Showing papers in "Evolution in 1999"

Perspective: highly variable loci and their interpretation in evolution and conservation.

Abstract: Although highly variable loci, such as microsatellite loci, are revolutionizing both evolutionary and conservation biology, data from these loci need to be carefully evaluated. First, because these loci often have very high within-population heterozygosity, the magnitude of differentiation measures may be quite small. For example, maximum GST values for populations with no common alleles at highly variable loci may be small and are at maximum less than the average within-population homozygosity. As a result, measures that are variation independent are recommended for highly variable loci. Second, bottlenecks or a reduction in population size can generate large genetic distances in a short time for these loci. In this case, the genetic distance may be corrected for low variation in a population and tests to detect bottlenecks are advised. Third, statistically significant differences may not reflect biologically meaningful differences both because the patterns of adaptive loci may not be correlated with highly variable loci and statistical power with these markers is so high. As an example of this latter effect, the statistical power to detect a one-generation bottleneck of different sizes for different numbers of highly variable loci is discussed. All of these concerns need to be incorporated in the utilization and interpretation of patterns of highly variable loci for both evolutionary and conservation biology.

Correlation of pairwise genetic and geographic distance measures: inferring the relative influences of gene flow and drift on the distribution of genetic variability

Abstract: Attempts to relate estimates of regional FST to gene flow and drift via Wright's (1931) equation FST ≈ 1/ (4Nm + 1) are often inappropriate because most natural sets of populations probably are not at equilibrium (McCauley 1993), as assumed by the island model upon which the equation is based, or ineffective because the influences of gene flow and drift are confounded in the product Nm. Evaluations of the association between genetic (FST ) and geographic distances separating all pairwise populations combinations in a region allows one to test for regional equilibrium, to evaluate the relative influences of gene flow and drift on population structure both within and between regions, and to visualize the behavior of the association across all degrees of geographic separation. Tests of the model using microsatellite data from 51 populations of eastern collared lizards (Crotaphytus collaris collaris) collected from four distinct geographical regions gave results highly consistent with predicted patterns of association based on regional differences in various historical and ecological factors that affect the amount of drift and gene flow. The model provides a prerequisite for and an alternative to regional FST analyses, which often simply assume regional equilibrium, thus potentially leading to erroneous and misleading inferences regarding regional population structure.

Perspective: the pace of modern life: measuring rates of contemporary microevolution.

Abstract: We evaluate methods for measuring and specifying rates of microevolution in the wild, with particular regard to studies of contemporary, often deemed "rapid," evolution. A considerable amount of ambiguity and inconsistency persists within the field, and we provide a number of suggestions that should improve study design, inference, and clarity of presentation. (1) Some studies measure change over time within a population (allochronic) and others measure the difference between two populations that had a common ancestor in the past (synchronic). Allochronic studies can be used to estimate rates of "evolution," whereas synchronic studies more appropriately estimate rates of "divergence." Rates of divergence may range from a small fraction to many times the actual evolutionary rates in the component populations. (2) Some studies measure change using individuals captured from the wild, whereas others measure differences after rearing in a common environment. The first type of study can be used to specify "phenotypic" rates and the later "genetic" rates. (3) The most commonly used evolutionary rate metric, the darwin, has a number of theoretical shortcomings. Studies of microevolution would benefit from specifying rates in standard deviations per generation, the haldane. (4) Evolutionary rates are typically specified without an indication of their precision. Readily available methods for specifying confidence intervals and statistical significance (regression, bootstrapping, randomization) should be implemented. (5) Microevolutionists should strive to accumulate time series, which can reveal temporal shifts in the rate of evolution and can be used to identify evolutionary patterns. (6) Evolutionary rates provide a convenient way to compare the tempo of evolution across studies, traits, taxa, and time scales, but such comparisons are subject to varying degrees of confidence. Comparisons across different time scales are particularly tenuous. (7) A number of multivariate rate measures exist, but considerable theoretical development is required before their utility can be determined. We encourage the continued investigation of evolutionary rates because the information they provide is relevant to a wide range of theoretical and practical issues.

Parasite-mediated selection against inbred Soay sheep in a free-living, island population

Abstract: Parasites are thought to provide a selective force capable of promoting genetic variation in natural populations. One rarely considered pathway for this action is via parasite-mediated selection against inbreeding. If parasites impose a fitness cost on their host and the offspring of close relatives have greater susceptibility to parasites due to the increased homozygosity that results from inbreeding, then parasite-mediated mortality may select against inbred individuals. This hypothesis has not yet been tested within a natural vertebrate population. Here we show that relatively inbred Soay sheep (Ovis aries), as assessed by microsatellite heterozygosity, are more susceptible to parasitism by gastrointestinal nematodes, with interactions indicating greatest susceptibility among adult sheep at high population density. During periods of high overwinter mortality on the island of Hirta, St. Kilda, Scotland, highly parasitised individuals were less likely to survive. More inbred individuals were also less likely to survive, which is due to their increased susceptibility to parasitism, because survival was random with respect to inbreeding among sheep that were experimentally cleared of their gastrointestinal parasite burden by anthelminthic treatment. As a consequence of this selection, average microsatellite heterozygosity increases with age in St. Kildan Soay sheep. We suggest that parasite-mediated selection acts to maintain genetic variation in this small island population by removing less heterozygous individuals.

