03343nas a2200265   4500008004100000245007300041210006900114300001200183490000700195520248900202653003502691653002802726653001902754653001902773653000902792653001602801653001602817653002502833653002402858653003102882653002402913653002402937100002202961856009402983       2003                        eng d00aThe behavioral ecology of threshold evolution in a polyphenic beetle0 abehavioral ecology of threshold evolution in a polyphenic beetle  a841-8540 v143 aFacultative expression of alternative male morphologies is thought to allow individual males to select the phenotype with the highest fitness gain given their competitive status relative to other males with which they compete for females. Choice of, or switching between, morphs commonly relies on developmental threshold responses. Evolutionary changes in developmental threshold responses are thought to provide an important avenue for phenotypic diversification and the evolution of morphological and behavioral novelties. However, the extent to which alternative male phenotypes and their underlying threshold responses actually evolve in natural populations is unclear. Likewise, the ecological factors that shape the evolution of threshold responses in natural populations are unexplored for most organisms, as are the consequences of such modifications for patterns of morphological diversity. I examined the ecological basis of rapid threshold evolution in exotic populations of the horn-polyphenic dung beetle Onthophagus taurus. Male O. taurus vary continuously in body size as a function of larval feeding conditions. Only males that exceed a critical threshold body size develop a pair of long horns on their heads, whereas males below this threshold remain hornless. Populations in two exotic ranges of this species, the eastern United States and western Australia, have diverged in the mean threshold body size, which has resulted in the evolution of highly divergent and novel horn length–body size allometries in these populations. Populations in a third and previously unstudied exotic range of O. taurus in eastern Australia exhibit threshold body sizes roughly intermediate between the eastern U.S. and western Australian populations. I tested three hypothesis to explain how differences in ecological and demographic factors can drive allometric divergences between populations, using data derived from comparative, standardized sampling of a large number of populations in each exotic range. Results suggest that differences in the intensity of both intra- and interspecific competition have contributed to the evolution of divergent thresholds in these populations. My results do not support the hypothesis that shifts in threshold body sizes to larger body sizes are a consequence of increases in the mean body size of competing males. I discuss my results in the context of Onthophagus mating systems and the evolutionary implications of threshold evolution.10aadaptive phenotypic plasticity10adevelopmental threshold10aexotic species10ahorned beetles10amale10aOnthophagus10aPolyphenism10aresource competition10asex ratio evolution10astatus-dependent selection10athreshold evolution10a–male competition1 aMoczek, Armin, P.  uhttps://scarabnet.myspecies.info/behavioral-ecology-threshold-evolution-polyphenic-beetle02476nas a2200133   4500008004100000245004600041210004600087300001200133490000600145520207300151100002202224700002202246856007402268       2003                        eng d00aRapid evolution of a polyphenic threshold0 aRapid evolution of a polyphenic threshold  a259-2680 v53 aPolyphenisms 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.1 aMoczek, Armin, P.1 aNijhout, Frederik  uhttps://scarabnet.myspecies.info/rapid-evolution-polyphenic-threshold