Multihost parasites can infect different types of hosts or even different host species. Epidemiological models have shown the importance of the diversity of potential hosts for understanding the dynamics of infectious disease (e.g., the importance of reservoirs), but the consequences of this diversity for virulence and transmission evolution remain largely overlooked. Here, I present a general theoretical framework for the study of life-history evolution of multihost parasites. This analysis highlights the importance of epidemiology (the relative quality and quantity of different types of infected hosts) and between-trait constraints (both within and between different hosts) to parasite evolution. I illustrate these effects in different transmission scenarios under the simplifying assumption that parasites can infect only two types of hosts. These simple but contrasted evolutionary scenarios yield new insights into virulence evolution and the evolution of transmission routes among different hosts. Because many of the pathogens that have large public-health and agricultural impacts have complex life cycles, an understanding of their evolutionary dynamics could hold substantial benefits for management.

The spread of genes and individuals through space in populations is relevant in many biological contexts. I study, via systems of reaction-diffusion equations, the spatial spread of advantageous alleles through structured populations. The results show that the temporally asymptotic rate of spread of an advantageous allele, a kind of invasion speed, can be approximated for a class of linear partial differential equations via a relatively simple formula, c = 2rD̄, that is reminiscent of a classic formula attributed to R. A. Fisher. The parameters r and D̄ represent an asymptotic growth rate and an average diffusion rate, respectively, and can be interpreted in terms of eigenvalues and eigenvectors that depend on the population's demographic structure. The results can be applied, under certain conditions, to a wide class of nonlinear partial differential equations that are relevant to a variety of ecological and evolutionary scenarios in population biology. I illustrate the approach for computing invasion speed with three examples that allow for heterogeneous dispersal rates among different classes of individuals within model populations.

The advent of multiple regression analyses of natural selection has facilitated estimates of both the direct and indirect effects of selection on many traits in numerous organisms. However, low power in selection studies has possibly led to a bias in our assessment of the levels of selection shaping natural populations. Using calculations and simulations based on the statistical properties of selection coefficients, we find that power to detect total selection (the selection differential) depends on sample size and the strength of selection relative to the opportunity of selection. The power of detecting direct selection (selection gradients) is more complicated and depends on the relationship between the correlation of each trait and fitness and the pattern of correlation among traits. In a review of 298 previously published selection differentials, we find that most studies have had insufficient power to detect reported levels of selection acting on traits and that, in general, the power of detecting weak levels of selection is low given current study designs. We also find that potential publication bias could explain the trend that reported levels of direct selection tend to decrease as study sizes increase, suggesting that current views of the strength of selection may be inaccurate and biased upward. We suggest that studies should be designed so that selection is analyzed on at least several hundred individuals, the total opportunity of selection be considered along with the pattern of selection on individual traits, and nonsignificant results be actively reported combined with an estimate of power.

Where the evolution of a trait is affected by selection at more than one hierarchical level, it is often useful to compare the magnitude of selection at each level by asking how much of the total evolutionary change is attributable to each level of selection. Three statistical partitioning techniques, each designed to answer this question, are compared, in relation to a simple multilevel selection model in which a trait's evolution is affected by both individual and group selection. None of the three techniques is wholly satisfactory: one implies that group selection can operate even if individual fitness is determined by individual phenotype alone, whereas the other two imply that group selection can operate even if there is no variance in group fitness. This has significant implications both for our understanding of what the term “multilevel selection” means and for the traditional concept of group selection.

Like many phenotypic traits, biomechanical systems are defined by both an underlying morphology and an emergent functional property. The relationship between these levels may have a profound impact on how selection for functional performance is translated into morphological evolution. In particular, complex mechanical systems are likely to be highly redundant, because many alternative morphologies yield equivalent functions. We suggest that this redundancy weakens the relationship between morphological and functional diversity, and we illustrate this effect using an evolutionary model of the four-bar lever system of labrid fishes. Our results demonstrate that, when traits are complex, the morphological diversity of a clade may only weakly predict its mechanical diversity. Furthermore, parallel or convergent selection on function does not necessarily produce convergence in morphology. Empirical observations suggest that this weak form-function relationship has contributed to the morphological diversity of labrid fishes, as functionally equivalent species may nevertheless possess morphologically distinct jaws. We suggest that partial decoupling of morphology and mechanics due to redundancy is a major factor in morphological diversification.

