A test of the short-term advantage of sexual reproduction

Early research on anther-smut disease: A fuller view of science?

The anther-smut host-pathogen system has provided extensive insights into the evolutionary ecology of disease resistance, transmission modes, host shifts, pathogen specialization, and disease evolution in metapopulations. It also has led to unexpected insights into sex ratio distorters, sex chromosome evolution, and transposable elements in fungi. In addition, anther-smut disease played a major role in Linnaeus' germ theory and the correspondence on parasitic castration between Darwin and Becker, one of the first female botanists. Here, we explicitly highlight some of the realities in the process of science, using an unusual autobiographical approach to describe how we came to collaborate on this system in the 1980s. Using perspectives from our different career stages, we present a surprising narrative that could not be deduced from merely reading the published papers. While our work was grounded in previous ecological and evolutionary theory, it was the product as much of empirical failures and intellectual roadblocks, as the result of a progressive scientific method. Our experiences illustrate not only the "human dimension of science" but more importantly show that linear sequences of hypothesis testing do not necessarily lead to new study systems and new ideas. We suggest there is a need to re-evaluate the scientific method in ecology and evolution, especially where the challenge is to engage in a productive dialog between natural history and theory.

Genetic diversity and disease: The past, present, and future of an old idea

Why do infectious diseases erupt in some host populations and not others? This question has spawned independent fields of research in evolution, ecology, public health, agriculture, and conservation. In the search for environmental and genetic factors that predict variation in parasitism, one hypothesis stands out for its generality and longevity: genetically homogeneous host populations are more likely to experience severe parasitism than genetically diverse populations. In this perspective piece, I draw on overlapping ideas from evolutionary biology, agriculture, and conservation to capture the far-reaching implications of the link between genetic diversity and disease. I first summarize the development of this hypothesis and the results of experimental tests. Given the convincing support for the protective effect of genetic diversity, I then address the following questions: (1) Where has this idea been put to use, in a basic and applied sense, and how can we better use genetic diversity to limit disease spread? (2) What new hypotheses does the established disease-diversity relationship compel us to test? I conclude that monitoring, preserving, and augmenting genetic diversity is one of our most promising evolutionarily informed strategies for buffering wild, domesticated, and human populations against future outbreaks.

The evolution of sex along an environmental gradient

Although temporally changing environments generally favor sex and recombination, the effects of spatial environmental heterogeneity have been less explored. In this article, we use a classical model of adaptation along with an environmental gradient to study the selective forces acting on reproductive mode evolution in the central and marginal parts of the distribution range of a species. The model considers a polygenic trait under stabilizing selection (the optimal trait value changing across space) and includes a demographic component imposing range limits. The results show that in the central part of the range (where populations are well adapted), recombination tends to increase the mean fitness of offspring in regimes where drift is sufficiently strong (generating a benefit for sex), while it has the opposite effect when the effect of drift stays negligible. However, these effects remain weak and are easily overwhelmed by slight intrinsic fitness differences between sexuals and asexuals. In agreement with previous results, asexuality may be favored in marginal populations, as it can preserve adaptation to extreme conditions. However, a substantial advantage of asexuality is possible only in conditions maintaining a strong maladaptation of sexuals at range limits (high effective environmental gradient, weak selection at loci coding for the trait).

Identifying the fitness consequences of sex in complex natural environments

In the natural world, sex prevails, despite its costs. Although much effort has been dedicated to identifying the intrinsic costs of sex (e.g., the cost of males), few studies have identified the ecological fitness consequences of sex. Furthermore, correlated biological traits that differ between sexuals and asexuals may alter these costs, or even render the typical costs of sex irrelevant. We conducted a large-scale, multisite, reciprocal transplant using multiple sexual and asexual genotypes of a native North American wildflower to show that sexual genotypes have reduced lifetime fitness, despite lower herbivory. We separated the effects of sex from those of hybridity, finding that overwinter survival is elevated in asexuals regardless of hybridity, but herbivores target hybrid asexuals more than nonhybrid asexual or sexual genotypes. Survival is lowest in homozygous sexual lineages, implicating inbreeding depression as a cost of sex. Our results show that the consequences of sex are shaped not just by sex itself, but by complex natural environments, correlated traits, and the identity and availability of mates.

