SEXUAL SELECTION AGAINST HYBRIDS BETWEEN SYMPATRIC STICKLEBACK SPECIES: EVIDENCE FROM A FIELD EXPERIMENT

Sexual selection against viable, fertile hybrids may contribute to reproductive isolation between recently diverged species. If so, then sexual selection may be implicated in the speciation process. Laboratory measures of the mating success of hybrids may underestimate the amount of sexual selection against them if selection pressures are habitat specific. Male F₁ hybrids between sympatric benthic and limnetic sticklebacks (Gasterosteus aculeatus complex) do not suffer a mating disadvantage when tested in the laboratory. However, in the wild males choose different microhabitats and parental females tend to be found in the same habitats as conspecific males. This sets up the opportunity for sexual selection against male hybrids because they must compete with parental males for access to parental females. To test for sexual selection against adult F₁ hybrid males, we examined their mating success in enclosures in their preferred habitat (open, unvegetated substrate) where limnetic males and females also predominate. We found significantly reduced mating success in F₁ hybrid males compared with limnetic males. Thus, sexual selection, like other mechanisms of postzygotic isolation between young sister species, may be stronger in a wild setting than in the laboratory because of habitat-specific selection pressures. Our results are consistent with, but do not confirm, a role for sexual selection in stickleback speciation.     

The role of diversification in causing the correlates of dioecy

Dioecy is reported to be correlated with a number of ecological traits, including tropical distribution, woody growth form, plain flowers, and fleshy fruits. Previous analyses have concentrated on determining whether dioecy is more likely to evolve in lineages possessing these traits, rather than considering the speciation and extinction rates of dioecious lineages with certain combinations of traits. To address the association between species richness in dioecious lineages as a function of the ecological traits, we compared the evolutionary success (i.e., relative species richness) of dioecious focal lineages with that of their nondioecious sister groups. This test was repeated for the evolutionary success of randomly chosen nondioecious lineages (control lineages) compared with their nondioecious sister groups. If the possession of certain ecological traits enhances the evolutionary success of dioecious lineages, we predict an association between the presence of these traits and relative species richness in the former, but not latter, set of sister-group comparisons. Dioecious focal lineages with a higher number of these traits experienced higher evolutionary success in sister-group comparisons, whereas no trend was found for the control focal lineages. The increase in evolutionary success was especially true for dioecious focal lineages that had a tropical distribution or fleshy fruit. We discuss how these results provide strong support for differential evolutionary success theories for the correlations between dioecy and the ecological traits considered.     

Population genetics of Bull Trout (Salvelinus confluentus) in the upper Athabasca River basin

Freshwater ecosystems are negatively impacted by a variety of anthropogenic stressors, with concomitant elevated rates of population decline for freshwater aquatic vertebrates. Because reductions in population size and extent can negatively impact genetic diversity and gene flow, which are vital for sustained local adaptation, it is important to measure these characteristics in threatened species that may yet be rescued from extinction. Across its native range, Bull Trout (Salvelinus confluentus) extent and abundance are in decline due to historic overharvest, invasive non‐native species, and habitat loss. In Alberta''s Eastern Slope region, populations at the range margin have progressively been lost, motivating us to better understand the amount and distribution of genetic variation in headwater habitats and some downstream sites where they continue to persist. Across this region, we sampled 431 Bull Trout from 20 sites in the Athabasca and Saskatchewan River basins and assayed 10 microsatellite loci to characterize within‐ and among‐population genetic variation. The Saskatchewan and Athabasca River basins contained similar levels of heterozygosity but were differentiated from one another. Within the Athabasca River basin, five genetically differentiated clusters were found. Despite the evidence for genetic differentiation, we did not observe significant isolation‐by‐distance patterns among these sites. Our findings of ample genetic diversity and no evidence for hybridization with non‐native Brook Trout in headwater habitats provide motivation to ameliorate downstream habitats and remove anthropogenic barriers to connectivity towards the goal of long‐term persistence of this species.     

