Does Development Bias Evolution?

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Emlen 9723637 Many beetle species have "horns" which are only produced by males. These horns can be extreme (as dramatic as the antlers of elk), and investment by males into the production of horns is presumed to be substantial. In many beetle species only some of the males produce horns, other males being completely hornless. In most species of the genus Onthophagus (Coleoptera: Scarabaeidae), male horn expression is facultative, and depends on whether or not a male grows larger than a threshold body size: males growing larger than this size produce horns, whereas males not attaining this size do not. Facultative expression of male horns provides an unusual opportunity to study how horns develop, and whether horns are "costly" to males to produce. Prior research revealed that males allocating developmental resources to horn production were forced to compensate by reducing allocation to other traits, including eyes. This suggests that there may be allocation tradeoffs operating during beetle development which restrict the range of beetle shapes that can be produced, and these may also restrict the directions of evolution within the genus. Here the PI extends his prior work by (1) using manipulative experiments on developing larvae to confirm the existence and nature of resource allocation tradeoffs, and (2) testing whether the biases predicted to arise from such a mechanism are manifest in the shape variation among taxa. This study will enhance our understanding of the relationship between developmental mechanisms and evolutionary change, as well as improve our understanding of the "costs" associated with the production of exaggerated secondary sexual characters like beetle horns.