, and variability in pelvic shape. This reveals that human childbirth is affected by a complex combination of evolutionary, ecological, and biocultural factors, which variably constrain maternal pelvic form and fetal growth. Our review demonstrates that it is unwarranted to reject the obstetrical dilemma hypothesis entirely because several of its fundamental assumptions have not been successfully discounted despite claims to the contrary. As such, the obstetrical dilemma remains a tenable hypothesis that can be used productively to guide evolutionary research.

Natural Selection on Adults Has Trait-Dependent Consequences for Juvenile Evolution in Dragonflies

AbstractAlthough natural selection often fluctuates across ontogeny, it remains unclear what conditions enable selection in one life-cycle stage to shape evolution in others. Organisms that undergo metamorphosis are useful for addressing this topic because their highly specialized life-cycle stages cannot always evolve independently despite their dramatic life-history transition. Using a comparative study of dragonflies, we examined three conditions that are hypothesized to allow selection in one stage to affect evolution in others. First, we tested whether lineages with less dramatic metamorphosis (e.g., hemimetabolous insects) lack the capacity for stage-specific evolution. Rejecting this hypothesis, we found that larval body shape evolves independently from selection on adult shape. Next, we evaluated whether stage-specific evolution is limited for homologous and/or coadapted structures. Indeed, we found that selection for larger wings is associated with the evolution of coadapted larval sheaths that store developing wing tissue. Finally, we assessed whether stage-specific evolution is restricted for traits linked to a single biochemical pathway. Supporting this hypothesis, we found that species with more wing melanization in the adult stage have evolved weaker melanin immune defenses in the larval stage. Thus, our results collectively show that natural selection in one stage imposes trait-dependent constraints on evolution in others.

Genomic architecture and evolutionary antagonism drive allelic expression bias in the social supergene of red fire ants

Supergene regions maintain alleles of multiple genes in tight linkage through suppressed recombination. Despite their importance in determining complex phenotypes, our empirical understanding of early supergene evolution is limited. Here we focus on the young ‘social’ supergene of fire ants, a powerful system for disentangling the effects of evolutionary antagonism and suppressed recombination. We hypothesize that gene degeneration and social antagonism shaped the evolution of the fire ant supergene, resulting in distinct patterns of gene expression. We test these ideas by identifying allelic differences between supergene variants, characterizing allelic expression across populations, castes and body parts, and contrasting allelic expression biases with differences in expression between social forms. We find strong signatures of gene degeneration and gene-specific dosage compensation. On this background, a small portion of the genes has the signature of adaptive responses to evolutionary antagonism between social forms.

Red fire ants (Solenopsis invicta) are native to South America, but the species has spread to North America, Australia and New Zealand where it can be an invasive pest. A reason for this species’ invasiveness types of colonies : one with a single egg-laying queen and another with several queens. However, it is not possible to simply add more queens to a colony with one queen. Instead, the number of queens in a colony is controlled genetically, by a chromosome known as the ‘social chromosome’.

Like many other animals, red fire ants are diploid: their cells have two copies of each chromosome, which can carry two different versions of each gene. The social chromosome is no different, and it comes in two variants, SB and Sb. Each ant can therefore have either two SB chromosomes, leading to a colony with a single queen; or one SB chromosome and one Sb chromosome, leading to a colony with multiple queens. Ants with two copies of the Sb variant die when they are young, so the Sb version is inherited in a similar way to how the Y chromosome is passed on in humans. However, the social chromosome in red fire ants appeared less than one million years ago, making it much younger than the human Y chromosome, which is 180 million years old. This makes the social chromosome a good candidate for examining the early evolution of special chromosome variants that are only inherited.

How differences between the SB and the Sb chromosomes are evolving is an open question, however. Perhaps each version of the social chromosome has been optimised through natural selection to one colony type. Another suggestion is that the Sb chromosome has degenerated over time because its genes cannot be ‘reshuffled’ as they would be on normal chromosomes.

Martinez-Ruiz et al. compared genetic variants on the SB and Sb chromosomes, along with their expression in different types of ant colonies. The analysis showed that the Sb variant is in fact breaking down because of the lack of gene shuffling. This loss is compensated by intact copies of the same genes found on the SB variant, which explains why ants with the Sb variant can only survive if they also carry the SB version. Only a handful of genes on the social chromosomes appear to have been optimised by natural selection. Therefore Martinez-Ruiz et al. concluded the differences between the two chromosomes that lead to different colony types are collateral effects of Sb’s inability to reshuffle its genes.