Ecological speciation in sticklebacks: environment-dependent hybrid fitness

Abstract: "Ecological" speciation occurs when reproductive isolation evolves as a consequence of divergent selection between populations exploiting different resources or environments. We tested this hypothesis of speciation in a young stickleback species pair by measuring the direct contribution of ecological selection pressures to hybrid fitness. The two species (limnetic and benthic) are strongly differentiated morphologically and ecologically, whereas hybrids are intermediate. Fitness of hybrids is high in the laboratory, especially F1 and F2 hybrids (backcrosses may show some breakdown). We transplanted F1 hybrids to enclosures in the two main habitats in the wild to test whether the distribution of resources available in the environment generates a hybrid disadvantage not detectable in the laboratory. Hybrids grew more slowly than limnetics in the open water habitat and more slowly than benthics in the littoral zone. Growth of F1 hybrids was inferior to the average of the parent species across both habitats, albeit not significantly. The contrast between laboratory and field results supports the hypothesis that mechanisms of F1 hybrid fitness in the wild are primarily ecological and do not result from intrinsic genetic incompatibilities. Direct selection on hybrids contributes to the maintenance of sympatric stickleback species and may have played an important role in their origin.

Perspective: spontaneous deleterious mutation

Abstract: Mildly deleterious mutation has been invoked as a leading explanation for a diverse array of observations in evolutionary genetics and molecular evolution and is thought to be a significant risk of extinction for small populations. However, much of the empirical evidence for the deleterious-mutation process derives from studies of Drosophila melanogaster, some of which have been called into question. We review a broad array of data that collectively support the hypothesis that deleterious mutations arise in flies at rate of about one per individual per generation, with the average mutation decreasing fitness by about only 2% in the heterozygous state. Empirical evidence from microbes, plants, and several other animal species provide further support for the idea that most mutations have only mildly deleterious effects on fitness, and several other species appear to have genomic mutation rates that are of the order of magnitude observed in Drosophila. However, there is mounting evidence that some organisms have genomic deleterious mutation rates that are substantially lower than one per individual per generation. These lower rates may be at least partially reconciled with the Drosophila data by taking into consideration the number of germline cell divisions per generation. To fully resolve the existing controversy over the properties of spontaneous mutations, a number of issues need to be clarified. These include the form of the distribution of mutational effects and the extent to which this is modified by the environmental and genetic background and the contribution of basic biological features such as generation length and genome size to interspecific differences in the genomic mutation rate. Once such information is available, it should be possible to make a refined statement about the long-term impact of mutation on the genetic integrity of human populations subject to relaxed selection resulting from modern medical procedures.

Heterosis and outbreeding depression in interpopulation crosses spanning a wide range of divergence.

Abstract: The intertidal copepod Tigriopus californicus was used as a model organism to look at effects of crossing distance on fitness and to investigate the genetic mechanisms responsible. Crosses were conducted between 12 pairs of populations spanning a broad range of both geographic distance (5 m to 2007 km) and genetic distance (0.2% to 22.3% sequence divergence for a 606-bp segment of the mitochondrial COI gene). For each pair of populations, three fitness components (hatching number, survivorship number, and metamorphosis number) were measured in up to 16 cohorts including parentals, reciprocal F1 , F2 , F3 , and first-generation backcross hybrids. Comparisons of each set of cohorts allowed estimation of within- and between-locus gene interaction. Relative to parentals, F1 hybrids showed a trend toward increased fitness, with no correspondence with population divergence, and a decrease in variance, which in some cases correlated with population divergence. In sharp contrast, F2 hybrids had a decrease in fitness and an increase in variance that both corresponded to population divergence. Genetic interpretation of these patterns suggests that both the beneficial effects of dominance and the detrimental effects of breaking up coadaptation are magnified by increasing evolutionary distance between populations. Because there is no recombination in T. californicus females, effects of recombination can be assessed by comparing F1 hybrid males and females backcrossed to parentals. Both recombinant and nonrecombinant backcross hybrids showed a decline in fitness correlated with population divergence, indicating that segregation among chromosomes contributes to the breakup of coadaptation. Although there was no difference in mean fitness between the two backcross types, recombinational backcrosses showed greater variance for fitness than nonrecombinational backcrosses, suggesting that the breakup of parental gene ombinations within chromosomes has both beneficial and detrimental effects.

Reproductive isolation between sympatric races of pea aphids. i. gene flow restriction and habitat choice.

Abstract: Determining the extent and causes of barriers to gene flow between genetically divergent populations or races of single species is an important complement to post facto analyses of the causes of reproductive isolation between recognized species. Sympatric populations of pea aphids (Acyrthosiphon pisum Harris, Homoptera: Aphididae) on alfalfa and red clover are highly genetically divergent and locally adapted. Here, hierarchical estimates of population structure based on Fst suggest that gene exchange between closely adjacent aphid populations on the two hosts is highly restricted relative to that among fields of the same host plant. Although these host-associated races are presently considered to be the same subspecies, they appear to be significantly reproductively isolated, suggesting incipient speciation. Habitat (host) choice was investigated as the first in a temporal series of factors that could reduce gene exchange between these sympatric populations. Field studies of winged colonists to newly planted fields of each host suggest pronounced habitat fidelity. This result was verified using replicated observations of the host choice behavior of different aphid genotypes for which the relative demographic performance on each host was known. These laboratory observations of behavior revealed a strong genetic correlation between habitat choice (or acceptance) and the relative performance in each habitat. Because mating occurs on the host plant, habitat choice in this system leads to assortative mating and is therefore a major cause of reproductive isolation between the sympatric pea aphid populations on alfalfa and clover. However, the extent of dispersal between hosts estimated from the field study of winged colonists (9-11%) is too great to be consistent with the genetic divergence estimated between the races. This suggests that barriers to gene flow other than host choice also exist, such as selection against migrants or hybrids in the parental environments, hybrid sterility, or hybrid breakdown.