Fitness consequences of occasional outcrossing in a functionally asexual plant (Oenothera biennis)

Many clonal organisms occasionally outcross, but the long-term consequences of such infrequent events are often unknown. During five years, representing three to five plant generations, we followed 16 experimental field populations of the forb, Oenothera biennis, originally planted with the same 18 original genotypes. Oenothera biennis usually self fertilizes, which, due to its genetic system (permanent translocation heterozygosity), results in seeds that are clones of the maternal plant. However, rare outcrossing produces genetically novel offspring (but without recombination or increased heterozygosity). We sought to understand whether novel genotypes produced through natural outcrossing had greater fecundity or different multigenerational dynamics compared to our original genotypes. We further assessed whether any differences in fitness or abundances through time between original and novel genotypes were exaggerated in the presence vs. absence of insect herbivores. Over the course of the experiment, we genotyped >12,500 plants using microsatellite DNA markers to identify and track the frequency of specific genotypes and estimated fecundity on a subset (>3,000) of plants. The effective outcrossing rate was 7.3% in the first year and ultimately 50% of the plants were of outcrossed origin by the final year of the experiment. Lifetime fruit production per plant was on average 32% higher across all novel genotypes produced via outcrossing compared to the original genotypes, and this fecundity advantage was significantly enhanced in populations lacking herbivores. Among 43 novel genotypes that were abundant enough to phenotype with replication, plants produced nearly 30% more fruits than the average of their specific two parental genotypes, and marginally more fruits (8%) than their most fecund parent. Mean per capita fecundity of novel genotypes predicted their relative frequencies at the end of the experiment. Novel genotypes increased more dramatically in herbivore-present compared to suppressed populations (45% vs. 27% of all plants), countering the increased competition from dandelions (Taraxacum officinale) that resulted from herbivore suppression. Increased interspecific competition likely also lead to the lower realized fitness of novel vs. original genotypes in herbivore-suppressed populations. These results demonstrate that rare outcrossing and the generation of novel genotypes can create high-fecundity progeny, with the biotic environment influencing the dynamical outcome of such advantages.

Sex as a strategy against rapidly evolving parasites

Why is sex ubiquitous when asexual reproduction is much less costly? Sex disrupts coadapted gene complexes; it also causes costs associated with mate finding and the production of males who do not themselves bear offspring. Theory predicts parasites select for host sex, because genetically variable offspring can escape infection from parasites adapted to infect the previous generations. We examine this using a facultative sexual crustacean, Daphnia magna, and its sterilizing bacterial parasite, Pasteuria ramosa. We obtained sexually and asexually produced offspring from wild-caught hosts and exposed them to contemporary parasites or parasites isolated from the same population one year later. We found rapid parasite adaptation to replicate within asexual but not sexual offspring. Moreover, sexually produced offspring were twice as resistant to infection as asexuals when exposed to parasites that had coevolved alongside their parents (i.e. the year two parasite). This fulfils the requirement that the benefits of sex must be both large and rapid for sex to be favoured by selection.

