Animals that are brightly colored have intrigued scientists since the time of Darwin, because it seems surprising that prey should have evolved to be clearly visible to predators. Often this self-advertisement is explained by the prey being unprofitable in some way, with the conspicuous warning coloration helping to protect the prey because it signals to potential predators that the prey is unprofitable. However, such signals only work in this way once predators have learned to associate the conspicuous color with the unprofitability of the prey. The evolution of warning coloration is still widely considered to be a paradox, because it has traditionally been assumed that the very first brightly colored individuals would be at an immediate selective disadvantage because of their greater conspicuousness to predators that are naive to the meaning of the signal. As a result, it has been difficult to understand how a novel conspicuous color morph could ever avoid extinction for long enough for predators to become educated about the signal. Thus, the traditional view that the evolution of warning coloration is difficult to explain rests entirely on assumptions about the foraging behavior of predators. However, we review recent evidence from a range of studies of predator foraging decisions, which refute these established assumptions. These studies show that: (1) Many predators are so conservative in their food preferences that even very conspicuous novel prey morphs are not necessarily at a selective disadvantage. (2) The survival and spread of novel color morphs can be simulated in field and aviary experiments using real predators (birds) foraging on successive generations of artificial prey populations. This work demonstrates that the foraging preferences of predators can regularly (though not always) result in the increase to fixation of a novel morph appearing in a population of familiar-colored prey. Such fixation events occur even if both novel and familiar prey are fully palatable and despite the novel food being much more conspicuous than the familiar prey. These studies therefore provide strong empirical evidence that conspicuous coloration can evolve readily, and repeatedly, as a result of the conservative foraging decisions of predators.

The behavior of the maximum body size (body length) in an evolving clade is exemplified by the evolutionary histories of Bivalvia, Cetacea, and Camerata (Crinoidea). Changes of the maximum size with time track closely diversification history: when a clade diversifies exponentially, the maximum size also increases exponentially, and when the number of species changes irregularly (at varying rates), the maximum size also changes in that manner. However, within any given clade, the maximum body size changes at lower rates than diversity does. The observed shifts in maximum body size approximate the rate of diversification per unit of time to the power of about 0.5.

Although homoploid hybrid speciation is increasingly recognized as an important phenomenon in plant evolution, its role in adaptive radiations is poorly documented. We studied a clade of seven extant species of Scaevola that are endemic to the Hawaiian Islands and show substantial ecological and morphological diversity. We estimated the genealogies for alleles isolated from multiple accessions of each species at four nuclear loci: the ITS region, and the introns of three nuclear genes, LEAFY (LFY), NITRATE REDUCTASE (NIA), and GLYCERALDEHYDE 3-PHOSPHATE DEHYDROGENASE (G3PDH). For five of the seven species, there was complete concordance among the genealogies estimated from the four loci and, when all four regions were combined, the relationships among these five species were fully resolved. Inclusion of alleles from the remaining two species, S. procera and S. kilaueae, resulted in incongruence among loci, which appears to reflect a history of hybridization. Based on the distribution of alleles, we infer that S. procera is the result of a homoploid hybrid speciation event between S. gaudichaudii and S. mollis and that S. kilaueae is probably the result of hybrid speciation between S. coriacea and S. chamissoniana. In each case the inferred hybridization is consistent with morphological, ecological, and geographic information. We conclude that homoploid hybrid speciation may be more common than is perceived and may play a role in generating novel combinations of adaptive traits that arise during island radiations.

We determined whether supernumerary B chromosomes were nonrandomly distributed among major angiosperm lineages and among lineages within families, as well as the identity of lineages with unusually high B-chromosome frequencies (hot spots). The incidence of B chromosomes for each taxon was gathered from databases showing species with and without these chromosomes (among species with known chromosome numbers). Heterogeneity was found at all ranks above the species level. About 8% of monocots had B chromosomes versus 3% for eudicots; they were rare in nonmonocot basal angiosperms. Significant heterogeneity in B-chromosome frequency occurred among related orders, families within orders, and major taxa within families. There were many B-chromosome hot spots, including Liliales and Commelinales at the order level. At the family level, there was a trend suggesting that B-chromosome frequencies are positively correlated with genome size.

