We study the spatial adaptive dynamics of a continuous trait that measures individual investment in altruism. Our study is based on an ecological model of a spatially heterogeneous population from which we derive an appropriate measure of fitness. The analysis of this fitness measure uncovers three different selective processes controlling the evolution of altruism: the direct physiological cost, the indirect genetic benefits of cooperative interactions, and the indirect genetic costs of competition for space. In our model, habitat structure and a continuous life cycle makes the cost of competing for space with relatives negligible. Our study yields a classification of adaptive patterns of altruism according to the shape of the costs of altruism (with decelerating, linear, or accelerating dependence on the investment in altruism). The invasion of altruism occurs readily in species with accelerating costs, but large mutations are critical for altruism to evolve in selfish species with decelerating costs. Strict selfishness is maintained by natural selection only under very restricted conditions. In species with rapidly accelerating costs, adaptation leads to an evolutionarily stable rate of investment in altruism that decreases smoothly with the level of mobility. A rather different adaptive pattern emerges in species with slowly accelerating costs: high altruism evolves at low mobility, whereas a quasi-selfish state is promoted in more mobile species. The high adaptive level of altruism can be predicted solely from habitat connectedness and physiological parameters that characterize the pattern of cost. We also show that environmental changes that cause increased mobility in those highly altruistic species can beget selection-driven self-extinction, which may contribute to the rarity of social species.

Explaining the uneven distribution of species among lineages is one of the oldest questions in evolution. Proposed correlations between biological traits and species diversity are routinely tested by making comparisons between phylogenetic sister clades. Several recent studies have used nested sister-clade comparisons to test hypotheses linking continuously varying traits, such as body size, with diversity. Evaluating the findings of these studies is complicated because they differ in the index of species richness difference used, the way in which trait differences were treated, and the statistical tests employed. In this paper, we use simulations to compare the performance of four species richness indices, two choices about the branch lengths used to estimate trait values for internal nodes and two statistical tests under a range of models of clade growth and character evolution. All four indices returned appropriate Type I error rates when the assumptions of the method were met and when branch lengths were set proportional to time. Only two of the indices were robust to the different evolutionary models and to different choices of branch lengths and statistical tests. These robust indices had comparable power under one nonnull scenario. Regression through the origin was consistently more powerful than the t-test, and the choice of branch lengths exerts a strong effect on both the validity and power. In the light of our simulations, we re-evaluate the findings of those who have previously used nested comparisons in the context of species richness. We provide a set of simple guidelines to maximize the performance of phylogenetically nested comparisons in tests of putative correlates of species richness.

We extracted bacterial isolates of similar colony morphology from spatially located soil samples within 1 ha of old-growth forest. The same soil samples were used to prepare growth medium. Each isolate was then cultured in each medium and its growth recorded. There was no overall tendency for isolates to grow more successfully in their home site (i.e., the medium derived from the soil sample from which they had been extracted). Most isolates grew very poorly, however, and when the analysis was restricted to the minority of vigorous isolates there was clear evidence of local adaptation: isolates tended to grow better at their home site than did isolates from elsewhere and grew better at their home site than they did at other sites. The variation of growth within the 1-ha plot made up a complex fitness landscape of peaks, ridges, and valleys. Most of the vigorous isolates were found at or near a local fitness (growth) peak, although seldom at a global peak. In consequence, there was a tendency for growth to diminish away from the home site. The home isolate was about 50% more fit than average at its home site; fitness diminished exponentially away from the home site at a rate of 0.0577 per meter. These figures are similar to those previously reported for plants. This selection gradient has matched the bacterial assemblage to the edaphic structure of the environment, although the fit is far from perfect.

Host shifts of plant-feeding insects and parasites promote adaptational changes that may result in the formation of host races, an assumed intermediate stage in sympatric speciation. Here, we report on genetically differentiated and host-adapted races of the fungal endophyte Epichloë bromicola, which presumably emerged after a shift from the grass Bromus erectus to other Bromus hosts. Fungi of the genus Epichloë (Ascomycota) and related anamorphs of Neotyphodium are widespread endophytes of cool-season grasses. Sexually reproducing strains sterilize the host by formation of external fruiting structures (stromata), whereas asexual strains are asymptomatic and transmitted via seeds. In E. bromicola, strains infecting B. erectus are sexual, and strains from two woodland species, B. benekenii and B. ramosus, are asexual and seed transmitted. Analyses of amplified fragment length polymorphism fingerprinting and of intron sequences of the tub2 and tef1 genes of 26 isolates from the three Bromus hosts collected at natural sites in Switzerland and nearby France demonstrated that isolates are genetically differentiated according to their host, indicating that E. bromicola does not form a single, randomly mating population. Phylogenetic analyses of sequence data did not unambiguously resolve the exact origin of asexual E. bromicola strains, but it is likely they arose from within sexual populations on B. erectus.