FITNESS CONSEQUENCES OF MULTIPLE PATERNITY IN WILD RADISH, RAPHANUS SATIVUS

In natural populations, wild radish plants typically mate with 6-8 pollen donors, and seeds of individual fruits are usually sired by 1-4 fathers. Since radish fruits are indehiscent and gravity-dispersed, progeny are most likely to compete with a mixture of full and half siblings. The fitness consequences of single and multiple paternity were investigated in a greenhouse experiment. Seeds of every possible cross in a 5 times 5 reciprocal diallel mating design were assigned to one of three competition regimes (four full siblings, four maternal half siblings, or four unrelated individuals per pot) or were grown as singletons. After 14 weeks, the aboveground biomass of all plants was harvested and oven-dried. The dry weight of singletons was more than three times that of progeny grown in competition, indicating that intraspecific competition had occurred. Full- and half-sib progenies did not differ in mean dry weight. Thus, there was no evidence that multiple paternity enhances this aspect of maternal fitness. However, the competition regime dramatically affected the coefficient of variation in dry weight of progeny within a pot. Weight hierarchies were much more pronounced in pots of half sibs and unrelated neighbors than in pots of full sibs. Also variance in dry weight attributable to sire was greatest in the half-sib and "unrelated neighbors" competition regimes. These results suggest that weight hierarchies reinforce genetic differences among the offspring.

EXPERIMENTAL STUDIES OF THE EVOLUTIONARY SIGNIFICANCE OF SEXUAL REPRODUCTION. V. A FIELD TEST OF THE SIB-COMPETITION LOTTERY HYPOTHESIS

Sexually and asexually derived tillers of Anthoxanthum odoratum were planted directly in the field to test the hypothesis that competition among groups of sexual and asexual siblings favors the maintenance of sexual reproduction in populations. The results showed a substantial fitness advantage for sexual tillers. However, in contrast with the models, the advantage of sex did not increase with increasing numbers of colonists in the patch, there were multiple survivors among colonists, and an advantage was observed even for singly planted tillers. When a truncation-selection scheme was imposed ex post facto on the data, the relative performance of sexual tillers was similar to that predicted by the Bulmer (1980) model, suggesting that sib-competition models fail due to the violation of the assumption of truncation selection. The advantage of sex was not correlated with the presence of other species, total percentage cover, or species diversity, although sites where sex was favored were physically clustered.

THE EFFECT OF DISTANCE FROM THE PARENTAL SITE ON OFFSPRING PERFORMANCE AND INBREEDING DEPRESSION IN IMPATIENS CAPENSIS: A TEST OF THE LOCAL ADAPTATION HYPOTHESIS

If microgeographic variation in selection within a natural plant population has resulted in local adaptation, then offspring fitness should decline with distance from the parental site. If outcrossed progeny are less well-adapted to the parental environment than inbred progeny, but perform better in environments different from that of the parent, then the fitness of inbred progeny relative to outcrossed progeny should decrease with dispersal distance from the parent. To test these predictions, we collected seedlings at 10-m intervals from a 40 times 40-m permanent grid in a natural population of Impatiens capensis, grew them in a greenhouse, and crossed them to produce outcrossed chasmogamous seeds. Seedlings from outcrossed chasmogamous and self-fertilized cleistogamous seeds were planted back into the source population in the original site of their maternal parents and in arcs 3 and 12 m from the parental location and censused weekly for survival and reproduction. The fitness of inbred offspring declined significantly and the magnitude of observed inbreeding depression increased with distance from the parental site, supporting the local adaptation hypothesis.

Infection dynamics in coexisting sexual and asexual host populations: support for the Red Queen hypothesis

The persistence of sexual reproduction is a classic problem in evolutionary biology. The problem stems from the fact that, all else equal, asexual lineages should rapidly replace coexisting sexual individuals due to the cost of producing males in sexual populations. One possible countervailing advantage to sexual reproduction is that, on average, outcrossed offspring are more resistant than common clones to coevolving parasites, as predicted under the Red Queen hypothesis. In this study, we evaluated the prevalence of infection by a sterilizing trematode (Microphallus sp.) in a natural population of freshwater snails that was composed of both sexual and asexual individuals (Potamopyrgus antipodarum). More specifically, we compared the frequency of infection in sexual and asexual individuals over a 5-year period at four sites at a natural glacial lake (Lake Alexandrina, South Island, New Zealand). We found that at most sites and over most years, the sexual population was less infected than the coexisting asexual population. Moreover, the frequency of uninfected sexual females was periodically greater than two times the frequency of uninfected asexual females. These results give clear support for a fluctuating parasite-mediated advantage to sexual reproduction in a natural population.