We investigated the relationship between flowering time and sexual allocation in wild-type Arabidopsis thaliana and in genetically similar lineages with single-locus mutations of floral induction genes. We examined whether the mechanisms of growth and development that govern resource investment would permit the independent evolution of reproductive phenology and sexual allocation, or whether constraints, manifested as pleiotropic effects of the single mutations, would link these two life-history traits. Flowering times differed significantly among genotypes, and, as expected, later flowering times were associated with larger vegetative size. Later flowering genotypes produced heavier floral parts (larger petals, in particular), and allocated a significantly lower proportion of biomass to androecia, especially in final allocations that included fruit biomass. At least part of this pleiotropic covariation of flowering time and sexual allocation is likely to be mediated by vegetative size and the rate of resource supply to growing reproductive tissues, because the larger fruits of late-flowering genotypes required the same time, or proportionately less time than the difference in biomass, to mature. Because fruit mass is considered an investment in female function, sexual allocation measured at the end of a growing season tends to be highly female biased in angiosperms. We consider the implications of the pleiotropic association of flowering time, vegetative size, and sexual investment for the theory of sex allocation, and suggest that the idiosyncratic phenology of sexual investment in flowering plants creates a departure from a central assumption of Fisher's seminal sex allocation argument.

Genetic variation at microsatellite markers was used to quantify genetic structure and mating behavior in a severely fragmented population of the wind-pollinated, wind-dispersed temperate tree Fraxinus excelsior in a deforested catchment in Scotland. Remnants maintain high levels of genetic diversity, comparable with those reported for continuous populations in southeastern Europe, and show low interpopulation differentiation (Θ = 0.080), indicating that historical gene exchange has not been limited (Nm = 3.48). We estimated from seeds collected from all trees producing fruits in three of five remnants that F. excelsior is predominantly outcrossing (t_m = 0.971 ± 0.028). Use of a neighborhood model approach to describe the relative contribution of local and long-distance pollen dispersal indicates that pollen gene flow into each of the three remnants is extensive (46–95%) and pollen dispersal has two components. The first is very localized and restricted to tens of meters around the mother trees. The second is a long-distance component with dispersal occurring over several kilometers. Effective dispersal distances, accounting for the distance and directionality to mother trees of sampled pollen donors, average 328 m and are greater than values reported for a continuous population. These results suggest that the opening of the landscape facilitates airborne pollen movement and may alleviate the expected detrimental genetic effects of fragmentation.

The Mediterranean land snail genus Mastus (Beck, 1837) is highly divergent. Thirty-two Mastus species have been recorded throughout the genus range, and 23 of them are endemic to the islands of the Aegean Sea and mainland Greece. Of these, all 16 Mastus species reported from Crete are endemic to this island. A robust molecular phylogenetic framework based on mitochondrial and nuclear genes (1623 bp) allowed us to explore the temporal diversification pattern of lineages, using molecular clock approaches. Our results showed an initial radiation in the evolutionary history of the Cretan lineage, followed by a subsequent slowdown of lineage splitting rate. Using a dated major vicariant event of the Aegean area, we estimated the absolute time of the radiation event and proposed a biogeographic scenario accounting for the observed pattern. Additionally, we tried to infer the processes that led to the divergence of the Cretan Mastus species, by applying comparative methods in phylogenetically informated context. Overall, our results favoured a nonecological radiation scenario in the Cretan Mastus species due to an allopatric divergence of secondary sexual characters.