This work reveals how a special chromosome similar to the Y chromosome in humans evolved. It also shows how multiple complex evolutionary forces can shape a species’ genetic makeup and social forms.

Limits to Genomic Divergence Under Sexually Antagonistic Selection

Since the autosomal genome is shared between the sexes, sex-specific fitness optima present an evolutionary challenge. While sexually antagonistic selection might favor different alleles within females and males, segregation randomly reassorts alleles at autosomal loci between sexes each generation. This process of homogenization during transmission thus prevents between-sex allelic divergence generated by sexually antagonistic selection from accumulating across multiple generations. However, recent empirical studies have reported high male-female F_ST statistics. Here, we use a population genetic model to evaluate whether these observations could plausibly be produced by sexually antagonistic selection. To do this, we use both a single-locus model with nonrandom mate choice, and individual-based simulations to study the relationship between strength of selection, degree of between-sex divergence, and the associated genetic load. We show that selection must be exceptionally strong to create measurable divergence between the sexes and that the decrease in population fitness due to this process is correspondingly high. Individual-based simulations with selection genome-wide recapitulate these patterns and indicate that small sample sizes and sampling variance can easily generate substantial male-female divergence. We therefore conclude that caution should be taken when interpreting autosomal allelic differentiation between the sexes.

Little Evidence of Antagonistic Selection in the Evolutionary Strata of Fungal Mating-Type Chromosomes (Microbotryum lychnidis-dioicae)

Recombination suppression on sex chromosomes often extends in a stepwise manner, generating evolutionary strata of differentiation between sex chromosomes. Sexual antagonism is a widely accepted explanation for evolutionary strata, postulating that sets of genes beneficial in only one sex are successively linked to the sex-determining locus. The anther-smut fungus Microbotryum lychnidis-dioicae has mating-type chromosomes with evolutionary strata, only some of which link mating-type genes. Male and female roles are non-existent in this fungus, but mating-type antagonistic selection can also generate evolutionary strata, although the life cycle of the fungus suggests it should be restricted to few traits. Here, we tested the hypothesis that mating-type antagonism may have triggered recombination suppression beyond mating-type genes in M. lychnidis-dioicae by searching for footprints of antagonistic selection in evolutionary strata not linking mating-type loci. We found that these evolutionary strata (i) were not enriched in genes upregulated in the haploid phase, where cells are of alternative mating types, (ii) carried no gene differentially expressed between mating types, and (iii) carried no genes displaying footprints of specialization in terms of protein sequences (d_N/d_S) between mating types after recommended filtering. Without filtering, eleven genes showed signs of positive selection in the strata not linking mating-type genes, which constituted an enrichment compared to autosomes, but their functions were not obviously involved in antagonistic selection. Thus, we found no strong evidence that antagonistic selection has contributed to extending recombination suppression beyond mating-type genes. Alternative hypotheses should therefore be explored to improve our understanding of the sex-related chromosome evolution.

Synergism and Antagonism of Proximate Mechanisms Enable and Constrain the Response to Simultaneous Selection on Body Size and Development Time: An Empirical Test Using Experimental Evolution

Natural selection acts on multiple traits simultaneously. How mechanisms underlying such traits enable or constrain their response to simultaneous selection is poorly understood. We show how antagonism and synergism among three traits at the developmental level enable or constrain evolutionary change in response to simultaneous selection on two focal traits at the phenotypic level. After 10 generations of 25% simultaneous directional selection on all four combinations of body size and development time in Manduca sexta (Sphingidae), the changes in the three developmental traits predict 93% of the response of development time and 100% of the response of body size. When the two focal traits were under synergistic selection, the response to simultaneous selection was enabled by juvenile hormone and ecdysteroids and constrained by growth rate. When the two focal traits were under antagonistic selection, the response to selection was due primarily to change in growth rate and constrained by the two hormonal traits. The approach used here reduces the complexity of the developmental and endocrine mechanisms to three proxy traits. This generates explicit predictions for the evolutionary response to selection that are based on biologically informed mechanisms. This approach has broad applicability to a diverse range of taxa, including algae, plants, amphibians, mammals, and insects.