Costs of induced responses and tolerance to herbivory in male and female fitness components of wild radish.

Abstract: Theory predicts that plant defensive traits are costly due to trade-offs between allocation to defense and growth and reproduction. Most previous studies of costs of plant defense focused on female fitness costs of constitutively expressed defenses. Consideration of alternative plant strategies, such as induced defenses and tolerance to herbivory, and multiple types of costs, including allocation to male reproductive function, may increase our ability to detect costs of plant defense against herbivores. In this study we measured male and female reproductive costs associated with induced responses and tolerance to herbivory in annual wild radish plants (Raphanus raphanistrum). We induced resistance in the plants by subjecting them to herbivory by Pieris rapae caterpillars. We also induced resistance in plants without leaf tissue removal using a natural chemical elicitor, jasmonic acid; in addition, we removed leaf tissue without inducing plant responses using manual clipping. Induced responses included increased concentrations of indole glucosinolates, which are putative defense compounds. Induced responses, in the absence of leaf tissue removal, reduced plant fitness when five fitness components were considered together; costs of induction were individually detected for time to first flower and number of pollen grains produced per flower. In this system, induced responses appear to impose a cost, although this cost may not have been detected had we only quantified the traditionally measured fitness components, growth and seed production. In the absence of induced responses, 50% leaf tissue removal, reduced plant fitness in three out of the five fitness components measured. Induced responses to herbivory and leaf tissue removal had additive effects on plant fitness. Although plant sibships varied greatly (49-136%) in their level of tolerance to herbivory, costs of tolerance were not detected, as we did not find a negative association between the ability to compensate for damage and plant fitness in the absence of damage. We suggest that consideration of alternative plant defense strategies and multiple costs will result in a broader understanding of the evolutionary ecology of plant defense.

Evolution of multiple kinds of female sperm-storage organs in drosophila.

Abstract: Females of all species belonging to the family Drosophilidae have two kinds of sperm-storage organs: paired spherical spermathecae and a single elongate tubular seminal receptacle. We examined 113 species belonging to the genus Drosophila and closely allied genera and describe variation in female sperm-storage organ use and morphology. The macroevolutionary pattern of organ dysfunction and morphological divergence suggests that ancestrally both kinds of organs stored sperm. Loss of use of the spermathecae has evolved at least 13 times; evolutionary regain of spermathecal function has rarely if ever occurred. Loss of use of the seminal receptacle has likely occurred only once; in this case, all descendant species possess unusually elaborate spermathecae. Data further indicate that the seminal receptacle is the primary sperm-storage organ in Drosophila. This organ exhibits a pattern of strong correlated evolution with the length of sperm. The evolution of multiple kinds of female sperm-storage organs and the rapidly divergent and correlated evolution of sperm and female reproductive tract morphology are discussed.

Hierarchical comparison of genetic variance-covariance matrices. i. using the flury hierarchy.

Abstract: The comparison of additive genetic variance-covariance matrices (G-matrices) is an increasingly popular exercise in evolutionary biology because the evolution of the G-matrix is central to the issue of persistence of genetic constraints and to the use of dynamic models in an evolutionary time frame. The comparison of G-matrices is a nontrivial statistical problem because family structure induces nonindependence among the elements in each matrix. Past solutions to the problem of G-matrix comparison have dealt with this problem, with varying success, but have tested a single null hypothesis (matrix equality or matrix dissimilarity). Because matrices can differ in many ways, several hypotheses are of interest in matrix comparisons. Flury (1988) has provided an approach to matrix comparison in which a variety of hypotheses are tested, including the two extreme hypotheses prevalent in the evolutionary literature. The hypotheses are arranged in a hierarchy and involve comparisons of both the principal components (eigenvectors) and eigenvalues of the matrix. We adapt Flury's hierarchy of tests to the problem of comparing G- matrices by using randomization testing to account for nonindependence induced by family structure. Software has been developed for carrying out this analysis for both genetic and phenotypic data. The method is illustrated with a garter snake test case.

Maternal effects on offspring size: variation through early development of chinook salmon.

Abstract: We performed two breeding experiments with chinook salmon (Oncorhynchus tshawytscha) to explore maternal effects on offspring size. We estimated the magnitude of maternal effects as the differences between sire-offspring and dam-offspring regression slopes. Early in life, offspring size is largely influenced by maternal size, but this influence decreases through early development, with the maternal effect becoming negative at intermediate offspring ages (corresponding to a period of reduced growth of progeny hatching from large eggs) and converging on zero as offspring age. Also, egg size was positively correlated with early survival, but negatively correlated with maternal fecundity.