Life-history theory is based on the assumption that evolution is constrained by trade-offs among different traits that contribute to fitness. Such trade-offs should be evident from negative genetic correlations among major life-history traits. However, this expectation is not always met. Here I report the results of a life-table experiment designed to measure the broad-sense heritabilities of life-history traits and their genetic correlations in 19 different clones of the aphid Myzus persicae from Victoria, Australia. Most individual traits, as well as fitness calculated as the finite rate of increase from the life table, exhibited highly significant heritabilities. The pattern of genetic correlations revealed absolutely no evidence for life-history trade-offs. Rather, life histories were arranged along an axis from better to worse. Clones with shorter development times tended to have larger body sizes, higher fecundities, and larger offspring. The fitness of clones estimated from the life table in the laboratory tended to be positively associated with their abundance in the field. Fitness also increased significantly with heterozygosity at the seven microsatellite loci that were used to distinguish clones and estimate their frequencies in the field. I discuss these findings in light of a recent proposition that positive genetic correlations among life-history traits for which trade-offs are expected can be explained by genetic variation for resource acquisition ability that is maintained in populations by a cost of acquisition, and I propose ways to test for such a cost in M. persicae.

Although F₁ female hybrids between Anopheles gambiae and A. arabiensis are fully fertile, sterility is present in backcross females. Here we report the results of a study into the genetic basis of backcross female sterility. Using 23 markers, we performed quantitative trait loci (QTL) mapping analyses to identify chromosomal regions involved in hybrid female sterility. We found that female sterility in backcrosses in both directions is primarily caused by interspecific interactions between a heterozygous X chromosome and recessive autosomal factors. In addition, our data provide support for two theories implicated in Haldane's rule in a single taxon. A comparison with data from a previous study shows that male hybrid sterility QTL are present in higher numbers than female hybrid sterility QTL. Furthermore, autosomal female sterility factors tend to be recessive, supporting the dominance theory for female sterility. Finally, our data indicate a very large effect of the X chromosome from both species on hybrid female sterility, despite the fact that the X chromosome represents less than 9% of the genome. However, this could be the result of a lack of introgression of the X chromosome between A. gambiae and A. arabiensis, rather than a faster evolution of sterility factors on the X chromosome.

Absolute constraints are limitations on genetic variation that preclude evolutionary change in some aspect of the phenotype. Absolute constraints may reflect complete absence of variation, lack of genetic variation that extends the range of phenotypes beyond some limit, or lack of additive genetic variation. This last type of absolute constraint is bidirectional, because the mean cannot evolve to be larger or smaller. Most traits do possess genetic variation, so bidirectional absolute constraints are most likely to be detected in a multivariate context, where they would reflect combinations of traits, or dimensions in phenotype space that cannot evolve. A bidirectional absolute constraint will cause the additive genetic covariance matrix (G) to have a rank less than the number of traits studied. In this study, we estimate the rank of the G-matrix for 20 aspects of wing shape in Drosophila melanogaster. Our best estimates of matrix rank are 20 in both sexes. Lower 95% confidence intervals of rank are 17 for females and 18 for males. We therefore find little evidence of bidirectional absolute constraints. We discuss the importance of this result for resolving the relative roles of selection and drift processes versus constraints in the evolution of wing shape in Drosophila.