Incongruence of trees derived from different genes may have resulted from recombination at some time in the recent history of host strains. Reciprocal inoculations of host plant seedlings showed that asexual isolates from B. benekenii and B. ramosus were incapable of infecting B. erectus, whereas the sexual isolates from B. erectus retained the assumed ancestral trait of broad compatibility with Bromus host seedlings. Because all isolates were interfertile in experimental crosses, asexual strains may not be considered independent biological species. We suggest that isolates infecting B. benekenii and B. ramosus represent long-standing host races or incipient species that emerged after host shifts and that may evolve through host-mediated reproductive isolation toward independent species.

A molecular phylogenetic study of the plant genus Coreocarpus was conducted using nuclear (ITS) and plastid (rpl16 intron) DNA sequences, with phylogenies of the nuclear and plastid sequences highly congruent in defining a monophyletic group of six species (core Coreocarpus), although three other species often placed within the genus were excluded. Relationships within the genus are largely but not totally concordant with prior biosystematic studies. Despite strong molecular support, no morphological characters uniting the six species of core Coreocarpus have been identified; retention of plesiomorphic characters and the genetic lability of characters are two probable factors contributing to lack of consistent defining characters. The age of the core Coreocarpus is estimated at 1 million years because the basal species is endemic to a volcanic island that emerged in the past million years. Mapping the results of earlier breeding studies on the molecular phylogeny showed that use of cross-compatibility as a criterion for species delimitation would result in the recognition of paraphyletic species. Prior field, morphological, and biosystematic studies provided no indication of past hybridization in the evolution of Coreocarpus, and species in the genus appeared to be well defined morphologically. However, three instances of incongruence were observed. Two of these were between the nuclear and plastid partitions, and the third was between the morphological species assignment of one accession and the molecular data. If hybridization accounts for incongruence between the nuclear and plastid data, it occurred between species that now appear to be cross-incompatible and allopatric. The incongruence between morphological species assignment and the molecular data could be the result of parallel fixation of characters that have a simple genetic basis. This study suggests that the evolutionary history of Coreocarpus is much more complex than indicated from prior biosystematic investigations and that biosystematic and molecular phylogenetic studies may complement each other for elucidating the evolution and phylogeny of a group.

Camellia japonica L. (Theaceae), an insect- and bird-pollinated, broad-leaved evergreen tree, is widely distributed in Japan and the southern Korean peninsula. The species has a relatively even age distribution within populations, which may influence the spatial genetic structure of different age classes relative to species with typical L-shaped age distributions. To determine whether the internal spatial genetic structure found in seedlings and young individuals carries over into adults, we used allozyme loci, F-statistics, spatial autocorrelation statistics (Moran's I), and coancestry measures to examine changes in genetic structure among seven age classes in a population (60-m × 100-m area) in southern Korea. In seedlings, weak but significant positive values of Moran's I-statistics and coancestry measures were found for distances less than 14 m, which is consistent with a mechanism of limited seed dispersal combined with overlapping seed shadows. This spatial structure, however, dissipates in older age classes, and in adults genetic variation has an essentially random spatial distribution. Morisita's index of dispersion of individuals in each age class showed that seedlings and juveniles are more highly clustered than are older individuals. These results suggest that self-thinning changes the spatial relationships of individuals, and thus genotypes. A multilocus estimate of F_ST (0.008) shows a small but statistically significant difference in allele frequencies among age classes. In summary, intrapopulation genetic structure within and among age classes of C. japonica was significant but weak. Despite presumably limited seed dispersal, weak spatial genetic structure in juveniles suggests overlapping seed shadows followed by self-thinning during recruitment. The present study also demonstrates that studies of spatial genetic structure focusing on limited numbers of generations may not be sufficient to reveal the entire picture of genetic structure in populations with overlapping generations.