A comparison of inbreeding depression in life-history and morphological traits in animals.

Abstract: The current study tests the hypothesis that life-history traits (closely related to fitness) show greater inbreeding depression than morphological traits (less closely related to fitness). The mean and median slope of the standardized coefficient of inbreeding depression (the slope of the linear relationship between F and the trait value) for life-history and morphological traits were compared. Slopes for life-history traits were higher than those for morphological traits. At F = 0.25 (full-sibling mating), life-history traits experienced a median reduction of 11.8% in trait value, whereas morphological traits showed a depression in trait value of approximately 2.2%.

Correlated trophic specialization and genetic divergence in sympatric lake whitefish ecotypes (coregonus clupeaformis): support for the ecological speciation hypothesis.

Abstract: There is ample empirical evidence that phenotypic diversification in an adaptive radiation is the outcome of divergent natural selection related to differential resource use. In contrast, the role of ecological forces in favoring and maintaining reproductive isolation in nature remains poorly understood. If the same forces driving phenotypic divergence are also responsible for speciation, one would predict a correlation between the extent of trophic specialization (reflecting variable intensity of divergent natural selection) and that of reproductive isolation being reached in a given environment. We tested this hypothesis by comparing the extent of morphological and genetic differentiation between sympatric dwarf and normal whitefish ecotypes (Coregonus sp.) from six lakes of the St. John River basin (eastern Canada and northern Maine). Eight meristic variables, 19 morphometric variables, and six microsatellite loci were used to quantify morphological and genetic differentiation, respectively. Dwarf and normal ecotypes in each lake differed primarily by traits related to trophic specialization, but the extent of differentiation varied among lakes. Significant but variable genetic divergence between ecotypes within lakes was also observed. A negative correlation was observed between the extent of gene flow between ecotypes within a lake and that of their morphological differentiation in trophic-related traits. The extent of reproductive isolation reached between dwarf and normal whitefish ecotypes appears to be driven by the potential for occupying distinct trophic niches and, thus, by the same selective forces driving tropic specialization in each lake. These results therefore support the hypothesis of ecological speciation.

Phylogenetic analysis of trait evolution and species diversity variation among angiosperm families.

Abstract: Angiosperm families differ greatly from one another in species richness (S). Previous studies have attributed significant components of this variation to the influence of pollination mode (biotic/abiotic) and growth form (herbaceous/woody) on speciation rate, but these results suffer difficulties of interpretation because all the studies ignored the phylogenetic relationships among families. We use a molecular phylogeny of the angiosperm families to reanalyse correlations between S and family-level traits and use reconstructions of trait evolution to interpret the results. We confirm that pollination mode and growth form are correlated with S and show that the majority of changes in pollination mode involved a change from biotic to abiotic pollination with an associated fall in speciation rate. The majority of growth form changes involved the evolution of herbaceousness from woodiness with a correlated rise in speciation rate. We test the hypothesis of Ricklefs and Renner (1994) that "evolutionary flexibility" rather than other trait changes triggered increased speciation rates in some families, but find little support for the hypothesis.

Genetic relationships between parasite virulence and transmission in the rodent malaria plasmodium chabaudi.

Abstract: Many parasites evolve to become virulent rather than benign mutualists. One of the major theoretical models of parasite virulence postulates that this is because rapid within-host replication rates are necessary for successful transmission (parasite fitness) and that virulence (damage to the host) is an unavoidable consequence of this rapid replication. Two fundamental assumptions underlying this so-called evolutionary trade-off model have rarely been tested empirically: (1) that higher replication rates lead to higher levels of virulence; and (2) that higher replication rates lead to higher transmission. Both of these relationships must have a genetic basis for this evolutionary hypothesis to be relevant. These assumptions were tested in the rodent malaria parasite, Plasmodium chabaudi, by examining genetic relationships between virulence and transmission traits across a population of eight parasite clones isolated from the wild. Each clone was injected into groups of inbred mice in a controlled laboratory environment, and replication rate (measured by maximum asexual parasitemia), virulence (measured by live-weight loss and degree of anemia in the mouse), and transmission (measured by density of sexual forms, gametocytes, in the blood and proportion of mosquitoes infected after taking a blood-meal from the mouse) were assessed. It was found that clones differed widely in these traits and these clone differences were repeatable over successive blood passages. Virulence traits were strongly phenotypically and genetically (i.e., across clones) correlated to maximum parasitemia thus supporting the first assumption that rapid replication causes higher virulence. Transmission traits were also positively phenotyp- ically and genetically correlated to parasitemia, which supports the second assumption that rapid replication leads to higher transmission. Thus, two assumptions of the parasite-centered trade-off model of the evolution of virulence were shown to be justified in malaria parasites.

Accelerated evolution of land snails mandarina in the oceanic bonin islands: evidence from mitochondrial dna sequences.