The ecology of sexual reproduction

Sexual reproduction is widely regarded as one of the major unexplained phenomena in biology. Nonetheless, while a general answer may remain elusive, considerable progress has been made in the last few decades. Here, we first review the genesis of, and support for, the major ecological hypotheses for biparental sexual reproduction. We then focus on the idea that host-parasite coevolution can favour cross-fertilization over uniparental forms of reproduction, as this hypothesis currently has the most support from natural populations. We also review the results from experimental evolution studies, which tend to show that exposure to novel environments can select for higher levels of sexual reproduction, but that sex decreases in frequency after populations become adapted to the previously novel conditions. In contrast, experimental coevolution studies suggest that host-parasite interactions can lead to the long-term persistence of sex. Taken together, the evidence from natural populations and from laboratory experiments point to antagonistic coevolution as a potent and possibly ubiquitous force of selection favouring cross-fertilization and recombination.

The maintenance of sex: Ronald Fisher meets the Red Queen

Background

Sex in higher diploids carries a two-fold cost of males that should reduce its fitness relative to cloning, and result in its extinction. Instead, sex is widespread and clonal species face early obsolescence. One possible reason is that sex is an adaptation that allows organisms to respond more effectively to endless changes in their environment. The purpose of this study was to model mutation and selection in a diploid organism in an evolving environment and ascertain their support for sex.

Results

We used a computational approach to model finite populations where a haploid environment subjects a diploid host to endlessly evolving change. Evolution in both populations is primarily through adoption of novel advantageous mutations within a large allele space. Sex outcompetes cloning by two complementary mechanisms. First, sexual diploids adopt advantageous homozygous mutations more rapidly than clonal ones under conditions of lag load (the gap between the actual adaptation of the diploid population and its theoretical optimum). This rate advantage can offset the higher fecundity of cloning. Second, a relative advantage to sex emerges where populations are significantly polymorphic, because clonal polymorphism runs the risk of clonal interference caused by selection on numerous lines of similar adaptation. This interference extends allele lifetime and reduces the rate of adaptation. Sex abolishes the interference, making selection faster and elevating population fitness. Differences in adaptation between sexual and clonal populations increase markedly with the number of loci under selection, the rate of mutation in the host, and a rapidly evolving environment. Clonal interference in these circumstances leads to conditions where the greater fecundity of clones is unable to offset their poor adaptation. Sexual and clonal populations then either co-exist, or sex emerges as the more stable evolutionary strategy.

Conclusions

Sex can out-compete clones in a rapidly evolving environment, such as that characterized by pathogens, where clonal interference reduces the adaptation of clonal populations and clones adopt advantageous mutations more slowly. Since all organisms carry parasitic loads, the model is of potentially general applicability.

The evolution of sex and recombination

The evolution and maintenance of sexual reproduction, and the associated process of genetic recombination, are still controversial issues. Two recent books have provided overviews of the ideas and observations in this field. This article reviews some of the major ideas that have been proposed to account for sex and recombination, and comments on the results of attempts at empirical tests. While there is now an impressive body of well-formulated evolutionary models, it has proved hard to discriminate between them, either experimentally or by means of comparative data. It may well be that there is no unitary selective advantage to sex and recombination, but that a variety of forces are involved.