Plants of Lycium californicum, L. exsertum, and L. fremontii produce flowers that are either male-sterile (female) or hermaphroditic, and populations are morphologically gynodioecious. As is commonly found in gynodioecious species, flowers on female plants are smaller than those on hermaphrodites for a number of floral traits. Floral size dimorphism has often been hypothesized to be the result of either a reduction in female flower size that allows reallocation to greater fruit and seed production, or an increase in hermaphroditic flower size due to the increased importance of pollinator attraction and pollen export for hermaphroditic flowers. We provide a test of these two alternatives by measuring 11 floral characters in eight species of Lycium and using a phylogeny to reconstruct the floral size shifts associated with the evolution of gender dimorphism. Our analyses suggest that female flowers are reduced in size relative to the ancestral condition, whereas flowers on hermaphrodites have changed only slightly in size. Female and hermaphroditic flowers have also diverged both from one another and from ancestral cosexual species in several shape characteristics. We expected sexual dimorphism to be similar among the three dimorphic taxa, as gender dimorphism evolved only a single time in the ancestor of the American dimorphic lineage. While the floral sexual dimorphism is broadly similar among the three dimorphic species, there are some species-specific differences. For example, L. exsertum has the greatest floral size dimorphism, whereas L. fremontii had the greatest size-independent dimorphism in pistil characters. To determine the degree to which phylogenetic uncertainty affected reconstruction of ancestral character states, we performed a sensitivity analysis by reconstructing ancestral character states on alternative topologies. We argue that investigations such as this one, that examine floral evolution from an explicitly phylogenetic perspective, provide new insights into the study of the evolution of floral sexual dimorphism.

Morphological divergence among species may be constrained by the pattern of genetic variances and covariances among traits within species. Assessing the existence of such a relationship in nature requires information on the stability of intraspecific correlation and covariance structure and the correspondence of this structure to the pattern of evolutionary divergence within a lineage. Here, we investigate these issues for nine morphological traits and 15 species of stalk-eyed flies in the genus Diasemopsis. Within-species matrices for these traits were generated from phenotypic data for all the Diasemopsis species and from genetic data for a single Diasemopsis species, D. dubia. The among-species pattern of divergence was assessed by calculating the evolutionary correlations for all pairwise combinations of the morphological traits along the phylogeny of these species. Comparisons of intraspecific matrices reveal significant similarity among all species in the phenotypic correlations matrices but not the covariance matrices. In addition, the differences in correlation structure that do exist among species are not related to their phylogenetic placement or change in the means of the traits. Comparisons of the phenotypic and phylogenetic matrices suggest a strong relationship between the pattern of evolutionary change among species and both the intraspecific correlation structure and the stability of this structure among species. The phenotypic and the phylogenetic matrices are significantly similar, and pairs of traits whose intraspecific correlations are more stable across taxa exhibit stronger coevolution on the phylogeny. These results suggest either the existence of strong constraints on the pattern of evolutionary change or a consistent pattern of correlated selection shaping both the phenotypic and phylogenetic matrices. The genetic correlation structure for D. dubia, however, does not correspond with patterns found in the phenotypic and phylogenetic data. Possible reasons for this disagreement are discussed.

Fish occupy a range of hydrological habitats that exert different demands on locomotor performance. We examined replicate natural populations of the rainbow fishes Melanotaenia eachamensis and M. duboulayi to determine if colonization of low-velocity (lake) habitats by fish from high-velocity (stream) habitats resulted in adaptation of locomotor morphology and performance. Relative to stream conspecifics, lake fish had more posteriorly positioned first dorsal and pelvic fins, and shorter second dorsal fin bases. Habitat dimorphism observed between wild-caught fish was determined to be heritable as it was retained in M. eachamensis offspring raised in a common garden. Repeated evolution of the same heritable phenotype in independently derived populations indicated body shape divergence was a consequence of natural selection. Morphological divergence between hydrological habitats did not support a priori expectations of deeper bodies and caudal peduncles in lake fish. However, observed divergence in fin positioning was consistent with a family-wide association between habitat and morphology, and with empirical studies on other fish species. As predicted, decreased demand for sustained swimming in lakes resulted in a reduction in caudal red muscle area of lake fish relative to their stream counterparts. Melanotaenia duboulayi lake fish also had slower sustained swimming speeds (U_crit) than stream conspecifics. In M. eachamensis, habitat affected U_crit of males and females differently. Specifically, females exhibited the pattern observed in M. duboulayi (lake fish had faster U_crit than stream fish), but the opposite association was observed in males (stream males had slower U_crit than lake males). Stream M. eachamensis also exhibited a reversed pattern of sexual dimorphism in U_crit (males slower than females) relative to all other groups (males faster than females). We suggest that M. eachamensis males from streams responded to factors other than water velocity. Although replication of muscle and U_crit phenotypes across same habitat populations within and/or among species was suggestive of adaptation, the common garden experiment did not confirm a genetic basis to these associations. Kinematic studies should consider the effect of the position and base length of dorsal fins.

