Body size, sexual size dimorphism, and Rensch's rule in adult hydropsychid caddisflies (Trichoptera: Hydropsychidae)

Body size influences most biological processes from metabolic rates to the outcome of interspecific interactions. Within a species, sexual size dimorphism (SSD) reflects either differential selection on body size of males and females or phylogenetic inertia. Among taxa, SSD should decrease as body size increases when females are the larger sex — a pattern known as Rensch's rule. We examined body size, SSD, and Rensch's rule among 29 species of adult hydropsychid caddisflies (Trichoptera: Hydropsychidae) and 12 closely related caddisfly species. Females were almost always larger than males in all species examined. Body size variation among genera was greater than variance among species. In contrast, the greatest variance in SSD was among species within a genus. Contrary to Rensch's rule, the degree of SSD did not change as body size increased among genera. Observed body size patterns suggest that hydropsychid caddisfly species within a genus may be subjected to similar selective pressures during the larval stage, but this issue remains to be investigated. In addition, our data suggest that hydropsychids may violate Rensch's rule, a pattern not often reported. Our data provide a basis for proposing and testing hypotheses about the ecology and evolution of hydropsychid caddisflies.

SEXUAL DIMORPHISM AND ADAPTIVE SPECIATION: TWO SIDES OF THE SAME ECOLOGICAL COIN

Models of adaptive speciation are typically concerned with demonstrating that it is possible for ecologically driven disruptive selection to lead to the evolution of assortative mating and hence speciation. However, disruptive selection could also lead to other forms of evolutionary diversification, including ecological sexual dimorphisms. Using a model of frequency-dependent intraspecific competition, we show analytically that adaptive speciation and dimorphism require identical ecological conditions. Numerical simulations of individual-based models show that a single ecological model can produce either evolutionary outcome, depending on the genetic independence of male and female traits and the potential strength of assortative mating. Speciation is inhibited when the genetic basis of male and female ecological traits allows the sexes to diverge substantially. This is because sexual dimorphism, which can evolve quickly, can eliminate the frequency-dependent disruptive selection that would have provided the impetus for speciation. Conversely, populations with strong assortative mating based on ecological traits are less likely to evolve a sexual dimorphism because females cannot simultaneously prefer males more similar to themselves while still allowing the males to diverge. This conflict between speciation and dimorphism can be circumvented in two ways. First, we find a novel form of speciation via negative assortative mating, leading to two dimorphic daughter species. Second, if assortative mating is based on a neutral marker trait, trophic dimorphism and speciation by positive assortative mating can occur simultaneously. We conclude that while adaptive speciation and ecological sexual dimorphism may occur simultaneously, allowing for sexual dimorphism restricts the likelihood of adaptive speciation. Thus, it is important to recognize that disruptive selection due to frequency-dependent interactions can lead to more than one form of adaptive splitting.

Divergent selection maintains adaptive differentiation despite high gene flow between sympatric rainbow smelt ecotypes (Osmerus mordax Mitchill)

In this study, we investigate the relative role of historical factors and evolutionary forces in promoting population differentiation in a new case of sympatric dwarf and normal ecotypes of the rainbow smelt (Osmerus mordax Mitchill) in Lac Saint-Jean (Québec, Canada). Our first objective was to test the hypothesis that the evolution of sympatric smelt ecotypes in Lac Saint-Jean has been contingent upon the secondary contact between two evolutionary lineages in postglacial times. Secondly, the QST method was applied to test the null hypothesis that the extent of phenotypic differences relative to that of neutral marker variation would be similar in comparisons involving populations within and among ecotypes. Thirdly, we applied a quantitative-genetic method as an exploratory assessment as to whether the amount of gene flow observed between populations could affect divergence in adaptive traits under specific conditions. This study revealed a unique situation of dwarf and normal smelt ecotypes that are, respectively, characterized by selmiparous and iteroparous life histories and the occurrence in each of two genetically distinct populations that synchronously use the same spawning habitat in two tributaries. Historical contingency has apparently played little role in the origin of these populations. In contrast, an important role of divergent natural selection in driving their phenotypic divergence was suggested. While divergent selection has apparently been strong enough to maintain phenotypic differentiation in the face of migration, this study suggests that gene flow has been sufficiently important to modulate the extent of adaptive differentiation being achieved between ecotypes, unless the extent of stabilizing selection acting on smelt ecotypes is much more pronounced than usually reported in natural populations.