Abstract: An endemic land snail genus Mandarina of the oceanic Bonin (Ogasawara) Islands shows exceptionally rapid evolution not only of morphological and ecological traits, but of DNA sequence. A phylogenetic relationship based on mitochondrial DNA (mtDNA) sequences suggests that morphological differences equivalent to the differences between families were produced between Mandarina and its ancestor during the Pleistocene. The inferred phylogeny shows that species with similar morphologies and life habitats appeared repeatedly and independently in different lineages and islands at different times. Sequential adaptive radiations occurred in different islands of the Bonin Islands and species occupying arboreal, semiarboreal, and terrestrial habitat arose independently in each island. Because of a close relationship between shell morphology and life habitat, independent evolution of the same life habitat in different islands created species possesing the same shell morphology in different islands and lineages. This rapid evolution produced some incongruences between phylogenetic relationship and species taxonomy. Levels of sequence divergence of mtDNA among the species of Mandarina is extremely high. The maximum level of sequence divergence at 16S and 12S ribosomal RNA sequence within Mandarina are 18.7% and 17.7%, respectively, and this suggests that evolution of mtDNA of Mandarina is extremely rapid, more than 20 times faster than the standard rate in other animals. The present examination reveals that evolution of morphological and ecological traits occurs at extremely high rates in the time of adaptive radiation, especially in fragmented environments.

Rapid and repeated invasions of fresh water by the copepod Eurytemora affinis

Abstract: Invasions of fresh water by marine organisms have been of great interest to evolutionary biologists and paleontologists because they typically constitute major evolutionary transitions. Recent (< 200 years) invasions of fresh water by brackish or marine species offer an opportunity to understand mechanisms underlying these events, but pathways of invasion from salt water have not been confirmed using genetic data. This study employed mito- chondrial DNA sequences (652 base pairs from the cytochrome oxidase I (COI) gene) to reconstruct the geographic and evolutionary history of freshwater invasion by the common estuarine and saltmarsh crustacean Eurytemora affinis (Copepoda; Poppe 1880). Phylogenetic analysis of populations from North America, Europe, and Asia revealed at least eight independent invasions of fresh water from genetically distinct lineages. At least five of these freshwater invasions most likely arose independently in different river drainages, recently from saltwater sources within each river drainage. An analysis of molecular variance (AMOVA) was performed at three geographic scales (among continents, among drainages, and within drainages) to assess the hierarchical distribution of genetic variance. Results indicated that 52% of the genetic variance was explained by differences among drainages, 43% by differences among continents, but only 5% by differences within drainages, thus supporting geographic patterns of invasions inferred from the phylogeny. Physiological experiments were performed to determine whether adults and larvae from saltwater populations could tolerate freshwater conditions. Transfer to zero salinity resulted in high mortalities, but with some survival to the second generation in one population. This study provides genetic evidence and physiological support for rapid transitions from a saline life history into fresh water, with repeated invasions on a global scale.

The quantitative and molecular genetic architecture of a subdivided species

Abstract: In an effort to elucidate the evolutionary mechanisms that determine the genetic architecture of a species, we have analyzed 17 populations of the microcrustacean Daphnia pulex for levels of genetic variation at the level of life-history characters and molecular markers in the nuclear and mitochondrial genomes. This species is highly subdivided, with approximately 30% of the variation for nuclear molecular markers and 50% of the variation for mitochondrial markers being distributed among populations. The average level of genetic subdivision for quantitative traits is essentially the same as that for nuclear markers, which superficially suggests that the life-history characters are diverging at the neutral rate. However, the existence of a strong correlation between the levels of population subdivision and broadsense heritabilities of individual traits argues against this interpretation, suggesting instead that the among-population divergence of some quantitative traits (most notably body size) is being driven by local adaptation to different environments. The fact that the mean phenotypes of the individual populations are also strongly correlated with local levels of homozygosity indicates that variation in local inbreeding plays a role in population differentiation. Rather than being a passive consequence of local founder effects, levels of homozygosity may be selected for directly for their effects on the phenotype (adaptive inbreeding depression). There is no relationship between the levels of variation within populations for molecular markers and quantitative characters, and this is explained by the fact that the average standing genetic variation for life-history characters in this species is equivalent to only 33 generations of variation generated by mutation.

Phylogeography of the pantropical sea urchin eucidaris in relation to land barriers and ocean currents