The painted turtle, Chrysemys picta, is currently recognized as a continentally distributed polytypic species, ranging across North America from southern Canada to extreme northern Mexico. We analyzed variation in the rapidly evolving mitochondrial control region (CR) in 241 turtles from 117 localities across this range to examine whether the painted turtle represents a continentally distributed species based on molecular analysis. We found strong support for the novel hypothesis that C. p. dorsalis is the sister group to all remaining Chrysemys, with the remaining Chrysemys falling into a single, extremely wide-ranging and genetically undifferentiated species. Given our goal of an evolutionarily accurate taxonomy, we propose that two evolutionary lineages be recognized as species within Chrysemys: C. dorsalis (Agassiz 1857) in the southern Mississippi drainage region, and C. picta (Schneider 1783) from the rest of the range of the genus. Neither molecular nor recent morphological analyses argue for the hybrid origin of C. p. marginata as previously proposed. Within C. picta, we find evidence of at least two independent range expansions into previously glaciated regions of North America, one into New England and the other into the upper Midwest. We further find evidence of a massive extinction/recolonization event across the Great Plains/Rocky Mountain region encompassing over half the continental United States. The timing and extent of this colonization is consistent with a recently proposed regional aridification as the Laurentide ice sheets receded approximately 14,000 years ago, and we tentatively propose this paleoclimatological event as a major factor shaping genetic variation in Chrysemys.

We used partial sequences of the cytochrome b mitochondrial DNA (mtDNA) gene, obtained from 76 individuals representing 45 populations of Iberian Salamandra salamandra plus 15 sequences of additional species of Salamandra and related genera, to investigate contact zones. These zones, identified by earlier allozymic and morphological analyses, are between populations of viviparous (S. s. bernardezi and S. s. fastuosa) and ovoviviparous (S. s. gallaica and S. s. terrestris) salamanders. The distribution of mtDNA and nuclear markers is mostly concordant at one contact zone (between S. s. gallaica and S. s. bernardezi), but at another (between S. s. fastuosa and S. s. terrestris) the markers are offset by about 250 km. The observed geographic variation fits a model of mtDNA capture. Among the potential mechanisms responsible for such discordance we favor a combination of range shifts due to climatic fluctuations and biased genetic admixture across moving contact zones. We apply our findings to the issue of possible homoplasy in the evolution of viviparity and conclude that viviparity likely arose only once within S. salamandra.

Carotenoids cannot be synthesized by birds and thus have to be ingested with food, suggesting that carotenoid-based plumage coloration is environmentally determined. However signaling functions ascribed to plumage imply that plumage coloration is the outcome of an evolutionary process based on genetic variation. By means of a cross-fostering design we show significant effects of both a common rearing environment and the brood from which a nestling originally came from (common origin) on the plumage coloration of nestling great tits (Parus major). This demonstration of origin-related variation in carotenoid-based plumage coloration suggests that the observed variation of the trait has a partial genetic basis. Consistent with environmental determination of this trait, we also found a significant positive correlation between the color saturation of nestlings and their foster-father's plumage. There was no significant correlation between nestling plumage coloration and the food quantity provided to the nestlings by the male, the female, or both parents. This suggests that the nestling-foster father correlation arises by the carotenoid quantity ingested rather than the food quantity per se. No significant nestling-true father correlation was found, which suggests that nestling plumage coloration did not indirectly evolve due to sexual selection. Consistent with this result there was no significant correlation between the nestling's plumage color and its coloration as a breeding adult the following year, suggesting that nestling plumage color is a different trait than the first year plumage.

A simple, deterministic analysis predicts that accumulation of Dobzhansky-Muller incompatibilities by a spatially structured population strongly depends on the number of negative interactions of an allele. If an allele can be incompatible with alleles at only one locus, incompatibilities accumulate linearly with time. In contrast, if an allele can participate in multiple pairwise incompatibilities with alleles at different loci, the expected number of incompatibilities eventually increases quadratically.

We employ a simple model to show that social selection can lead to prezygotic reproductive isolation. The evolution of social discrimination causes the congealing of phenotypically similar individuals into different, spatially distinct tribes. However, tribal formation is only obtained for certain types of social behavior: altruistic and selfish acts can produce tribes, whereas spiteful and mutualistic behaviors never do. Moreover, reduced hybrid fitness at tribal borders leads to the selection of mating preferences, which then spread to the core areas of the respective tribes. Unlike models of resource competition, our model generates reproductive isolation in an ecologically homogeneous environment. We elaborate on how altruistic acts can lead to reproductive isolation, but also predict that certain types of competition can lead to the speciation effect. Our theory provides a framework for how individual-level interactions mold lineage diversification, with parapatric speciation as a possible end product.