The ecology of individuals: incidence and implications of individual specialization

Most empirical and theoretical studies of resource use and population dynamics treat conspecific individuals as ecologically equivalent. This simplification is only justified if interindividual niche variation is rare, weak, or has a trivial effect on ecological processes. This article reviews the incidence, degree, causes, and implications of individual-level niche variation to challenge these simplifications. Evidence for individual specialization is available for 93 species distributed across a broad range of taxonomic groups. Although few studies have quantified the degree to which individuals are specialized relative to their population, between-individual variation can sometimes comprise the majority of the population's niche width. The degree of individual specialization varies widely among species and among populations, reflecting a diverse array of physiological, behavioral, and ecological mechanisms that can generate intrapopulation variation. Finally, individual specialization has potentially important ecological, evolutionary, and conservation implications. Theory suggests that niche variation facilitates frequency-dependent interactions that can profoundly affect the population's stability, the amount of intraspecific competition, fitness-function shapes, and the population's capacity to diversify and speciate rapidly. Our collection of case studies suggests that individual specialization is a widespread but underappreciated phenomenon that poses many important but unanswered questions.

REGULAR ARTICLES / ARTICLES RÉGULIERSSize-related predation reduces intramorph competition in paedomorphic Alpine newts

Evolutionary theory assumes that facultative paedomorphosis in newts and salamanders is adaptive in allowing either a younger age at maturity or resource partitioning between the heterochronic morphs. In newt populations that only take the metamorphic ontogenetic pathway, juveniles are terrestrial and avoid food competition with larvae and breeding adults. In contrast, in populations where paedomorphosis occurs, branchiate newts of all sizes coexist in the aquatic habitats, posing the question of whether intramorph competition exists and its relationship with the evolution of paedomorphosis. We studied size-related predation in such a size-structured community of branchiate Alpine newts (Triturus alpestris) inhabiting a deep alpine lake. Although gape limitation may explain such size-related predation, individuals also exhibited selectivity according to prey size. Amongst small prey that were within the capture range of all newt size classes, smaller newts preyed on smaller items than did larger ones. We assume that such decisions favour the coexistence of different-sized individuals. It is suspected that such size-selective predation on items which are avoided by water-living metamorphs allows the maintenance of facultative paedomorphosis, in favouring resource partitioning between morphs.

Heterochrony, maternal effects, and phenotypic variation among sympatric pupfishes

Variation in ontogeny can produce phenotypic variation both within and among species. I investigated whether changes in timing and rate of growth were a source of phenotypic variation in a putative incipient species group of pupfish (Cyprinodon spp.). On San Salvador Island, Bahamas, sympatric forms of pupfish differ in morphology but show only partial reproductive isolation in the laboratory. Offspring from two forms and two geographical areas and their hybrids were bred in the laboratory, and ontogenetic trajectories of their feeding morphology were followed until maturity. In the Bahamian pupfish the two forms grow along similar size but not shape trajectories. Two heterochronic parameters, onset and rate of growth, alter shape trajectories in the Bahamian pupfish. Similar forms from different geographical areas (Florida and the Bahamas) grow along parallel shape trajectories, differing only in one heterochronic parameter, the onset shape. Hybrids within and between the pupfish forms produced intermediate feeding morphologies that were influenced by their maternal phenotype, suggesting that maternal effects may be a source of phenotypic variation in shape that can persist to maturity. In Cyprinodon, small changes in multiple heterochronic parameters translate into large phenotypic differences in feeding morphology.

Incipient speciation in sympatric Nicaraguan crater lake cichlid fishes: sexual selection versus ecological diversification

The growing body of empirical evidence for sympatric speciation has been complemented by recent theoretical treatments that have identified evolutionary conditions conducive to speciation in sympatry. The Neotropical Midas cichlid (Amphilophus citrinellum) fits both of the key characteristics of these models, with strong assortative mating on the basis of a colour polymorphism coupled with trophic and ecological differentiation derived from a polymorphism in their pharyngeal jaws. We used microsatellite markers and a 480 bp fragment of the mitochondrial DNA control region to study four polymorphic populations of the Midas cichlid from three crater lakes and one large lake in Nicaragua in an investigation of incipient sympatric speciation. All populations were strongly genetically differentiated on the basis of geography. We identified strong genetic separation based on colour polymorphism for populations from Lake Nicaragua and one crater lake (Lake Apoyo), but failed to find significant genetic structuring based on trophic differences and ecological niche separation in any of the four populations studied. These data support the idea that sexual selection through assortative mating contributes more strongly or earlier during speciation in sympatry than ecological separation in these cichlids. The long-term persistence of divergent cichlid ecotypes (as measured by the percentage sequence divergence between populations) in Central American crater lakes, despite a lack of fixed genetic differentiation, differs strikingly from the patterns of extremely rapid speciation in the cichlids in Africa, including its crater lakes. It is unclear whether extrinsic environmental factors or intrinsic biological differences, e.g. in the degree of phenotypic plasticity, promote different mechanisms and thereby rates of speciation of cichlid fishes from the Old and New Worlds.

