Rapid evolution of a polyphenic threshold

Polyphenisms are thought to play an important role in the evolution of phenotypic diversity and the origin of morphological and behavioral novelties. However, the extent to which polyphenic developmental mechanisms evolve in natural populations is unknown. Here we contrast patterns of male phenotype expression in native and exotic and ances- tral and descendant populations of the horn polyphenic bee- tle, Onthophagus taurus . Males in this species express two alternative morphologies in response to larval feeding condi- tions. Favorable conditions cause males to grow larger than a threshold body size and to develop a pair of horns on their heads. Males that encounter relatively poor conditions do not reach this threshold size and remain hornless. We show that exotic and native populations of O. taurus differ significantly in the body size threshold that separates alternative male phenotypes. Comparison with archival museum collections and additional samples obtained from the native range of O. taurus suggests that allometric differences between exotic and native populations do not reflect preexisting variation in the native range of this species. Instead, our data sug- gest that threshold divergences between exotic and native populations have evolved in less than 40 years since the introduction to a new habitat and have proceeded in oppo- site directions in two exotic ranges of this species. Finally, we show that the kind and magnitude of threshold diver- gence between native and exotic populations are similar to differences normally observed between species. Our re- sults support the view that certain components of the de- velopmental control mechanism that underlie polyphenic development can evolve rapidly in natural populations and may provide important avenues for phenotypic differentia- tion and diversification in nature. We discuss the role of developmental control mechanisms in the origin of allo- metric diversification and explore potential evolutionary mechanisms that could drive scaling relationship evolution in nature.