Factors influencing diversification in angiosperms: at the crossroads of intrinsic and extrinsic traits

Recent studies indicate that both key innovations and available area influence species richness in angiosperms. Available area has been observed to have the greatest effect, however, and appears to alter the "carrying capacity" of a lineage rather than alter diversification rates. Here, we review and weigh the evidence of predictors of angiosperm diversification and further dissect how area can place ecological limits on diversification of angiosperms, specifically addressing the following: (1) theoretical mechanisms by which particular intrinsic and extrinsic traits may affect diversification in angiosperm families; (2) evidence that the amount of available area determines the ecological limits on lineages; and (3) geographical distribution of diversification hotspots in angiosperms, concentrating on the effects of zygomorphy, noncontiguous area, and latitude. While we found that dispersal to numerous noncontiguous areas is most important in spurring diversification, diversification of tropical and zygomorphic families appears to be elevated by the generation of more species per given area.     

Nonrandom extinction leads to elevated loss of angiosperm evolutionary history

The phylogenetic clustering of extinction may jeopardize the existence of entire families and genera, which can result in elevated reductions of evolutionary history (EH), trait diversity, and ecosystem functioning. Analyses of globally threatened birds and mammals suggest current extinction threats will result in a much higher loss of EH than random extinction scenarios, while the analyses of the taxonomical distribution of regionally rare plants find the opposite pattern. The disproportionately high number of rare plant species within species-rich families potentially suggests that lower losses of plant EH will be sustained than expected under random extinction. We show that at a global scale, this is not the case. Species-poor (especially monotypic) angiosperm families are more often at risk of extinction than expected. Because these high-risk species-poor families are as evolutionarily distinct as other families, the expected family-level EH plausibly lost in the next 100 years exceeds that predicted from random extinction by up to approximately 1165 million years.     

Ecology and evolution join forces to good effect

The 3rd regular meeting of the Canadian Society of Ecology and Evolution was held at the University of British Columbia, Vancouver, Canada from 11 to 14 May 2008.     

Conservation genetics of prickly sculpin (Cottus asper) at the periphery of its distribution range in Peace River, Canada

Populations at the edge of their range often invoke taxonomic confusion and are increasingly considered to harbour cryptic genetic diversity of significant adaptive potential. In the Peace River region of northwestern Canada, three sculpin species have been reported: spoonhead (Cottus ricei), slimy (Cottus cognatus) and prickly (Cottus asper) sculpin. Prickly sculpin occurrence in this region represents the most eastern edge of its distribution, but its status has remained uncertain following its initial discovery in 1989. These populations may represent an independently evolving lineage of special conservation concern, or be the consequence of an ongoing range expansion, possibly accompanied by interspecific hybridization with local species. Using a combination of mtDNA sequencing and microsatellite analyses, we did not find peripheral population differentiation or interspecific hybridization, suggesting that the Albertan Peace River population belongs to the same genetic group as its western counterparts. Future studies will benefit from a greater understanding of whether demographically independent prickly sculpin populations established in Alberta without the typical genetic signatures of expansion at the periphery of their range.     

Evolutionary ecology of specialization: insights from phylogenetic analysis

In this Special feature, we assemble studies that illustrate phylogenetic approaches to studying salient questions regarding the effect of specialization on lineage diversification. The studies use an array of techniques involving a wide-ranging collection of biological systems (plants, butterflies, fish and amphibians are all represented). Their results reveal that macroevolutionary examination of specialization provides insight into the patterns of trade-offs in specialized systems; in particular, the genetic mechanisms of trade-offs appear to extend to very different aspects of life history in different groups. In turn, because a species may be a specialist from one perspective and a generalist in others, these trade-offs influence whether we perceive specialization to have effects on the evolutionary success of a lineage when we examine specialization only along a single axis. Finally, how geographical range influences speciation and extinction of specialist lineages remains a question offering much potential for further insight.