Geographic reversal of sexual size dimorphism in Sepsis punctum

The differential equilibrium model of sexual size dimorphism (SSD) explains dimorphism as the evolutionary outcome of consistent differences in natural and sexual selection between the sexes. The magnitude of SSD can often vary significantly across and within species but there are very few examples of intra-species directional variation in SSD.

Here, we comprehensively examine a unique cross-continental reversal in SSD in the dung fly, Sepsis punctum. First, using common garden laboratory experiments, we established that SSD is male-biased in four European populations and female-biased in three North American populations. Next, using standardised regression methods, we estimated the relative strength of selection for the three major fitness components of the differential equilibrium model (viability, fecundity and sexual selection) under laboratory conditions.

We found that although there was strong fecundity selection for body size in all populations, there was no difference between the continents. Similarly, adult viability in the lab varied but not significantly. However, estimated sexual (pairing success) and fecundity selection on males (clutch size of female partner) under three different operational sex ratios (OSR) and find that the intensity of sexual selection is significantly stronger in European vs. North American populations, increasing with male body size and OSR in the former only.

For details,
see Puniamoorthy et al. 2012 Evolution
Puniamoorthy et al. 2012 J. Evol. Biol.