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PubMed1981

Models of speciation by sexual selection on polygenic traits.

The joint evolution of female mating preferences and secondary sexual characters of males is modeled for polygamous species in which males provide only genetic material to the next generation and females have many potential mates to choose among. Despite stabilizing natural selection on males, various types of mating preferences may create a runaway process in which the outcome of phenotypic evolution depends critically on the genetic variation parameters and initial conditions of a population. Even in the absence of genetic instability, rapid evolution can result from an interaction of natural and sexual selection with random genetic drift along lines of equilibria. The models elucidate genetic mechanisms that can initiate or contribute to rapid speciation by sexual isolation and divergence of secondary sexual characters.

Journal Article↗
PubMed1979

Simultaneous hermaphroditism and sexual selection.

Theory about the evolution of sexual behavior in dioecious species is based on the general assumption that egg production is limited by a female's ability to garner resources to make eggs, not by a lack of sperm to fertilize them. Reproductive success for males is thus limited by access to females (and their eggs). I suggest that egg production by simultaneous hermaphrodites also obeys this principle-that fertilized egg production by an individual is not limited by sperm availability, but by resources allocated to eggs. If true, this suggests that sperm competition (reproduction success through male function) and a form of male-female conflict have played important roles in the evolution of hermaphroditism.

Journal Article↗
PubMed1988

Sexual selection and canine dimorphism in New World monkeys.

Social and ecological factors are important in shaping sexual dimorphism in Anthropoidea, but there is also a tendency for body-size dimorphism and canine dimorphism to increase with increased body size (Rensch's rule) (Rensch: Evolution Above the Species Level. London: Methuen, 1959.) Most ecologist interpret Rensch's rule to be a consequence of social and ecological selective factors that covary with body size, but recent claims have been advanced that dimorphism is principally a consequence of selection for increased body size alone. Here we assess the effects of body size, body-size dimorphism, and social structure on canine dimorphism among platyrrhine monkeys. Platyrrhine species examined are classified into four behavioral groups reflecting the intensity of intermale competition for access to females or to limiting resources. As canine dimorphism increases, so does the level of intermale competition. Those species with monogamous and polyandrous social structures have the lowest canine dimorphism, while those with dominance rank hierarchies of males have the most canine dimorphism. Species with fission-fusion social structures and transitory intermale breeding-season competition fall between these extremes. Among platyrrhines there is a significant positive correlation between body size and canine dimorphism However, within levels of competition, no significant correlation was found between the two. Also, with increased body size, body-size dimorphism tends to increase, and this correlation holds in some cases within competition levels. In an analysis of covariance, once the level of intermale competition is controlled for, neither molar size nor molar-size dimorphism accounts for a significant part of the variance in canine dimorphism. A similar analysis using body weight as a measure of size and dimorphism yields a less clear-cut picture: body weight contributes significantly to the model when the effects of the other factors are controlled. Finally, in a model using head and body length as a measure of size and dimorphism, all factors and the interactions between them are significant. We conclude that intermale competition among platyrrhine species is the most important factor explaining variations in canine dimorphism. The significant effects of size and size dimorphism in some models may be evidence that natural (as opposed to sexual) selection also plays a role in the evolution of increased canine dimorphism.

Animals↗
PubMed2005

Sexual selection: the importance of long-term fitness measures.

New results from a 20-year study of free-living song sparrows confirm that attractive males contribute more offspring than less attractive males. They also reveal that the offspring of preferred males produce more descendents themselves. Females prefer males with a large song repertoire, which further work shows is a condition-dependent indicator of male quality.

Animals↗
PubMed2005

Sexual selection: copycat mating in birds.

Female zebra finches may be influenced by the choices of other females when selecting mates, challenging the view that mate-choice copying should not occur in species with biparental care.

Animals↗
PubMed1991

Parasites and sexual selection: a macroevolutionary perspective.

The Hamilton-Zuk hypothesis postulates a causal link between parasitism and the evolution of epigamic traits by intersexual selection. Oversimplified assumptions about basic parasite biology, ambiguous formulation of the hypothesis, and poor communication between ethologists and parasitologists have hampered its testing. The hypothesis is supported at the microevolutionary level if females show significant preference for lightly or uninfected males, if intensity of infection reflects host resistance to parasites that depress host fitness by causing disease, and if intensity of infection is related to the degree of epigamic development. It must be shown that particular parasites cause disease, that the host population is polymorphic for resistance to infection by those species, and that female hosts are capable of distinguishing male hosts with low parasite loads due to heritable aspects of host resistance from males that are uninfected due to chance. The macroevolutionary prediction of the hypothesis, that species displaying strongly developed epigamic characters should host "more parasites" than species with weakly developed epigamic traits, contradicts the microevolutionary dynamic of the hypothesis, and is too ambiguous. We propose a macroevolutionary prediction based on understanding the evolutionary origin of epigamic traits and the evolutionary origin of each host-parasite association. Associations originating in the ancestor in which the epigamic trait appeared corroborate the hypothesis most strongly; those originating prior to the evolution of the epigamic trait corroborate it weakly; those beginning after the origin of the epigamic trait could not have been involved in the origin and spread of the epigamic trait.

Animals↗
PubMed2004

The evolution of premating isolation: local adaptation and natural and sexual selection against hybrids.

Although reinforcement is ostensibly driven by selection against hybrids, there are often other components in empirical cases and theoretical models of reinforcement that may contribute to premating isolation. One of these components is local adaptation of a trait used in mate choice. I use several different comparisons to assess the roles that local adaptation and selection against hybrids may play in reinforcement models. Both numerical simulations of exact recursion equations and analytical weak selection approximations are employed. I find that selection against hybrids may play a small role in driving preference evolution in a reinforcement model where the mating cue is separate from loci causing hybrid incompatibilities. When females have preferences directly for purebreds of their own population, however, selection against hybrids can play a large role in premating isolation evolution. I present some situations in which this type of selection is likely to exist. This work also illustrates shortfalls of using a weak selection approach to address questions about reinforcement.

Adaptation, Biological↗
PubMed2004

Schizophrenia as one extreme of a sexually selected fitness indicator.

Schizophrenia remains an evolutionary paradox. Its delusions, hallucinations and other symptoms begin in adolescence or early adulthood and so devastate sexual relationships and reproductive success that selection should have eliminated the disorder long ago. Yet it persists as a moderately heritable disorder at a global 1% prevalence--too high for new mutations at a few genetic loci. We suggest that schizophrenia persists and involves many loci because it is the unattractive, low-fitness extreme of a highly variable mental trait that evolved as a fitness ("good genes") indicator through mutual mate choice. Here we show that this hypothesis explains many key features of schizophrenia and predicts that some families carry modifier alleles that increase the indicator's neurodevelopmental sensitivity to heritable fitness and condition. Such alleles increase the extent to which high-fitness family members develop impressive courtship abilities and achieve high reproductive success, but also increase the extent to which low-fitness family members develop schizophrenia. Here we introduce this fitness indicator model of schizophrenia, discuss its explanatory power, explain how it resolves the evolutionary paradox, discuss its implications for gene hunting, and identify some empirically testable predictions as directions for further research.

Biological Evolution↗