Abstract: The pantropical sea urchin genus Eucidaris contains four currently recognized species, all of them allopatric: E. metularia in the Indo-West Pacific, E. thouarsi in the eastern Pacific, E. tribuloides in both the western and eastern Atlantic, and E. clavata at the central Atlantic islands of Ascension and St. Helena. We sequenced a 640-bp region of the cytochrome oxidase I (COI) gene of mitochondrial DNA to determine whether this division of the genus into species was confirmed by molecular markers, to ascertain their phylogenetic relations, and to reconstruct the history of possible dispersal and vicariance events that led to present-day patterns of species distribution. We found that E. metularia split first from the rest of the extant species of the genus. If COI divergence is calibrated by the emergence of the Isthmus of Panama, the estimated date of the separation of the Indo-West Pacific species is 4.7-6.4 million years ago. This date suggests that the last available route of genetic contact between the Indo-Pacific and the rest of the tropics was from west to east through the Eastern Pacific Barrier, rather than through the Tethyan Sea or around the southern tip of Africa. The second cladogenic event was the separation of eastern Pacific and Atlantic populations by the Isthmus of Panama. Eucidaris at the outer eastern Pacific islands (Galapagos, Isla del Coco, Clipperton Atoll) belong to a separate clade, so distinct from mainland E. thouarsi as to suggest that this is a different species, for which the name E. galapagensis is revived from the older taxonomic literature. Complete lack of shared alleles in three allozyme loci between island and mainland populations support their separate specific status. Eucidaris galapagensis and E. thouarsi are estimated from their COI divergence to have split at about the same time that E. thouarsi and E. tribuloides were being separated by the Isthmus of Panama. Even though currents could easily convey larvae between the eastern Pacific islands and the American mainland, the two species do not appear to have invaded each other's ranges. Conversely, the central Atlantic E. clavata at St. Helena and Ascension is genetically similar to E. tribuloides from the American and African coasts. Populations on these islands are either genetically connected to the coasts of the Atlantic or have been colonized by extant mitochondrial DNA lineages of Eucidaris within the last 200,000 years. Although it is hard to explain how larvae can cross the entire width of the Atlantic within their competent lifetimes, COI sequences of Eucidaris from the west coast of Africa are very similar to those of E. tribuloides from the Caribbean. FST statistics indicate that gene flow between E. metularia from the Indian Ocean and from the western and central Pacific is restricted. Low gene flow is also evident between populations of E. clavata from Ascension and St. Helena. Rates of intraspecific exchange of genes in E. thouarsi, E. galapagensis, and E. tribuloides, on the other hand, are high. The phylogeny of Eucidaris confirms Ernst Mayr's conclusions that major barriers to the dispersal of tropical echinoids have been the wide stretch of deep water between central and eastern Pacific, the cold water off the southwest coast of Africa, and the Isthmus of Panama. It also suggests that a colonization event in the eastern Pacific has led to speciation between mainland and island populations.

Local maladaptation in the anther-smut fungus microbotryum violaceum to its host plant silene latifolia: evidence from a cross-inoculation experiment.

Abstract: Conventional wisdom holds that parasites evolve more rapidly than their hosts and are therefore locally adapted, that is, better at exploiting sympatric than allopatric hosts. We studied local adaptation in the insect-transmitted fungal pathogen Microbotryum violaceuin and its host plant Silene latifolia. Infection success was tested in sympatric (local) and allopatric (foreign) combinations of pathogen and host from 14 natural populations from a metapopulation. Seedlings from up to 10 seed families from each population were exposed to sporidial suspensions from each of four fungal strains derived from the same population, from a near-by population (< 10 km distance), and from two populations at an intermediate (< 30 km) and remote (< 170 km) distance, respectively. We obtained significant pathogen X plant interactions in infection success (proportion of diseased plants) at both fungal population and strain level. There was an overall pattern of local maladaptation of this pathogen: average fungal infection success was significantly lower on sympatric hosts (mean proportion of diseased plants = 0.32 + 0.03 SE) than on allopatric hosts (0.40 + 0.02). Five of the 14 fungal populations showed no strong reduction in infection success on sympatric hosts, and three even tended to perform better on sympatric hosts. This pattern is consistent with models of time-lagged cycles predicting patterns of local adaptation in host-parasite systems to emerge only on average. Several factors may restrict the evolutionary potential of this pathogen relative to that of its host. First, a predominantly selfing breeding system may limit its ability to generate new virulence types by sexual recombination, whereas the obligately outcrossing host S. latifolia may profit from rearrangement of resistance alleles by random mating. Second, populations often harbor only a few infected individuals, so virulence variation may be further reduced by drift. Third, migration rates among host plant populations are much higher than among pathogen populations, possibly because pollinators prefer healthy over diseased plants. Migration among partly isolated populations may therefore introduce novel host plant resistance variants more often than novel parasite virulence variants. That migration contributes to the coevolutionary dynamics in this system is supported by the geographic pattern of infectivity. Infection success increased over the first 10-km range of host-pathogen population distances, which is likely the natural range of gene exchange.

Pollinator-mediated competition, reproductive character displacement, and the evolution of selfing in arenaria uniflora (caryophyllaceae).

Abstract: Ecological factors that reduce the effectiveness of cross-pollination are likely to play a role in the frequent evolution of routine self-fertilization in flowering plants. However, we lack empirical evidence linking the reproductive assurance value of selfing in poor pollination environments to evolutionary shifts in mating system. Here, we investigated the adaptive significance of prior selfing in the polymorphic annual plant Arenaria uniflora (Caryophyllaceae), in which selfer populations occur only in areas of range overlap with congener A. glabra. To examine the hypothesis that secondary contact between the two species contributed to the evolution and maintenance of selfing, we used field competition experiments and controlled hand-pollinations to measure the female fitness consequences of pollinator-mediated interspecific interactions. Uniformly high fruit set by selfers in the naturally pollinated field arrays confirmed the reproductive assurance value of selfing, whereas substantial reductions in outcrosser fruit set (15%) and total seed production (20-35%) in the presence of A. glabra demonstrated that pollinator-mediated interactions can provide strong selection for self-pollination. Heterospecific pollen transfer, rather than competition for pollinator service, appears to be the primary mechanism of pollinator-mediated competition in Arenaria. Premating barriers to hybridization between outcrossers and A. glabra are extremely weak. The production of a few inviable hybrid seeds after heterospecific pollination and intermediate seed set after mixed pollinations indicates that A. glabra pollen can usurp A. uniflora ovules. Thus, any visit to A. uniflora by shared pollinators carries a potential female fitness cost. Moreover, patterns of fruit set and seed set in the competition arrays relative to controls were consistent with the receipt of mixed pollen loads, rather than a lack of pollinator visits. Competition through pollen transfer favors preemptive self-pollination and may be responsible for the evolution of a highly reduced floral morphology in A. uniflora selfers as well as their current geographical distribution.