Most work on adaptive speciation to date has focused on the role of low hybrid fitness as the force driving reinforcement (the evolution of premating isolation after secondary contact that reduces the likelihood of matings between populations). However, recent theoretical work has shown that postmating, prezygotic incompatibilities may also be important in driving premating isolation. We quantified premating, postmating-prezygotic, and early postzygotic fitness effects in crosses among three populations: Drosophila persimilis, D. pseudoobscura USA (sympatric to D. persimilis), and D. pseudoobscura Bogotá (allopatric to D. persimilis). Interspecific matings were more likely to fail when they involved the sympatric populations than when they involved the allopatric populations, consistent with reinforcement. We also found that failure rate in sympatric mating trials depended on whether D. persimilis females were paired with D. pseudoobscura males or the reverse. This asymmetry most likely indicates differences in discrimination against heterospecific males by females. By measuring egg laying rate, fertilization success and hatching success, we also compared components of postmating-prezygotic and early postzygotic isolation. Postmating-prezygotic fitness costs were small and not distinguishable between hetero- and conspecific crosses. Early postzygotic fitness effects due to hatching success differences were also small in between-population crosses. There was, however, a postzygotic fitness effect that may have resulted from an X-linked allele found in one of the two strains of D. pseudoobscura USA. We conclude that the postmating-prezygotic fitness costs we measured probably did not drive premating isolation in these species. Premating isolation is most likely driven in sympatric populations by previously known hybrid male sterility.

Drosophila polymorpha is a widespread species that exhibits abdominal pigmentation variation throughout its range. To gain insight into this variation we combined phenotypic and genotypic data to test a series of nested hypotheses. First, we tested the null hypothesis that geographic variation in pigmentation is due to neutral factors. We used nested clade analysis to examine the distribution of haplotypes from a nuclear and a mitochrondrial locus. Restricted gene flow via isolation by distance, the primary inference of this phylogeographic analysis, was then used to generate and test the hypothesis of increasing average abdominal pigmentation difference with increasing geographic distance. We found no correlation between geographic distance and phenotypic distance. We then tested the hypothesis that pigmentation is affected by environmental differences among localities. We found a significant effect of habitat type on the average abdominal pigmentation phenotype of different localities. Finally, we tested the hypothesis that pigmentation in D. polymorpha is associated with desiccation resistance. We found that dark individuals of both sexes survived significantly longer in a desiccating environment than light individuals. These patterns combined lead us to hypothesize that abdominal pigmentation variation in D. polymorpha is important in mediating the organism's interactions with local ecological factors.

Both ornaments and weapons of sexual selection frequently exhibit prolific interspecific diversity of form. Yet, most studies of this diversity have focused on ornaments involved with female mate choice, rather than on the weapons of male competition. With few exceptions, the mechanisms of divergence in weapon morphology remain largely unexplored. Here, we characterize the evolutionary radiation of one type of weapon: beetle horns. We use partial sequences from four nuclear and three mitochondrial genes to develop a phylogenetic hypothesis for a worldwide sample of 48 species from the dung beetle genus Onthophagus (Coleoptera: Scarabaeidae). We then use these data to test for multiple evolutionary origins of horns and to characterize the evolutionary radiation of horns. Although our limited sampling of one of the world's most species-rich genera almost certainly underestimates the number of evolutionary events, our phylogeny reveals prolific evolutionary lability of these exaggerated sexually selected weapons (more than 25 separate gains and losses of five different horn types). We discuss these results in the context of the natural history of these beetles and explore ways that sexual selection and ecology may have interacted to generate this extraordinary diversity of weapon morphology.

In Georgia (USA) the soldier beetle, Chauliognathus pennsylvanicus (Coleoptera; Cantharidae), exhibits clinal variation in the length of the spot on its elytron. This suggests that the viability of phenotypes varies by habitat. Evidence of viability selection comes from within-site changes in the spot length distribution across a breeding season. When males with spots of intermediate length became less frequent, they became disproportionately less likely to mate, consistent with either a loss of vigor among remaining males or female rejection of disfavored phenotypes. Persistent, daily courtship by males provides females with the opportunity to track changes in male phenotype frequency and to exercise choice for phenotypes favored under natural selection. A laboratory experiment in which the frequency of one spot morph (long) or the other (short) was increased from 25% to 75% over a period of 30 days revealed that females possess a flexible preference that leads them to prefer whichever spot type has become more common over time. A haploid genetic model demonstrates that a flexible female preference for the locally favored male phenotype can be selected for when different viability alleles, genetically correlated with the male trait, are favored in different habitats that are linked by gene flow. Thus, migration between different kinds of habitat patches of a metapopulation could maintain the variation in male quality. This variation favors female choice for any trait that is directly or indirectly favored by natural selection. Such choice imparts positive frequency-dependent selection that could rapidly fix traits pleiotropically linked to viability. Rapid fixation would cause differentiation between populations of colonizing species as females exercise choice for mates favored under new ecological conditions.