Because low developmental stability may compromise the precision with which adaptations can be reached, the variability and genetic basis of developmental stability are important evolutionary parameters. Developmental stability is also an important clue to understanding how traits are regulated to achieve their phenotypic target value. However, developmental stability must be studied indirectly through proxy variables, such as fluctuating asymmetry, that are suggested to have noisy and often nonlinear relationships to the underlying variable of interest. In this paper we first show that mean-standardized measures of variance and covariance in fluctuating asymmetry, unlike heritabilities, repeatabilities, and correlations, are linearly related to corresponding measures of variation in underlying developmental stability. We then examine the variational properties of developmental stability in a population of the Neotropical vine, Dalechampia scandens (Euphorbiaceae). By studying fluctuating asymmetry in a large number of floral characters in both selfed and outcrossed individuals in a diallel design, we assemble strong evidence that both additive genetic and individual variation and covariation in developmental stability are virtually absent in this population.

Phototropins are blue-light photoreceptor molecules mediating the capacity for phototropism or bending toward or away from directional light. Like the red-light sensing phytochromes that control shade avoidance, phototropins modulate developmental plasticity in plant architecture. Yet, unlike phytochromes, the adaptive significance of phototropins has been largely a topic of conjecture. In Arabidopsis thaliana, phototropism of seedling and plant stems is under the control of two paralogous genes, PHOT1 and PHOT2, that encode different phototropins with partially redundant light response qualities. The PHOT1 gene product interacts with the NPH3 gene product to cause phototropic bending over a broad range of light intensity, from very weak light in the soil to stronger light in the aerial environment. The PHOT2 gene product modulates shoot bending in response to light of higher intensity only. We compared the fitness of wild-type, phot1, phot2, and nph3 genotypes over a range of light conditions in the field. Seeds were sown in the field on the soil surface and left bare or covered with either gravel or bark mulch chips. Plantings were made under full sun and dense canopy cover. Rates of seedling emergence, survival to flowering, and total seed set were measured. All mutant genotypes had significantly reduced lifetime fitness compared to wild-type. Consistent with their different fluence rate sensitivities, phot1 and phot2 signaling pathways affected fitness at discrete life-cycle stages. Fitness costs of phot1 and nph3 were expressed mainly during seedling emergence from the soil whereas that of phot2 was expressed solely after emergence. Surprisingly, the only significant genotype-by-environment interaction for fitness occurred during emergence: genotypes blind to dim blue light (phot1 and nph3) had poor emergence in the open, but not in the shade. Possibly, the loss of negative phototropism in seedling roots of mutant genotypes reduced establishment success in open (dry soil) conditions. Results show that phototropin-modulated pathways are adaptive and that their evolution has involved functional specialization. However, mechanism(s) of selection on these pathways remain a mystery.

Recent work on a diverse array of echinoderm species has demonstrated, as is true in amphibians, that thyroid hormone (TH) accelerates development to metamorphosis. Interestingly, the feeding larvae of several species of sea urchins seem to obtain TH through their diet of planktonic algae (exogenous source), whereas nonfeeding larvae of the sand dollar Peronella japonica produce TH themselves (endogenous source). Here we examine the effects of TH (thyroxine) and a TH synthesis inhibitor (thiourea) on the development of Dendraster excentricus, a sand dollar with a feeding larva. We report reduced larval skeleton lengths and more rapid development of the juvenile rudiment in the exogenous TH treatments when compared to controls. Also, larvae treated with exogenous TH reached metamorphic competence faster at a significantly reduced juvenile size, representing the greatest reduction in juvenile size ever reported for an echinoid species with feeding larvae. These effects of TH on D. excentricus larval development are strikingly similar to the phenotypically plastic response of D. excentricus larvae reared under high food conditions. We hypothesize that exogenous (algae-derived) TH is the plasticity cue in echinoid larvae, and that the larvae use ingested TH levels as an indicator for larval nutrition, ultimately signaling the attainment of metamorphic competence. Furthermore, our experiments with the TH synthesis inhibitor thiourea indicate that D. excentricus larvae can produce some TH endogenously. Endogenous TH production might, therefore, be a shared feature among sand dollars, facilitating the evolution of nonfeeding larval development in that group. Mounting evidence on the effects of thyroid hormones in echinoderm development suggests life-history models need to incorporate metamorphic hormone effects and the evolution of metamorphic hormone production.