Neoteny and progenesis as two heterochronic processes involved in paedomorphosis in Triturus alpestris (Amphibia: Caudata)

Current theories on the evolution of paedomorphosis suppose that several ontogenetic pathways have appeared according to different selective pressures. The aim of this study was to find out whether two distinct processes can lead to paedomorphosis in the Alpine newt, Triturus alpestris. In this respect, we compared age structures of paedomorphic and metamorphic individuals in two newt populations where the two forms lived syntopically. Whereas paedomorphosis resulted in a slower rate of somatic development in one population, it resulted in an acceleration of sexual maturation in the other population. These processes correspond to neoteny and progenesis, respectively. These results suggest that phenotypic plasticity can result from contrasted ontogenetic pathways between two populations of the same species. They give support to models that consider gonadic development as the target of selection under different environmental pressures.

Energetics, cost reduction and functional consequences of fish morphology

Cost reduction strategies are often invoked as explanations when studies of adaptation fail to find predicted costs. This might seem discouraging, offering little opportunity for further investigation. In this paper, we demonstrate that cost reduction strategies can themselves be investigated by arguments from design. Recent work on inducible morphological defences has shown that hydrodynamical disadvantages (e.g. high drag) in fishes can be compensated for by standard metabolic rate (SMR) adjustments. Here, we theoretically investigate the possibilities and limitations for swimming cost compensation through SMR adjustment. We continue by modelling how intraspecific power curve variation affects the optimal swimming velocity between food patches. Our results show that, even though SMR modifications may compensate for hydrodynamical disadvantages, low-drag fishes will nevertheless have a marked advantage under high food abundance. The relative advantage will decrease with decreasing food levels. We also show that hydrodynamical properties of fishes can be used to predict their propensity to become foraging (or swimming) specialists. Low-drag fishes can use a broad range of swimming velocities without substantial increases in swimming cost, whereas the cost of deviating from the optimal swimming velocity increases markedly in high-drag fishes. The results have important implications for the evolution of morphological diversity in fishes.

Nested biological variation and speciation

The modes of speciation that are thought to have contributed most to the generation of biodiversity require population differentiation as the initial stage in the speciation process. Consequently, a complete understanding of the mechanisms of speciation requires that the process be examined not just after speciation is complete, or nearly so, but also much earlier. Because reproductive isolation defines biological species, and it evolves slowly, study of the process may require a prohibitive span of time. Even if speciation could be observed directly, selection of populations in the process of speciation is typically difficult or impossible, because those that will ultimately undergo speciation cannot be distinguished from those that will differentiate but never assume the status of new biological species. One means of circumventing this problem is to study speciation in taxa comprising several sibling species, at least one of which exhibits extensive population differentiation. We illustrate this approach by exploring patterns of population variation in the post-glacial radiation of the threespine stickleback, Gasterosteus aculeatus . We focus on lacustrine populations and species within this complex, demonstrating parallel axes of divergence within populations, among populations and among species. The pattern that emerges is one of parallel relationships between phenotype and fitness at all three hierarchical levels, a pattern that facilitates exploration of the causes and consequences of speciation and secondary contact. A second outcome of this exploration is the observation that speciation can be the consequence of a cascade of effects, beginning with selection on trophic or other characteristics that in turn force the evolution of other population characteristics that precipitate speciation. Neither of these conclusions could have been reached without comparative studies of wild populations at several hierarchical levels, a conclusion reinforced by a brief survey of similar efforts to elucidate the process of speciation. We address the issues most likely to be resolved using this approach, and suggest that comparisons of natural variation within taxa at several hierarchical levels may substantially increase our understanding of the speciation process.

Adaptive individual differences within single populations

Phenotypic differences can exist between species, between local populations of the same species and between individuals within single local populations. At all scales, phenotypic differences can be either adaptive or non–adaptive. Using natural selection to explain differences between closely related species was controversial during the 1940s but had become common by the 1960s. Similarly, the adaptive nature of differences between local populations was initially controversial but had become widely accepted by the 1980s. The interpretation of differences at the finest scale—between individuals within single populations—is still unresolved. This paper reviews studies of adaptive individual differences in resource use and response to risk. A general conceptual framework for thinking about adaptive individual differences within populations can unite subjects as seemingly different as speciation and personality psychology.

Adaptive evolution via a major gene effect: paedomorphosis in the Mexican axolotl

Although adaptive evolution is thought to depend primarily on mutations of small effect, major gene effects may underlie many of the important differences observed among species in nature. The Mexican axolotl (Ambystoma mexicanum) has a derived mode of development that is characterized by metamorphic failure (paedomorphosis), an adaptation for an entirely aquatic life cycle. By using an interspecific crossing design and genetic linkage analysis, a major quantitative trait locus for expression of metamorphosis was identified in a local map of amplified fragment length polymorphisms. These data are consistent with a major gene hypothesis for the evolution of paedomorphosis in A. mexicanum.