The evolution of sex is favoured during adaptation to new environments

Both theory and experiments have demonstrated that sex can facilitate adaptation, potentially yielding a group-level advantage to sex. However, it is unclear whether this process can help solve the more difficult problem of the maintenance of sex within populations. Using experimental populations of the facultatively sexual rotifer Brachionus calyciflorus, we show that rates of sex evolve to higher levels during adaptation but then decline as fitness plateaus. To assess the fitness consequences of genetic mixing, we directly compare the fitnesses of sexually and asexually derived genotypes that naturally occur in our experimental populations. Sexually derived genotypes are more fit than asexually derived genotypes when adaptive pressures are strong, but this pattern reverses as the pace of adaptation slows, matching the pattern of evolutionary change in the rate of sex. These fitness assays test the net effect of sex but cannot be used to disentangle whether selection on sex arises because highly sexual lineages become associated with different allele combinations or with different allele frequencies than less sexual lineages (i.e., "short-" or "long-term" effects, respectively). We infer which of these mechanisms provides an advantage to sex by performing additional manipulations to obtain fitness distributions of sexual and asexual progeny arrays from unbiased parents (rather than from naturally occurring, and thereby evolutionarily biased, parents). We find evidence that sex breaks down adaptive gene combinations, resulting in lower average fitness of sexual progeny (i.e., a short-term disadvantage to sex). As predicted by theory, the advantage to sex arises because sexually derived progeny are more variable in fitness, allowing for faster adaptation. This "long-term advantage" builds over multiple generations, eventually resulting in higher fitness of sexual types.

The effect of sex on the mean and variance of fitness in facultatively sexual rotifers

The evolution of sex is a classic problem in evolutionary biology. While this topic has been the focus of much theoretical work, there is a serious dearth of empirical data. A simple yet fundamental question is how sex affects the mean and variance in fitness. Despite its importance to the theory, this type of data is available for only a handful of taxa. Here, we report two experiments in which we measure the effect of sex on the mean and variance in fitness in the monogonont rotifer, Brachionus calyciflorus. Compared to asexually derived offspring, we find that sexual offspring have lower mean fitness and less genetic variance in fitness. These results indicate that, at least in the laboratory, there are both short- and long-term disadvantages associated with sexual reproduction. We briefly review the other available data and highlight the need for future work.

Additive and interactive effects of plant genotypic diversity on arthropod communities and plant fitness

Recent research suggests that genetic diversity in plant populations can shape the diversity and abundance of consumer communities. We tested this hypothesis in a field experiment by manipulating patches of Evening Primrose (Oenothera biennis) to contain one, four or eight plant genotypes. We then surveyed 92 species of naturally colonizing arthropods. Genetically diverse plant patches had 18% more arthropod species, and a greater abundance of omnivorous and predacious arthropods, but not herbivores, compared with monocultures. The effects of genotypic diversity on arthropod communities were due to a combination of interactive and additive effects among genotypes within genetically diverse patches. Greater genetic diversity also led to a selective feedback, as mean genotype fitness was 27% higher in diverse patches than in monocultures. A comparison between our results and the literature reveals that genetic diversity and species diversity can have similar qualitative and quantitative effects on arthropod communities. Our findings also illustrate the benefit of preserving genetic variation to conserve species diversity and interactions within multitrophic communities.

Evolution of sex: why do organisms shuffle their genotypes?

Sexual processes alter associations among alleles. To understand the evolution of sex, we need to know both the short-term and long-term consequences of changing these genetic associations. Ultimately, we need to identify which evolutionary forces--for example, selection, genetic drift, migration--are responsible for building the associations affected by sex.

The influence of temperature and host availability on the host exploitation strategies of sexual and asexual parasitic wasps of the same species

In the hymenopteran parasitoid Venturia canescens, asexual (obligate thelytoky not induced by Wolbachia bacteria) and sexual (arrhenotokous) wasps coexist in field conditions despite the demographic cost incurred due to the production of males by sexual females. Arrhenotoky predominates in field conditions, whereas populations in indoor conditions (mills, granaries) are exclusively thelytokous. These differences in the relative abundance of the two modes of reproduction between environments suggest that the individuals of each reproductive mode may have developed strategies adapted to the conditions prevailing in each kind of habitat. The two environments contrast in temperature variability and in the spatial heterogeneity of host availability. In this study, we considered the combined effect of temperature and host availability on host patch exploitation by thelytokous and arrhenotokous V. canescens. As expected, arrhenotokous females were more sensitive to temperature changes. If the temperature decreased before foraging, they remained longer and exploited patches more thoroughly. This is consistent with the expected behaviour of parasitoids in response to signs of unfavourable conditions that entail increasing risk of time limitation or a reduced probability of attaining further patches. Both arrhenotokous and thelytokous females increased patch exploitation with host availability. However, unexpectedly, we found no difference in the way the two types of wasp responded to differences in host availability. Differences in the strategies adopted under different environmental conditions may indicate divergence of niche-specific life history traits between the two modes of reproduction. Niche displacement may partly account for the coexistence of these two modes of reproduction at a geographical scale.