Sympatric populations can diverge when variation in phenology or life cycle causes them to mate at distinctly different times. We report patterns consistent with this process (allochronic speciation) in North American gall-forming aphids, in the absence of a host or habitat shift. Pemphigus populi-transversus Riley and P. obesinymphae Aoki form a monophyletic clade within the North American Pemphigus group. They are sympatric on the eastern cottonwood, Populus deltoides (Salicaceae), but have distinctly different life cycles, with sexual stages offset by approximately six months. Field evidence indicates that intermediate phenotypes do not commonly occur, and mitochondrial and bacterial endosymbiont DNA sequences show no maternal gene flow between the two species. Because a genetically distinct population of P. obesinymphae occurs in the southwestern United States on Populus fremontii, we consider the possibility of an initial allopatric phase in the divergence. We discuss the likely origins of the host use patterns in P. obesinymphae, and the larger sequence of evolutionary changes that likely led to the sympatric divergence of P. populi-transversus and P. obesinymphae. A plausible interpretation at this stage of investigation is that a shift in timing of the life cycle in an ancestral population, correlated with an underlying phenological complexity in its host plant, spurred divergence between the incipient species.

We investigate the evolution of host association in a cryptic complex of mutualistic Crematogaster (Decacrema) ants that inhabits and defends Macaranga trees in Southeast Asia. Previous phylogenetic studies based on limited samplings of Decacrema present conflicting reconstructions of the evolutionary history of the association, inferring both cospeciation and the predominance of host shifts. We use cytochrome oxidase I (COI) to reconstruct phylogenetic relationships in a comprehensive sampling of the Decacrema inhabitants of Macaranga. Using a published Macaranga phylogeny, we test whether the ants and plants have cospeciated. The COI phylogeny reveals 10 well- supported lineages and an absence of cospeciation. Host shifts, however, have been constrained by stem traits that are themselves correlated with Macaranga phylogeny. Earlier lineages of Decacrema exclusively inhabit waxy stems, a basal state in the Pachystemon clade within Macaranga, whereas younger species of Pachystemon, characterized by nonwaxy stems, are inhabited only by younger lineages of Decacrema. Despite the absence of cospeciation, the correlated succession of stem texture in both phylogenies suggests that Decacrema and Pachystemon have diversified in association, or codiversified. Subsequent to the colonization of the Pachystemon clade, Decacrema expanded onto a second clade within Macaranga, inducing the development of myrmecophytism in the Pruinosae group. Confinement to the aseasonal wet climate zone of western Malesia suggests myrmecophytic Macaranga are no older than the wet forest community in Southeast Asia, estimated to be about 20 million years old (early Miocene). Our calculation of COI divergence rates from several published arthropod studies that relied on tenable calibrations indicates a generally conserved rate of approximately 1.5% per million years. Applying this rate to a rate-smoothed Bayesian chronogram of the ants, the Decacrema from Macaranga are inferred to be at least 12 million years old (mid-Miocene). However, using the extremes of rate variation in COI produces an age as recent as 6 million years. Our inferred timeline based on 1.5% per million years concurs with independent biogeographical events in the region reconstructed from palynological data, thus suggesting that the evolutionary histories of Decacrema and their Pachystemon hosts have been contemporaneous since the mid-Miocene. The evolution of myrmecophytism enabled Macaranga to radiate into enemy- free space, while the ants' diversification has been shaped by stem traits, host specialization, and geographic factors. We discuss the possibility that the ancient and exclusive association between Decacrema and Macaranga was facilitated by an impoverished diversity of myrmecophytes and phytoecious (obligately plant inhabiting) ants in the region.

Sexual communication can contribute to population divergence and speciation because of its effect on assortative mating. We examined the role of communication in assortative mating in the Enchenopa binotata species complex of treehoppers. These plant-feeding insects are a well studied case of sympatric speciation resulting from shifts to novel host-plant species. Shifting to hosts with different phenologies causes changes in life-history timing. In concert with high host fidelity, these changes reduce gene flow between populations on ancestral and novel hosts and facilitate a rapid response to divergent natural selection. However, some interbreeding can still occur because of partial overlap of mating periods. Additional behavioral mechanisms resulting in reproductive isolation may thus be important for divergence. In E. binotata, mating pairs form after an exchange of plant-borne vibrational signals. We used playback experiments to examine the relevance of inter- and intraspecific variation in male advertisement signals for female mate choice in a member of the E. binotata species complex. Female signals given in response to male signals provided a simple and reliable assay. Male species and male individual identity were important determinants of female responses. Females failed to respond to the signals of the two most closely related species in the complex, but they responded strongly to the signals of conspecific males, as well as to those of the most basal species in the complex. Communication systems in the E. binotata species complex can therefore play a role in reproductive isolation. Female responses were influenced by among-individual variation in male signals and females, suggesting the involvement of sexual selection in the evolution of these communication systems.

