Co-Authors: Allison Ozark, Samantha Swank, Raheyma Siddiqui, Akhil Ghosh, Dr. Michael A. Bell, Dr. Gregory J. Matthews, and Dr. Yoel E. Stuart
Abstract: Theory suggests that populations should expand their habitat- and resource-use niches when they are freed from interactions with competitors and predators—so-called “ecological release”. One way that ecological release can manifest is through increased differences between conspecific as they diverge into different niches. Theory predicts that divergent niche use should drive corresponding divergent adaptation in the niche-use traits, resulting in an increase in sexual dimorphism for those traits. This is sometimes called character displacement between the sexes. Using a dataset of 16 traits collected from 18 temporal samples spread across 16,500 years of evolution of the fossil stickleback fish, Gasterosteus doryssus, we tested the prediction that release from predation inferred in this system would result in character displacement between the sexes. We used data from populations of extant stickleback (Gasterosteus aculeatus) to build an imputation model to classify individual sex in fossil specimens. Then, we quantified the extent of sexual dimorphism at each temporal sample and tracked how dimorphism changed through time. Contrary to theoretical prediction, we found evidence for character displacement between the sexes in only two of 16 traits. Two traits reduced their sexual dimorphism, and the rest showed little change. This is not the only empirical example of an absence of a microevolutionary response of sexual dimorphism to ecological release, suggesting that one or more assumptions of the theory need to be revisited.
Preprint Forthcoming