Opposites attract? Mate choice for parasite evasion and the evolutionary stability of sex

If sex is naturally selected as a way to combat parasites, then sexual selection for disease resistance might increase the overall strength of selection for outcrossing. In the present study, we compared how two forms of mate choice affect the evolutionary stability of outcrossing in simultaneous hermaphrodites. In the first form, individuals preferred to mate with uninfected individuals (condition-dependent choice). In the second form, individuals preferred to mate with individuals that shared the least number of alleles in common at disease-resistance loci. The comparisons were made using individual-based computer simulations in which we varied parasite virulence, parasite transmission rate, and the rate of deleterious mutation at 500 viability loci. We found that alleles controlling both forms of mate choice spread when rare, but their effects on the evolutionary stability of sex were markedly different. Surprisingly, condition-dependent choice for uninfected mates had little effect on the evolutionary stability of sexual reproduction. In contrast, active choice for mates having different alleles at disease-resistance loci had a pronounced positive effect, especially under low rates of deleterious mutation. Based on these results, we suggest that mate choice that increases the genetic diversity of offspring can spread when rare in a randomly mating population, and, as an indirect consequence, increase the range of conditions under which sexual reproduction is evolutionarily stable.

Viral pathogens and the advantage of sex in the perennial grass Anthoxanthum odoratum

The ubiquity of sexual reproduction among plants and animals remains one of the major unresolved paradoxes of modern evolutionary biology. In order for sex to be maintained in populations, sex must confer immediate and substantial fitness benefits. Theoreticians have proposed numerous mechanisms to explain how such advantages arise, but experimental data are few. In one well-studied population of the perennial grass Anthoxanthum odoratum in a mown North Carolina field, sexual offspring have been found to have significantly higher fitness than asexual offspring. More recent field experiments show that an aphid-transmitted virus, barley yellow dwarf (BYDV)-strain SGV, specifically transmitted by Schizaphus graminum , frequently infects Anthoxanthum progeny soon after transplantation into the field, BYDV infection is asymptomatic in Anthoxanthum , but BYDV-inoculated clones planted directly in the field had significantly lower fitness than healthy controls. Sexual females have been hypothesized to gain a fitness advantage for their offspring in the presence of pathogens either by providing ‘an escape in time’ from pathogens preadapted to the parental genotype or through the production of rare genotypes, which escape frequency-dependent infection. When parental clones and seed-derived sexual offspring were planted in identical but separate arrays in sites near where the parent was collected, parental clones were twice as frequently infected as sexual offspring. Factors other than genetic variation may have contributed to differences in levels of infection between sexual and asexual progeny: in this experiment, clonally derived asexual offspring tillers were slightly larger than seed-derived sexual tillers; in field experiments, larger plants were more frequently infected than smaller plants. When different families were planted into a common site, there was evidence that genotypes were less frequently infected when locally rare than when common. Taken together, the data suggest that BYDV infection generates advantages for rare or sexually produced genotypes in Anthoxanthum . The pattern of infection is likely to result from a complex interaction between vector, host, and viral genetics and population structure, vector behaviour, and host and vector dispersal patterns. Sexually produced genotypes appear to benefit because they are both novel and rare, but the observed minority advantage was weak. Other viral, bacterial, and fungal pathogens in this Anthoxanthum population may act as frequency-dependent selective forces in different places in the field, collectively generating the substantial and observed overall fitness advantage of rare genotypes. Further study is needed to elucidate their role. Nevertheless, the data do show that viral pathogens, which are often asymptomatic, play a significant evolutionary role in plant populations.