Sexual Selection on male cuticular hydrocarbons via male-male competition and female choice

Traditional views of sexual selection assumed that male–male competition and female mate choice work in harmony, selecting upon the same traits in the same direction. However, we now know that this is not always the case and that these two mechanisms often impose conflicting selection on male sexual traits. Cuticular hydrocarbons (CHCs) have been shown to be linked to both social dominance and male attractiveness in several insect species. However, although several studies have estimated the strength and form of sexual selection imposed on male CHCs by female mate choice, none have established whether these chemical traits are also subject to sexual selection via male–male competition. Using a multivariate selection analysis, we estimate and compare sexual selection exerted by male–male competition and female mate choice on male CHC composition in the broad‐horned flour beetle Gnatocerus cornutus. We show that male–male competition exerts strong linear selection on both overall CHC abundance and body size in males, while female mate choice exerts a mixture of linear and nonlinear selection, targeting not just the overall amount of CHCs expressed but the relative abundance of specific hydrocarbons as well. We discuss the potential implications of this antagonistic selection with regard to male reproductive success.

Evolution of haploid selection in predominantly diploid organisms

Diploid organisms manipulate the extent to which their haploid gametes experience selection. Animals typically produce sperm with a diploid complement of most proteins and RNA, limiting selection on the haploid genotype. Plants, however, exhibit extensive expression in pollen, with actively transcribed haploid genomes. Here we analyze models that track the evolution of genes that modify the strength of haploid selection to predict when evolution intensifies and when it dampens the "selective arena" within which male gametes compete for fertilization. Considering deleterious mutations, evolution leads diploid mothers to strengthen selection among haploid sperm/pollen, because this reduces the mutation load inherited by their diploid offspring. If, however, selection acts in opposite directions in haploids and diploids ("ploidally antagonistic selection"), mothers evolve to reduce haploid selection to avoid selectively amplifying alleles harmful to their offspring. Consequently, with maternal control, selection in the haploid phase either is maximized or reaches an intermediate state, depending on the deleterious mutation rate relative to the extent of ploidally antagonistic selection. By contrast, evolution generally leads diploid fathers to mask mutations in their gametes to the maximum extent possible, whenever masking (e.g., through transcript sharing) increases the average fitness of a father's gametes. We discuss the implications of this maternal-paternal conflict over the extent of haploid selection and describe empirical studies needed to refine our understanding of haploid selection among seemingly diploid organisms.

Heterosis in age-specific selected populations of a seed beetle: sex differences in longevity and reproductive behavior

We tested mutation accumulation hypothesis for the evolution of senescence using short-lived and long-lived populations of the seed-feeding beetle, Acanthoscelides obtectus (Say), obtained by selection on early- and late-life for many generations. The expected consequence of the mutation accumulation hypothesis is that in short-lived populations, where the force of natural selection is the strongest early in life, the late-life fitness traits should decline due to genetic drift which increases the frequency of mutations with deleterious effects in later adult stages. Since it is unlikely that identical deleterious mutations will increase in several independent populations, hybrid vigor for late-life fitness is expected in offspring obtained in crosses among populations selected for early-life fitness traits. We tested longevity of both sexes, female fecundity and male reproductive behavior for hybrid vigor by comparing hybrid and nonhybrid short-lived populations. Hybrid vigor was confirmed for male virility, mating speed and copulation duration, and longevity of both sexes at late ages. In contrast to males, the results on female fecundity in short-lived populations did not support mutation accumulation as a genetic mechanism for the evolution of this trait. Contrary to the prediction of this hypothesis, male mating ability indices and female fecundity in long-lived populations exhibited hybrid vigor at all assayed age classes. We demonstrate that nonhybrid long-lived populations diverged randomly regarding female and male reproductive fitness, indicating that sexually antagonistic selection, when accompanied with genetic drift for female fecundity and male virility, might be responsible for overriding natural selection in the independently evolving long-lived populations.

Kin selection, genomics and caste-antagonistic pleiotropy

Kin selection is a fundamentally important process that affects the evolution of social behaviours. The genomics revolution now provides the opportunity to test kin selection theory using genomic data. In this commentary, we discuss previous studies that explored the link between kin selection and patterns of variation within the genome. We then present a new theory aimed at understanding the evolution of genes involved in the development of social insects. Specifically, we investigate caste-antagonistic pleiotropy, which occurs when the phenotypes of distinct castes are optimized by different genotypes at a single locus. We find that caste-antagonistic pleiotropy leads to narrow regions where polymorphism can be maintained. Furthermore, multiple mating by queens reduces the region in which worker-favoured alleles fix, which suggests that multiple mating impedes worker caste evolution. We conclude by discussing ways to test these and other facets of kin selection using newly emerging genomic data.