Adaptations conferring resistance to xenobiotics (antibiotics, insecticides, herbicides, etc.) are often costly to the organism's fitness in the absence of the selecting agent. In such conditions, and unless other mutations compensate for the costs of resistance, sensitive individuals are expected to out-reproduce resistant individuals and drive resistance alleles to a low frequency, with the rate and magnitude of this decline being proportional to the costs of resistance. However, this evolutionary dynamic is open to modification by other sources of selection acting on the relative fitness of susceptible and resistant individuals. Here we show parasitism not only as a source of selection capable of modifying the costs of organophosphate insecticide resistance in mosquitoes, but also that qualitatively different interactions (increasing or decreasing the relative fitness of resistant individuals) occurred depending on the particular form of resistance involved. As estimates of the parasite's fitness also varied according to its host's form of resistance, our data illustrate the potential for epidemiological feedbacks to influence the strength and direction of selection acting on resistance mutations in untreated environments.

Drosophila falleni belongs to the quinaria species group, whose species vary considerably in patterns of wing and abdominal pigmentation. Drosophila falleni itself exhibits substantial variation among wild flies in abdominal spotting patterns. A selection experiment revealed that natural populations of D. falleni harbor high levels of genetic variation for spot number; in 10 generations of selection modal spot number within populations declined from 18 (the modal number in wild-caught females) to as low as zero. Rearing flies at different temperatures shows that some of the variation among wild flies is likely to reflect variation in the environmental conditions under which they developed. Fitness assays did not reveal any cost of reduced spot number with respect to development time, adult survival, or female fecundity. However, spotless flies were almost twice as susceptible to infection by the nematode parasite Howardula aoronymphium. Thus, selection exerted by nematode parasites may influence pigmentation patterns and other, genetically correlated traits in natural populations D. falleni.

Results of intrapopulation studies of sexual selection and genetic variation and covariation underlying elements of the sex comb of Drosophila bipectinata are presented. The magnitude of the sex comb, a sexual ornament, varies significantly among Australasian populations, motivating research into the evolutionary mechanisms responsible for its incipient diversification. The comb is composed of stout black teeth on the front legs of males arranged in three distinct segments: C1, C2, and C3. Significant sexual selection in field populations in northeastern Queensland, Australia, was detected for increasing C2 and body size, and simultaneously for reducing comb positional fluctuating asymmetry. In contrast, sexual selection was not detected for other comb segments, nor for sternopleural bristle number or symmetry. Selection intensities for C2 and comb positional fluctuating asymmetry were similar in magnitude, and although they were opposite in sign, values across twelve sampling dates, or selection episodes, were uncorrelated. Heritability estimates for C2 were high and significant across years, whereas heritability estimates for comb positional asymmetry were small, and generally nonsignificant. The major sex comb segments (C1 and C2) were significantly and positively correlated genetically, indicating the potential for correlated evolution of these components of the comb under sexual selection. The original finding of a significant positive genetic correlation between the magnitude of this sex trait and its positional asymmetry indicates that the counteracting and independent selection pressures detected could contribute to the maintenance of genetic variation sustaining sexual selection. The study documents the simultaneous presence of sexual selection in nature and of heritable genetic variation underlying expression of the sex comb, fundamental conditions necessary for its adaptive diversification. Drosophila bipectinata may be a valuable model for studies of adaptive diversification and incipient speciation by sexual selection.

Theory suggests that frequency-dependent resource competition will disproportionately impact the most common phenotypes in a population. The resulting disruptive selection forms the driving force behind evolutionary models of niche diversification, character release, ecological sexual dimorphism, resource polymorphism, and sympatric speciation. However, there is little empirical support for the idea that intraspecific competition generates disruptive selection. This paper presents a test of this theory, using natural populations of the three-spine stickleback, Gasterosteus aculeatus. Sticklebacks exhibit substantial individual specialization associated with phenotypic variation and so are likely to experience frequency-dependent competition and hence disruptive selection. Using body size and relative gonad mass as indirect measures of potential fecundity and hence fitness, I show that an important aspect of trophic morphology, gill raker length, is subject to disruptive selection in one of two natural lake populations. To test whether this apparent disruptive selection could have been caused by competition, I manipulated population densities in pairs of large enclosures in each of five lakes. In each lake I removed fish from one enclosure and added them to the other to create paired low- and high-population-density treatments with natural phenotype distributions. Again using indirect measures of fitness, disruptive selection was consistently stronger in high-density than low-density enclosures. These results support long-standing theoretical arguments that intraspecific competition drives disruptive selection and thus may be an important causal agent in the evolution of ecological variation.