Current methods for measuring selection with longitudinal data have been developed with the assumption that episodes of selection are sequential. However, a number of empirical examinations have demonstrated that natural and sexual selection may act concurrently and in opposing directions. Other recent work has highlighted the difficulty of assigning fitness values for survival when reproduction and mortality within a population temporally overlap. I treat these as facets of a single problem; how to analyze selection where mortality and reproduction are concurrent. To address this problem, I formalize a method to estimate total fitness of individuals over a period of time utilizing longitudinal data. I then show how the fitness may be partitioned to provide two separate estimates of fitness for reproductive opportunity and reproductive success. In addition, another total fitness estimate for the period can be obtained from the two partitioned estimates. This procedure will allow calculation of total fitness where there are some missing datapoints for reproductive success of an individual. A simulation indicates that bias is generally low for the various fitness estimates. These methods should expand our ability to understand the interaction of different selection episodes.

Concurrent natural and sexual selection have been inferred from laboratory and comparative studies in a number of taxa, but are rarely measured in natural populations. Because the interaction of these two general categories of selection may be complex when they occur simultaneously, empirical evidence from natural populations would help us to understand this interaction and probably give us greater insight into each separate episode as well. In male sockeye salmon, sexual selection for larger body size has been indicated in both deep and shallow water habitats. However, in shallow habitats male sockeye are generally smaller and less deep-bodied than in deep habitats, a difference that has been ascribed to natural selection. We measured concurrent natural and sexual selection in two years on breeding male sockeye salmon with respect to body size, body shape, and time of arrival to the breeding grounds. Natural selection was variable in effect and sexual selection was variable in intensity in these two years. The patterns of selection also appear to be interdependent; areas where predation on spawning adults is not intense have yielded different patterns of sexual selection than those measured here. It appears that some of the body shape differences in sockeye salmon associated with different spawning habitats, which were previously attributed to selective mortality, may be a result of different patterns of sexual selection in the different habitats. Total selection resulting from the combination of both natural and sexual selection was less intense than either natural or sexual selection in most cases. Measurement of concurrent selection episodes in nature may help us to understand whether the pattern of differential sexual selection is common, and whether observed patterns of habitat-related differentiation may be due to differences in sexual selection.

Lineages that underwent rapid cladogenesis are attractive systems for the study of mechanisms underlying taxonomic, ecological, morphological, and behavioral diversification. Recently developed statistical methods provide insights into historical patterns of diversity and allow distinguishing bursts of cladogenesis from stochastic background rates in the presence of confounding factors such as extinction and incomplete taxon sampling. Here, we compare the dynamics of speciation in several marine fish lineages some of which were previously proposed to have undergone significant changes of cladogenesis through time. We tested for evidence of episodes of rapid cladogenesis using the constant rate and Monte Carlo constant rate tests that are robust to incomplete taxon sampling. These tests employ the statistic γ to measure the relative position of internal node in a chronogram. For the first time, we conducted a comparative analysis to address the behavior of the statistic under different chronogram-constructing methods (Langley-Fitch, nonparametric rate smoothing, and penalized likelihood). Although estimates of γ sometimes differ widely among methods, acceptance or rejection of the constant rate model within a particular clade appears to be robust to the choice of method. Bursts of cladogenesis were detected in 14 of 34 studied datasets. Some of these were previously proposed to represent marine fish “radiations,” whereas others are identified anew. Our results indicate that the wider application of tree shape methods that are able to detect significantly elevated rates of speciation is useful to more precisely define clades that underwent episodes of rapid cladogenesis in marine fish clades. Contrasting the patterns of phylogenetic diversification in marine fish lineages may facilitate the identification of common evolutionary trajectories versus idiosyncrasies, and ultimately help towards a better understanding of the factors and processes underlying speciation in the marine realm.

