Parapatric speciation along altitudinal gradients

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We aimed to test whether ecological speciation could result from differentiation of body size between populations adapted to different altitudinal conditions, focusing on the body size variation among Carabus (Ohomopterus) tosanus populations at different altitudes. Molecular phylogenetic analyses showed that C. tosanus was a monophyletic group diverged within Shikoku and that there were only small genetic divergence among populations at different altitudes, despite that body size was clearly smaller at elevations of about 1000 m or higher. Field study indicated that the smaller body sizes at higher altitudes was related to adaptive responses to temperature and food (earthworms) conditions which were limited at higher altitudes. Laboratory experiments showed that the differences in body length between populations from different altitudes were largely determined by genetic differences with a small effect of plastic responses to temperature. The analysis of heritability revealed that the difference between populations was attributable to maternal inheritance (probably due to genes on X chromosome). Mating experiments showed that the body length difference caused a mechanical reproductive isolation, suggesting that the body size differentiation can result in speciation.