Macroevolutionary patterns of bumblebee body size: detecting the interplay between natural and sexual selection

Bumblebees and other eusocial bees offer a unique opportunity to analyze the evolution of body size differences between sexes. The workers, being sterile females, are not subject to selection for reproductive function and thus provide a natural control for parsing the effects of selection on reproductive function (i.e., sexual and fecundity selection) from other natural selection. Using a phylogenetic comparative approach, we explored the allometric relationships among queens, males, and workers in 70 species of bumblebees (Bombus sp.). We found hyperallometry in thorax width for males relative to workers, indicating greater evolutionary divergence of body size in males than in sterile females. This is consistent with the hypothesis that selection for reproductive function, most probably sexual selection, has caused divergence in male size among species. The slope for males on workers was significantly steeper than that for queens on workers and the latter did not depart from isometry, providing further evidence of greater evolutionary divergence in male size than female size, and no evidence that reproductive selection has accelerated divergence of females. We did not detect significant hyperallometry when male size was regressed directly on queen size and our results thus add the genus Bombus to the increasing list of clades that have female-larger sexual size dimorphism and do not conform to Rensch's rule when analyzed according to standard methodology. Nevertheless, by using worker size as a common control, we were able to demonstrate that bumblee species do show the evolutionary pattern underlying Rensch's rule, that being correlated evolution of body size in males and females, but with greater evolutionary divergence in males.     

Physical Restraint or Stimulation? The Function(s) of the Modified Front Legs of Male Archisepsis diversiformis (Diptera, Sepsidae)

Four hypotheses that could explain the elaborate species-specific morphology of the clasping organs on the front legs of male Archisepsis diversiformis flies were tested: direct male–male combat, mechanical fit, male–female conflict of interests, and male stimulation of the female. Experimental modification of the shape of the male clasper and of the female wing base where the male clasped the female both strongly reduced the chances that a mount would result in copulation. This reduction was not predicted by the male–male combat hypothesis but was predicted by the others. Males in the field did nave to fight other males to remain mounted on females, as expected by the male–male combat hypothesis. Reduced male copulatory success was not due to inferior male ability to grasp and hold onto the female's wings, as predicted by the mechanical fit and male–female conflict hypotheses but to a reduction in the likelihood that the female would allow intromission, as predicted by the stimulation hypothesis. By a process of elimination, and in combination with data from a previous morphological study, the data support the hypothesis that the species-specific aspects of grasping organs in these flies function to stimulate females. Further behavioral data will be needed to test alternative possibilities.     

Locus- and population-specific selection and differentiation between incipient species of Anopheles gambiae

Anopheles gambiae, the primary mosquito vector of malaria in sub-Saharan Africa, is divided into 2 sympatric incipient species known as M form and S form. Recent genomic analysis of each form revealed that differentiation between forms is clustered into 3 unlinked regions of the genome. Here, we expand the investigation of these "genomic islands of speciation" to multiple populations, including all of the genes across one of the islands. Differentiation between the M and S forms in 2 of the islands is complete across all individuals in all populations, confirming that the M and S forms are reproductively isolated taxa. Differentiation at the third island (on chromosome 2R) is limited to Cameroon populations. There is reduced variation in the M form in Cameroon at this location and increased divergence to the outgroup Anopheles arabiensis, supporting an association of adaptation with reproductive isolation.     

Sex and differentiation: population genetic divergence and sexual dimorphism in Mexican goodeid fish

Genetic differentiation arises due to the interaction between natural and sexual selection, migration and genetic drift. A potential role of sexual selection in speciation has received much interest, although comparative studies are inconsistent in finding supporting evidence. A poorly tested prediction is that species subject to a higher intensity of sexual selection should show greater genetic differentiation amongst populations because females from these populations should be more choosy in mate choice. The Goodeinae is a group of endemic Mexican fishes in which female choice has driven some species to be morphologically sexually dimorphic, whereas others are relatively monomorphic. Here, we measured population divergence, using microsatellite loci, within four goodeid species which show contrasting levels of sexual dimorphism. We found higher levels of differentiation between populations of the more dimorphic species, implying less gene flow between populations. We also found evidence of higher levels of genetic differences between the sexes within populations of the dimorphic species, consistent with greater dispersal in males. Adjusted for geographic distance, the mean F(ST) for the dimorphic species is 0.25 compared with 0.16 for the less dimorphic species. We conclude that population differentiation is accelerated in more sexually dimorphic species, and that comparative phylogeography may provide a more powerful approach to detecting processes, such as an influence of sexual selection on differentiation, than broad-scale comparative studies.