Over the past 15 years, phylogenetic comparative methods (PCMs) have become standard in the study of life-history evolution. To date, most studies have focused on variation among species or higher taxonomic levels, generally revealing the presence of significant phylogenetic effects as well as residual variation potentially attributable to adaptive evolution. Recently, population-level phylogenetic hypotheses have become available for many species, making it possible to apply PCMs directly to the level at which experiments are typically used to test adaptive hypotheses. In this study, we present the results of PCMs applied to life-history variation among populations of the widespread and well-studied lizard Sceloporus undulatus. Using S. undulatus (which may represent four closely related species) as an example, we explore the benefits of using PCMs at the population level, as well as consider the importance of several thorny methodological problems including but not limited to nonindependence of populations, lack of sufficient variation in traits, and the typically small sample sizes dictated by the difficulty of collecting detailed demographic data. We show that phylogenetic effects on life-history variation among populations of S. undulatus appear to be unimportant, and that several classic trade-offs expected by theory and revealed by many interspecific comparisons are absent. Our results suggest that PCMs applied to variation in life-history traits below the species level may be of limited value, but more studies like ours are needed to draw a general conclusion. Finally, we discuss several outstanding problems that face studies seeking to apply PCMs below the species level.

Despite great interest in sexual selection, relatively little is known in detail about the genetic and environmental determinants of secondary sexual characters in natural populations. Such information is important for determining the way in which populations may respond to sexual selection. We report analyses of genetic and large- scale environmental components of phenotypic variation of two secondary sexual plumage characters (forehead and wing patch size) in the collared flycatcher Ficedula albicollis over a 22-year period. We found significant heritability for both characters but little genetic covariance between the two. We found a positive association between forehead patch size and a large-scale climatic index, the North Atlantic Oscillation (NAO) index, but not for wing patch. This pattern was observed in both cross-sectional and longitudinal data suggesting that the population response to NAO index can be explained as the result of phenotypic plasticity. Heritability of forehead patch size for old males, calculated under favorable conditions (NAO index ≥ median), was greater than that under unfavorable conditions (NAO index < median). These changes occurred because there were opposing changes in additive genetic variance (V_A) and residual variance (V_R) under favorable and unfavorable conditions, with V_A increasing and V_R decreasing in good environments. However, no such effect was detected for young birds, or for wing patch size in either age class. In addition to these environmental effects on both phenotypic and genetic variances, we found evidence for a significant decrease of forehead patch size over time in older birds. This change appears to be caused by a change in the sign of viability selection on forehead patch size, which is associated with a decline in the breeding value of multiple breeders. Our data thus reveal complex patterns of environmental influence on the expression of secondary sexual characters, which may have important implications for understanding selection and evolution of these characters.

One of the main tenets of modern life-history theory is the negative relationship (trade-off) between the number and quality of offspring produced. Theory predicts a negative genetic correlation between these traits since both are closely related to fitness of individuals. However, the genetic basis of the trade-off has only been tested to a limited extent in natural populations. We examined whether size and quality of offspring are negatively related to litter size in the bank vole Clethrionomys glareolus. First, we found a significant negative phenotypic correlation between the number and size of offspring at birth in both laboratory and field populations of the bank vole. Second, a larger size at birth decreased the maturation age of female offspring in the laboratory, and increased the probability of breeding and the size of the first litter in the field. Furthermore, manipulation of offspring size at weaning indicated that structural effects of birth size in mammals have a more profound effect on the expression of life-history traits than weaning size. Finally, in addition to the phenotypic negative correlation between the number and size of offspring, we found evidence for a negative genetic correlation between these two traits, which confirms the genetic basis of the trade-off. This negative genetic covariation may have considerable effects on the rate and direction of evolution of the two related life-historical traits.