Sex, size, competition and escape-strategies of reproduction and dispersal in Lasallia pustulata (Umbilicariaceae, Ascomycetes)

The lichen Lasallia pustulata has a mixed strategy of asexual and sexual reproduction. Close-dispersed, asexual, symbiotic isidia are produced early, when the thalli are small. The asexual propagules are subsequently supplemented by far-dispersed, sexually generated ascospores when the thalli grow larger. This observation is consistent with evolutionary stable strategy (ESS) models of dispersal allocations in heterocarpic plants accordin to which the production of far-dispersed propagules should increase as clutch size and sibcompetition in the local habitat increases. The observation is also consistent with the "tangled bank" or "elbow room" hypothesis for the maintenance of sexuality, according to which sex, by generating genetic variation, represents an escape from competition in biologically saturated environments. Thus the advantage of sex is density dependent. L. pustulata grows in densely packed populations where intraspecific competition results in self-thining and the development of distinct sizehierarchies.

Mutation-selection balance and the evolutionary advantage of sex and recombination

Mutation-selection balance in a multi-locus system is investigated theoretically, using a modification of Bulmer's infinitesimal model of selection on a normally-distributed quantitative character, taking the number of mutations per individual (n) to represent the character value. The logarithm of the fitness of an individual with n mutations is assumed to be a quadratic, decreasing function of n. The equilibrium properties of infinitely large asexual populations, random-mating populations lacking genetic recombination, and random-mating populations with arbitrary recombination frequencies are investigated. With 'synergistic' epistasis on the scale of log fitness, such that log fitness declines more steeply as n increases, it is shown that equilibrium mean fitness is least for asexual populations. In sexual populations, mean fitness increases with the number of chromosomes and with the map length per chromosome. With 'diminishing returns' epistasis, such that log fitness declines less steeply as n increases, mean fitness behaves in the opposite way. Selection on asexual variants and genes affecting the rate of genetic recombination in random-mating populations was also studied. With synergistic epistasis, zero recombination always appears to be disfavoured, but free recombination is disfavoured when the mutation rate per genome is sufficiently small, leading to evolutionary stability of maps of intermediate length. With synergistic epistasis, an asexual mutant is unlikely to invade a sexual population if the mutation rate per diploid genome greatly exceeds unity. Recombination is selectively disadvantageous when there is diminishing returns epistasis. These results are compared with the results of previous theoretical studies of this problem, and with experimental data.

On the universality of human nature and the uniqueness of the individual: the role of genetics and adaptation

The concept of a universal human nature, based on a species-typical collection of complex psychological adaptations, is defended as valid, despite the existence of substantial genetic variation that makes each human genetically and biochemically unique. These apparently contradictory facts can be reconciled by considering that (a) complex adaptations necessarily require many genes to regulate their development, and (b) sexual recombination makes it improbable that all the necessary genes for a complex adaptation would be together at once in the same individual, if genes coding for complex adaptations varied substantially between individuals. Selection, interacting with sexual recombination, tends to impose relative uniformity at the functional level in complex adaptive designs, suggesting that most heritable psychological differences are not themselves likely to be complex psychological adaptations. Instead, they are mostly evolutionary by-products, such as concomitants of parasite-driven selection for biochemical individuality. An evolutionary approach to psychological variation reconceptualizes traits as either the output of species-typical, adaptively designed development and psychological mechanisms, or as the result of genetic noise creating perturbations in these mechanisms.

Deleterious mutations and the evolution of sexual reproduction

The origin and maintenance of sexual reproduction continues to be an important problem in evolutionary biology. If the deleterious mutation rate per genome per generation is greater than 1, then the greater efficiency of selection against these mutations in sexual populations may be responsible for the evolution of sex and related phenomena. In modern human populations detrimental mutations with small individual effects are probably accumulating faster than they are being eliminated by selection.