Transposable elements (TEs), which promote various kinds of mutations, constitute a large fraction of the genome. How they invade natural populations and species is therefore of fundamental importance for understanding the dynamics of genetic diversity and genome composition. On the basis of 85 samples of natural populations of Drosophila simulans, we report the distributions of the genome insertion site numbers of nine TEs that were chosen because they have a low average number of sites. Most populations were found to have 0–3 insertion sites, but some of them had a significantly higher number of sites for a given TE. The populations located in regions outside Africa had the highest number of sites for all elements except HMS Beagle and Coral, suggesting a recent increase in the activity of some TEs associated with the colonization patterns of Drosophila simulans. The element Tirant had a very distinctive pattern of distribution: it was identified mainly in populations from East Africa and some islands in the Indian Ocean, and its insertion site number was low in all these populations. The data suggest that the genome of the entire species of Drosophila simulans may be being invaded by TEs from populations in which they are present in high copy number.

We show that a predator, the tumbling flower beetle Mordellistena convicta (Coleoptera: Mordellidae), has formed host races in response to a host-plant shift and subsequent host-race formation by its prey, the gall-inducing fly Eurosta solidaginis (Diptera: Tephritidae). This fly has formed two host races, one that induces stem galls on the ancestral host plant, Solidago altissima (Compositae), and another that induces stem galls on the closely related S. gigantea. We found that subpopulations of M. convicta that attack E. solidaginis galls on the different host plants have significantly different emergence times and, although slight, these allochronic differences are consistent across a range of temperatures. More importantly, we found that beetles assortatively mate according to their natal host plants, and female M. convicta preferentially attack and/or their offspring have higher survival in galls on natal host plants. Our data suggest that subpopulations of M. convicta that attack E. solidaginis galls on S. altissima and S. gigantea have formed host races. This is one of the first studies to demonstrate that a host shift and subsequent host-race formation by an herbivorous insect may have resulted in subsequent diversification by one of its natural enemies.

Although sperm competition is a pervasive selective force shaping the reproductive tactics of males, the mechanisms underlying different patterns of sperm precedence remain obscure. Parker et al. (1990) developed a series of linear models designed to identify two of the more basic mechanisms: sperm lotteries and sperm displacement; the models can be tested experimentally by manipulating the relative numbers of sperm transferred by rival males and determining the paternity of offspring. Here we show that tests of the model derived for sperm lotteries can result in misleading inferences about the underlying mechanism of sperm precedence because the required inverse transformations may lead to a violation of fundamental assumptions of linear regression. We show that this problem can be remedied by reformulating the model using the actual numbers of offspring sired by each male, and log-transforming both sides of the resultant equation. Reassessment of data from a previous study (Sakaluk and Eggert 1996) using the corrected version of the model revealed that we should not have excluded a simple sperm lottery as a possible mechanism of sperm competition in decorated crickets, Gryllodes sigillatus.

The fitness costs of high genetic load in wild populations have rarely been assessed under natural conditions. Such costs are expected to be greatest in small, bottlenecked populations, including those occurring near range edges. Britain is at the northwesterly range limit of the natterjack toad Bufo calamita. We compared fitness attributes in two populations of this amphibian with very different recent histories. Key larval fitness attributes in B. calamita, notably growth rate and metamorph production, were substantially higher in the large outbreeding population (Ainsdale) than in the small and isolated one (Saltfleetby). These differences were manifest under seminatural conditions, when larvae were reared in mesh cages within breeding ponds at the site of the small population, and were exacerbated by high stress treatments. The results indicate that genetic load effects can be sufficiently severe enough to predispose extinction over relatively short time frames, as predicted by extinction vortex models.

Palumbi et al. (2001) proposed a “three-times rule” that uses mitochondrial DNA (mtDNA) sequences to predict probabilities of monophyly for nuclear loci (i.e., whether the alleles within a taxon coalesce with one another before they coalesce with alleles from a sister taxon). They use neutral coalescent theory to infer these probabilities from the ratio of interspecific divergence to intraspecific variation of mtDNA. We show that the estimated probabilities have very wide confidence intervals because of the inherent stochasticity of the mtDNA coalescent process. Under neutrality, the true probability of monophyly can be much higher, or much lower, than predicted by the three-times rule. We also review recent empirical and theoretical studies that refute neutrality-based predictions concerning mtDNA variation and divergence. We conclude that the three-times rule is neither a useful test for neutral molecular evolution nor a reliable guide to genealogical species.