Evolution of body size in the genus Damaliscus: a comparison with hartebeest Alcelaphus spp.

In species with low levels of sexual size dimorphism, it may be relatively easy to detect the role of natural selection in the evolution of body size. Habitat primary production (HPP) appears to be a key factor in the divergence of size in the hartebeest clade ( Alcelaphus spp.), such that subspecies in less productive savannahs are smaller than those in richer ones. Here I test whether a similar pattern exists within the genus Damaliscus (topi and their allies). Basal skull length was used as a surrogate of body size and measured in the seven allopatric subspecies of Damaliscus . Means for each subspecies and sex were regressed against climatic factors as surrogates of HPP. Variation in skull length across Damaliscus taxa was less than in hartebeest. Two clusters were present in both sexes and corresponded to the distinction between the species, Damaliscus dorcas and Damaliscus lunatus . This may reflect differences in productivity between edaphic grasslands, occupied by all D. lunatus , and dry grasslands, occupied by D. dorcas . Mean annual rainfall was the best predictor of body size in males and showed a non-significant positive tendency in females. After accounting for phylogenetic effects, these correlations were both non-significant. Edaphic grasslands might be less dependent on precipitation for primary production because the impeded drainage of their soil prolongs water availability after the end of the rains. Furthermore, they are probably more consistent in productivity across African regions than secondary grasslands and savannah woodlands, which rely on rainfall for grass growth. These properties of edaphic grasslands may explain why size in Damaliscus appears to be less sensitive to variation in rainfall and less variable across subspecies than in Alcelaphus .     

Population divergence and morphometric integration in the greenfinch (Carduelis chloris) – evolution against the trajectory of least resistance?

Theory predicts that genetic and phenotypic correlations among traits may direct the process of short-term evolution by limiting the directions of variation available to natural selection to act on. We studied correlations between 14 skeletal traits in 10 geographically distinct and relatively young greenfinch (Carduelis chloris) populations to unravel whether the divergence among populations has occurred into directions predicted by the within-population correlations (cf. drift/correlated responses models), or whether it is better explained by ‘adaptive’ models, which predict no necessary association between within- and among-population correlations (allometries). We found that the within-population character correlations (or covariances) did not predict character divergence between populations. This was because the first eigenvector of the among-population correlation/covariance matrix, summarizing the major dimension of divergence, was a bipolar body:beak dimension, and distinct from the (≈ isometric) first eigenvector of within-population matrix. Hence, as the divergence among greenfinch populations cannot be satisfactorily accommodated by drift/correlated response models, an adaptive basis for divergence is suggested. The second major axis of within-population variation was a classical ‘group size’ factor revealing that beak size was more or less free to vary independently of body size. Consequently, even if the divergence among populations cannot be simply accommodated to expectations of drift and correlated response models, it is striking that the most pronounced size-independent (nonallometric) changes had occurred along the second largest dimension of variance. This could mean that selection pressures which shape integration within populations are the same as those that cause divergence among populations. A relaxed beak:body integration could also occur as a result of species level selection favouring taxa in which independent evolution of beak and body is made possible.     

Latitudinal compensation in female reproductive rate of a geographically widespread reef fish

Latitudinal variation in fitness-related traits has often been attributed to local adaptation to climates. In poikilotherms including fishes, lower temperatures and shorter reproductive seasons at high latitudes would be expected to cause a reduction in annual reproductive output of an individual. Theories of latitudinal compensation predict that organisms at high latitudes should evolve compensatory responses for these climatic effects. Therefore, latitudinal compensation in female reproductive rate (egg production rate), that individuals from high latitudes produce eggs at higher rates than those from lower latitudes, is likely to occur. I tested this hypothesis with a latitudinally widespread reef fish Pomacentrus coelestis that is a multiple batch spawner, from three different localities, from temperate to subtropical waters, within Japan. I used common-environment experiments at three different temperatures to compare reproductive capacity among local populations. In the experiments, average inter-spawning intervals were the shortest and average size-specific clutch weight was the heaviest in fish from the most northern locality across all temperatures, showing clear latitudinal clines. Thus, the northern fish can achieve higher reproductive output per unit time both by shortening inter-spawning intervals and increasing size-specific clutch weight. Additionally, faster egg production rate of the northern fish did not result from increased food consumption. This finding suggests that gross egg production efficiency was higher in the northern fish and that northern fish had a superior capacity for reproduction within a season. These results support the prediction that latitudinal compensation occurs in the female reproductive rate of P. coelestis. As the reproductive season of this species decreases drastically with increasing latitude, the observed cline in the reproductive rate must be an adaptive response to the local selective regime, i.e., length of the reproductive season. Such latitudinal compensation in female reproductive rates may be a common pattern in latitudinally widespread fishes.     

Body size and alternative mating tactics in the beewolf Philanthus zebratus (Hymenoptera; Sphecidae)

A study of the mating behaviour of males of the beewolf Philanthus zebratus revealed that in one population males display variability in mating tactics and that this variability is related to male body size. There was a tendency for large males to patrol the airspace above the nesting area while smaller males were territorial adjacent to it. The mean sizes of the two groups of males were significantly different, although the size ranges of the two groups overlapped. Only 2.5% of the males were observed to undertake both mating tactics, at different times. Observations are presented on daily and seasonal activity patterns and on the relative location of nests, territories, and patrolling males. A second population, with lower nest density, was observed for several days, revealing only territorial males. It is suggested that the presence of patrolling males is related to the higher nest density of the one population. The fact that patrolling males tend to be relatively large is possibly related to flight energetics or simply to the ability of large males to seize females, which are usually larger than males, in mid-air.