Does cosmopolitanism result from overconservative systematics? a case study using the marine sponge chondrilla nucula.

Abstract: The sponge species Chondrilla nucula has a simple morphology and a very wide geographical distribution. To verify whether the latter might be an artifact of the former, samples of this species were collected across 10,000 km of its range, in the Mediterranean, the Caribbean, and the southwestern atlantic. The classical (spicule morphology) and molecular (allozymes) systematic approaches were compared, to try to define the geographic limits between populations and detect possible cryptic species. We found five distinct genetic forms within C. nucula that sometimes showed morphological homogeneity and other times plasticity. The difference in size of spicules could not be related to the clear-cut genetic differences, suggesting that the use of spicule sizes for sponge systematics should be reappraised. The population of one of the genetic forms along 3000 km of the Brazilian coast was highly structured (FST = 0.21; Ne m = 0.96). Our results reject the null hypothesis of cosmopolitanism of C. nucula and indicate that the putative worldwide distribution of some marine sponges, and possibly many other benthic invertebrates, may be the result of overly conservative systematics. Cryptic species appear to be particularly prevalent when genera are well defined but species are characterized by only a few morphological characters.

Coincident biogeographic patterns: indo-west pacific ocean.

Abstract: The majority of tropical marine families demonstrate their greatest concentration of species within the relatively small East Indies Triangle. In every direction, the species diversity decreases with distance from the East Indies. Other patterns suggest that the East Indies is where the average generic age is youngest, where some historical routes of dispersal originate, where the most apomorphic species occur, where genetic diversity is the greatest, and where extinctions are likely to originate. These coincident patterns provide support for the hypothesis that the East Indies has been operating as a center of evolutionary radiation. The driving force for this dynamic system is apparently the predominance of successful speciation involving relatively large populations with higher genetic diversity. This mechanism fits the centrifugal speciation model that was proposed more than 50 years ago.

Natural selection on thermogenic capacity of high-altitude deer mice.

Abstract: Adaptive explanations that rely on physiological arguments are common, but tests of hypotheses about the significance of whole-animal physiological performance (e.g., aerobic capacities) are rare. We studied phenotypic selection on the thermogenic capacity (i.e., maximal rate of oxygen consumption [VO2 max] elicited via cold exposure) of high-altitude (~3800 m) deer mice (Peromyscus maniculatus). A high VO2 max equates to a high capacity for heat production and should favor survival in the cold environments prevalent at high altitude. Strong directional selection favored high VO2 max, at least in one year. The selection for increased VO2 max is consistent with predictions derived from incorporating our physiological data into a biophysical model. During another year, we found weak evidence of selection for decreased body mass. Nonlinear selection was not significant for any of the selection episodes we studied. The strong directional selection for VO2 max that we observed suggests that-given ample genetic variation-aerobic metabolism and perhaps endothermy may have evolved rapidly on the geological time scale.

Plant polyploidy and pollination: Floral traits and insect visits to diploid and tetraploid Heuchera grossulariifolia

Abstract: In many polyploid species, polyploids often have different suites of floral traits and different flowering times than their diploid progenitor species. We hypothesized that such differences in floral traits in polyploids may subsequently affect their interactions with pollinating and other insect visitors. We measured floral morphology and flowering phenology in 14 populations of diploid and autotetraploid Heuchera grossulariifolia Rydb. (Saxifragaceae), determined if repeated evolution of independent polyploid lineages resulted in differentiation in floral morphology among those lineages, and ascertained if there was a consistent pattern of differentiation among genetically similar diploid and autotetraploid populations. In addition, we evaluated the differences in suites of floral visitors within a natural community where diploids and autotetraploids occur sympatrically. Overall, flowers of autotetraploid plants were larger and shaped differently than those of diploids, had a different flowering phenology than that of diploids, and attracted different suites of floral visitors. In comparison with flowers of diploids, tetraploid floral morphology varied widely from pronounced differences between cytotypes in some populations to similar flower shapes and sizes between ploidal levels in other populations. Observations of floral visitors to diploids and autotetraploids in a natural sympatric population demonstrated that the cytotypes had different suites of floral visitors and six of the 15 common visitors preferentially visited one ploidy more frequently. Moreover, we also found that floral morphology differed among independent autotetraploid origins, but there was no consistent pattern of differentiation between genetically similar diploid and autotetraploid populations. Hence, the results suggest that the process of polyploidization creates the potential for attraction of different suites of floral visitors. Multiple origins of polyploidy also presents the opportunity for new or different plant-insect interactions among independent polyploid lineages. These differences in turn may affect patterns of gene flow between diploids and polyploids and also among plants of independent polyploid origin. Polyploidy, therefore, may result in a geographic mosaic of interspecific interactions across a species' range, contributing to diversification in both plant and insect groups.