The Bateman principle, which holds that oocytes are the limiting gamete in reproduction, is violated in a variety of species. Self-fertilizing hermaphrodites of the nematode Caenorhabditis elegans provide an example of a system in which sperm number limits lifetime reproductive output, in this species due to the protandrous nature of sperm production that in turn delays the onset of fertilization. This reproductive delay forms the basis of a trade-off between generation time and total fecundity, in which sperm number plays a pivotal role. I use an age-structured population model to describe the number of sperm that maximize fitness, given larval development time and rates of gamete production. The model predicts the evolution of sperm numbers that are consistent with empirical data for C. elegans provided that precocious larval sperm production is taken into account. Several testable hypotheses follow from the model regarding how natural selection and environmental variation may influence patterns of sperm production among populations or species with a similar mode of reproduction.

Genes may acquire nonsynonymous substitutions more rapidly when X-linked than when autosomal, but evidence for “fast-X evolution” has been elusive. Fast-X evolution could explain the disproportionate contribution of X-linked genes to hybrid sterility and other traits. Here, we use a comparative genomic approach, with sequences of 30–110 genes in four Drosophila species, to test for fast-X evolution. Specifically, the 3L autosome arm in D. melanogaster and D. simulans is homologous to the right arm of the X chromosome in D. pseudoobscura and D. miranda. We executed two paired comparisons to determine how often genes on this chromosome arm exhibit higher rates of nonsynonymous substitution in the D. pseudoobscura species group, as predicted by fast-X evolution. We found a statistically significant pattern consistent with fast-X evolution in one comparison and a similar trend in the other comparison. Variation in functional constraints across genes may have masked the signature of fast-X evolution in some previous studies, and we conclude paired comparisons are more powerful for examining rates of evolution of genes when X-linked over autosomal.

Juvenile growth is submaximal in many species, suggesting that a trade-off with juvenile growth must exist. In support of this, recent studies have demonstrated that rapid growth early in life results in decreased physiological performance. Theory clearly shows that for submaximal growth in juveniles to be optimal, the cost of growth must be nonlinear. However, nearly all of the empirical evidence for costs of growth comes from linear comparisons between fast- and slow-growing groups. It is consequently unclear whether any known cost can account for the evolution of submaximal juvenile growth. To test whether the cost of growth exhibits the logically necessary nonlinearity, we measured critical swimming speed (U_crit), the maximum speed sustained in incremental velocity trials, in Atlantic silversides, a species for which the costs and benefits of growth are well studied. To increase our ability to detect a nonlinear relationship between U_crit, a proxy for juvenile fitness, and growth, we manipulated ration levels to produce a broad range of growth rates (0.16 mm/day⁻¹ to 1.20 mm/day⁻¹). Controlling for size and age, we found that U_crit decreased precipitously as growth approached the physiological maximum. Using Akaike's information criterion, we show that swimming performance decreases with the square of growth rate, providing the first demonstration of a nonlinear cost of growth.

Cryptic female choice (CFC) refers to female-mediated processes occurring during or after copulation that result in biased sperm use in favor of preferred or compatible males. Despite recent empirical support for this hypothesis, evidence that CFC contributes towards the evolution of male body ornaments, in the same way that precopulatory female choice does, is currently lacking. Here, we tested the possibility that CFC selects for increased male attractiveness in the guppy Poecilia reticulata, a freshwater fish exhibiting internal fertilization. Specifically, we examined whether females are able to manipulate the number of sperm transferred or retained at copulation in favor of relatively attractive males. In support of this prediction, we found that following solicited copulations the number of sperm inseminated is influenced exclusively by the female's perception of relative male coloration, independent of any direct manipulation of males themselves. Because females prefer brightly colored males during precopulatory mate choice, our finding that colorful males are also favored as a consequence of enhanced insemination success indicates that cryptic female choice can reinforce precopulatory preferences for extravagant male ornaments.

Genetic divergence among conspecific subpopulations can be due to either low recurrent gene flow or recent divergence and no gene flow. Here we present a modification of an earlier method developed by Nielsen and Wakeley (2001), which accommodates a finite-site mutation model, to assess which of the two models of divergence is most likely given the observed data. We apply the method to nucleotide sequence data collected from the variable part of the mitochondrial control region in fin whales (Balaenoptera physalus) from the Atlantic coast off Spain and the Mediterranean Sea. Our estimations strongly favor a model of recurrent gene flow over a model of recent divergence and zero gene flow. We estimated the migration rate at two females per generation. While the estimated rate is high by evolutionary standards, exchange rates of this order of magnitude is low from an ecological and conservation perspective and entirely consistent with the current paucity of fin whale sightings in the Strait of Gibraltar today. Intensive commercial shore-based whaling during the 1920s removed substantial numbers of fin whales in the Strait of Gibraltar and this local population has seemingly since failed to recover.