The “Bateman gradient” provides a means for estimating the strength of sexual selection. Although widely used for this purpose, this approach has not been applied to examine the covariance between mate numbers and offspring numbers among alternative mating strategies. Differences in this covariance could exist if the average fitnesses of different mating phenotypes were unequal, as has been suggested for “alternative mating tactics.” We tested this hypothesis in Paracerceis sculpta, a sexually dimorphic marine isopod in which three male morphs coexist. We found no significant differences in sexual competency and no significant differences in Bateman gradients among morphs, that is, the average morph fitnesses were equivalent. However, with data pooled among morphs, we found a significant sex difference in Bateman gradients, as expected for dimorphic species; females gained no additional fitness from mating with multiple males, whereas male fitness increased with increasing mate numbers. In nature, the fitnesses of the three morphs are variable due to differences in the availability of receptive females. Our results suggest that differences in mate availability, not differences in sexual competency, are responsible for observed variance in fitness within, and for the equality of fitnesses among, the three male morphs in this species.
Part of the book: Crustacea
Populations with reduced gene flow and restricted population size are expected to show reduced genetic variation. Using starch gel electrophoresis, we examined allozyme variation at 12 loci in two species of freshwater, sphaeromatid isopods. Thermosphaeroma thermophilum, an endangered species, inhabits a single thermal spring in central New Mexico, USA; and T. milleri, inhabits a more complex thermal spring system in northern Chihuahua, México. We found no significant differences in allelic variation between the sexes within each species. Between species, electromorphs at each locus differed significantly in both number and moiety on the gel, with T. milleri showing greater polymorphism and greater heterozygosity than T. thermophilum. Nei’s unbiased genetic distance, calculated using the nine loci common to both populations (D = 0.75), was consistent with morphological classification of T. thermophilum and T. milleri as separate species, as well as with molecular analyses suggesting that these populations have been separated since the late Cretaceous (88 myr). Moreover, consistent with the theoretical expectation that small, isolated populations will exhibit reduced genetic variation, T. thermophilum, an endangered species, exhibited significantly less genetic variation than the more numerous and less confined T. milleri. We compare our results with other recent studies using this approach to understand the population genetics of natural populations.
Part of the book: Crustacea