Neutral genetic diversity in a metapopulation with recurrent local extinction and recolonization

Abstract: Many species exist as metapopulations in balance between local population extinction and recolonization, processes that may strongly affect the distribution of neutral genetic diversity within demes and in the metapopulation as a whole. In this paper we use both the infinite-alleles and the infinite-sites models to reframe Slatkin's propagule- pool and migrant-pool models in terms of mean within-deme and among-deme genetic diversity; the infinite-sites model is particularly relevant to DNA sequence data. Population turnover causes a major reduction in neutral genetic diversity within demes, -S, and in the metapopulation as a whole, XaT. This effect is particularly strong for propagule- pool colonization, in which colonists are drawn from a single extant deme. Because metapopulation dynamics affect both within-deme and total metapopulation diversity similarly, comparisons between species with different ecologies on the basis of ratios such as FST are difficult to interpret and absolute measures of divergence between populations should be used as well. Although the value of FST in a metapopulation with local extinction depends strongly on the mode of colonization, this has almost no effect on the numerator of the FST ratio, XrT - rs, so that FST is influenced mainly by the effect of the colonization mode on the denominator (XTT). Our results also indicate that it is inappropriate to use measures of average within-deme diversity in species with population turnover to estimate the scaled mutation rate, 0, because extinction can greatly reduce TrS. Finally, we discuss the effect of population turnover on the effective size of a metapopulation.

Copulatory behavior, genital morphology, and male fertilization success in water striders.

Abstract: Recent theoretical and empirical interest in postmating processes have generated a need for increasing our understanding of the sources of variance in fertilization success among males. Of particular importance is whether such postmating sexual selection merely reinforces the effects of premating sexual selection or whether other types of male traits are involved. In the current study, we document large intraspecific variation in last male sperm precedence in the water strider Gerris lateralis. Male relative paternity success was repeatable across replicate females, showing that males differ consistently in their ability to achieve fertilizations. By analyzing shape variation in male genital morphology, we were able to demonstrate that the shape of male intromittent genitalia was related to relative paternity success. This is the first direct experimental support for the suggestion that male genitalia evolve by postmating sexual selection. A detailed analysis revealed that different components of male genitalia had different effects, some affecting male ability to achieve sperm precedence and others affecting male ability to avoid sperm precedence by subsequent males. Further, the effects of the shape of the male genitalia on paternity success was in part dependent on female morphology, suggesting that selection on male genitalia will depend on the frequency distribution of female phenotypes. We failed to find any effects of other morphological traits, such as male body size or the degree of asymmetry in leg length, on fertilization success. Although males differed consistently in their copulatory behavior, copulation duration was the only behavioral trait that had any significant effect on paternity.

Hierarchical comparison of genetic variance-covariance matrices. ii coastal-inland divergence in the garter snake, thamnophis elegans.

Abstract: The time-scale for the evolution of additive genetic variance-covariance matrices (G-matrices) is a crucial issue in evolutionary biology. If the evolution of G-matrices is slow enough, we can use standard multivariate equations to model drift and selection response on evolutionary time scales. We compared the G-matrices for meristic traits in two populations of garter snakes (Thamnophis elegans) with an apparent separation time of 2 million years. Despite considerable divergence in the meristic traits, foraging habits, and diet, these populations show conservation of structure in their G-matrices. Using Flury's hierarchial approach to matrix comparisons, we found that the populations have retained the principal components (eigenvectors) of their G-matrices, but their eigenvalues have diverged. In contrast, we were unable to reject the hypothesis of equal environmental matrices (E-matrices) for these populations. We propose that a conserved pattern of multivariate stabilizing selection may have contributed to conservation of G- and E-matrix structure during the divergence of these populations.

Wolbachia and the evolution of reproductive isolation between Drosophila recens and Drosophila subquinaria

Abstract: Endosymbiotic bacteria of the genus Wolbachia are widespread among insects and in many cases cause cytoplasmic incompatibility in crosses between infected males and uninfected females. Such findings have been used to argue that Wolbachia have played an important role in insect speciation. Theoretical models, however, indicate that Wolbachia alone are unlikely to lead to stable reproductive isolation between two formerly nonspecific populations. Here we analyze the components of reproductive isolation between Drosophila recens, which is infected with Wolbachia, and its uninfected sister species Drosophila subquinaria. Laboratory pairings demonstrated that gene flow via matings between D. recens females and D. subquinaria males is hindered by behavioral isolation. Matings readily occurred in the reciprocal cross (D. quinaria females X D. recens males), but very few viable progeny were produced. The production of viable hybrids via this route was restored by antibiotic curing of D. recens of their Wolbachia symbionts, indicating that hybrid offspring production is greatly reduced by cytoplasmic incompatibility in the crosses involving infected D. recens males. Thus, behavioral isolation and Wolbachia-induced cytoplasmic incompatibility act as com- plementary asymmetrical isolating mechanisms between these two species. In accordance with Haldane's rule, hybrid females were fertile, whereas hybrid males invariably were sterile. Levels of mtDNA variation in D. recens are much lower than in either D. subquinaria or D. falleni, neither of which is infected with Wolbachia. The low haplotype diversity in D. recens is likely due to an mtDNA sweep associated with the spread of Wolbachia. Nevertheless, the existence of several mtDNA haplotypes in this species indicates that Wolbachia have been present as a potential isolating mechanism for a substantial period of evolutionary time. Finally, we argue that although Wolbachia by themselves are unlikely to bring about speciation, they can increase the rate of speciation in insects.