Evol Ecol Res 4: 587-601 (2002)     Full PDF if your library subscribes.

Threshold evolution in exotic populations of a polyphenic beetle

Armin P. Moczek,1* John Hunt,2 Douglas J. Emlen3 and Leigh W. Simmons2

1Department of Biology, Duke University, Box 90338, Durham, NC 27708-0338, USA, 2Department of Zoology, The University of Western Australia, Nedlands, WA 6907, Australia and 3Division of Biological Sciences, University of Montana, Missoula, MT 59812-1002, USA

Author to whom all correspondence should be addressed.
e-mail: arminmo@email.arizona.edu


Polyphenic development is thought to play an important role in the evolution of phenotypic diversity and morphological novelties, yet the evolution of polyphenisms has rarely been documented in natural populations. Here we compare the morphologies of male dung beetles (Onthophagus taurus; Coleoptera: Scarabaeidae) from populations introduced to Australia and the eastern United States. Males in this species express two alternative morphologies in response to larval feeding conditions. Males encountering favourable conditions grow larger than a threshold body size and develop a pair of horns on their heads, whereas males that encounter poor conditions do not reach this threshold size and remain hornless. Australian and US populations did not differ in overall body size ranges, but exhibited significant differences in the location of the critical body size threshold that separates alternative male morphs. Australian males remained hornless at much larger body sizes than males in US populations, resulting in substantial and significant differences in the average body size–horn length allometry between exotic populations, as well as significant differences in morph ratios. The phenotypic divergence observed between field populations was maintained in laboratory populations after two generations under identical environmental conditions, suggesting a genetic basis to allometric divergence in these populations. Divergence between exotic O. taurus populations was of a magnitude and kind typically observed between species. We use our results to examine potential causes of allometric divergence in onthophagine beetles, and discuss the evolutionary potential of threshold traits and polyphenic development in the origin of morphological and behavioural diversity.

Keywords: adaptive phenotypic plasticity, alternative tactics, developmental threshold, exotic species, horn polyphenism, Onthophagus, status-dependent selection, threshold evolution.

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