New World monkeys (NWM) display substantial variation (two orders of magnitude) in body size. Despite this, variation in skull size and associated shape show a conserved allometric relationship, both within and between genera. Maximum likelihood estimates of quantitative ancestral states were used to compare the direction of morphological differentiation with the phenotypic (p_max) and genetic (g_max) lines of least evolutionary resistance (LLER). Diversification in NWM skulls occurred principally along the LLER defined by size variation. We also obtained measures of morphological amount and pace of change using our skull data together with published genetic distances to test whether the LLER influenced the amount and pace of diversification. Moreover, data on an ecological factor (diet) was obtained from the literature and used to test the association of this niche-related measure with the morphological diversification. Two strategies were used to test the association of LLER with the morphological and dietary amount and pace of change, one focusing on both contemporary genera and maximum likelihood reconstructed ancestors and the other using only the 16 contemporary genera in a phylogenetic comparative analysis. Our results suggest that the LLER influenced the path, amount, and pace of morphological change. Evolution also occurred away from the LLER in some taxa but this occurred at a slower pace and resulted in a relatively low amount of morphological change. We found that longer branch lengths (time) are associated with larger differences in p_max orientation. However, on a macroevolutionary scale there is no such trend. Diet is consistently associated with both absolute size differences and morphological integration patterns, and we suggest that this ecological factor might be driving adaptive radiation in NWM. Invasion of diet-based adaptive zones involves changes in absolute size, due to metabolic and foraging constraints, resulting in simple allometric skull diversification along the LLER. While it is clear that evolutionary change occurred along the LLER, it is not clear whether this macroevolutionary pattern results from a conservation of within-population genetic covariance patterns or long-term adaptation along a size dimension or whether both constraints and selection were inextricably involved.

Seed production in many plants is pollen limited, likely because of unpredictable variation in the pollinator environment. One way for plants to escape the consequences of pollinator variability is to evolve mating systems, such as autonomous selfing, that assure reproduction without relying on pollinators. We explore this hypothesis through the construction and analysis of heuristic models of plant population dynamics in seed- or site-limited populations. Our analysis suggests several important points: the familiar rule that inbreeding depression greater than 0.5 maintains outcrossing significantly underestimates the threshold required under pollen limited conditions with prior selfing; variability in the pollination environment erodes the ability of inbreeding depression to maintain outcrossing; and variable pollination environments can result in stable intermediate rates of prior selfing. The results reflect the importance of geometric mean fitness (which in a variable environment is less than the arithmetic mean) in the face of temporal variation.

Divergent natural selection contributes to reproductive isolation among populations adapting to different habitats or resources if hybrids between populations are intermediate in phenotype and suffer an associated, environmentally dependent reduction in fitness. This prediction was tested using two host races of Mitoura butterflies. Thirty-five F₁ hybrid and parental lines were created, larvae were raised on the two host plants, and oviposition preferences were assayed in choice arenas. Larvae from both reciprocal hybrid crosses suffered a host-specific reduction in performance: when reared on incense cedar, hybrid survival was approximately 30% less than the survival of pure lines of the cedar-associated host race. The performance of hybrid larvae reared on the other host, MacNab cypress, was not reduced relative to parental genotypes. Females from both reciprocal hybrid crosses preferred to oviposit on incense cedar, the same host that resulted in the reduced survival of hybrid larvae. Thus, dominance is implicated in the inheritance of traits involved in both preference and performance, which do not appear to be genetically linked in Mitoura butterflies. Gene flow between host races may be reduced because the correlation between preference and performance that was previously described in parental populations is essentially broken by hybridization.