Body size evolution under character release in the ground beetleCarabus japonicus

Abstract

AbstractAimWe tested the hypothesis of character release in body size among allopatric populations of the carabid beetleCarabus japonicusby analysing geographical variation in body size in relation to habitat temperature and sympatry/allopatry with the larger congeneric speciesCarabus dehaanii.LocationThe main and satellite islands of Kyushu in the south‐western part of the Japanese archipelago.MethodsWe studied geographical variation in the body length and genital size ofC. japonicuspopulations at different sites to examine the effects of both habitat temperature and sympatry/allopatry withC. dehaanii. To determine whether the conditions for character release were fulfilled, we then estimated the heritable differences in body size by common garden rearing, and estimated the phylogenetic relationships among populations by molecular phylogenetic analysis.ResultsWhile body size was positively correlated with annual mean temperature, it was consistently small in sympatry but larger in some allopatric populations in warmer regions. The body size differences among populations were heritable. AllopatricC. japonicuspopulations on satellite islands were derived from the sympatric mainland populations. In a few sympatric areas, mitochondrial haplotypes were shared between the species because of introgressive hybridization, suggesting the occurrence of reproductive interference between the species. We also found that genital size was not affected by sympatry/allopatry, but was positively correlated with body size.Main conclusionsWe demonstrated that the increased body size of allopatricC. japonicushas evolved through a character release process. However, not all allopatric populations had enlarged body size, thus suggesting that some additional environmental factors are also involved. Geographical differentiation in body size and associated genital dimensions may result in prezygotic reproductive isolation among populations and promote allopatric differentiation leading to speciation.