Contrasting post-settlement selection results in many-to-one mapping of high performance phenotypes in the Hawaiian waterfall-climbing goby Sicyopterus stimpsoni

Natural selection drives adaptive evolution, but contrasting environmental pressures may lead to trade-offs between phenotypes that confer different performances. Such trade-offs may weaken the strength of selection and/or generate complex fitness surfaces with multiple local optima that correspond to different selection regimes. We evaluated how differences in patterns of phenotypic selection might promote morphological differences between subpopulations of the amphidromous Hawaiian waterfall-climbing goby, Sicyopterus stimpsoni. We conducted laboratory experiments on fish from the islands of Kaua‘i and Hawai‘i (the “Big Island”) to compare patterns of linear and nonlinear selection, and the opportunity for selection, that result from two contrasting pressures, predator evasion and waterfall climbing, which vary in intensity between islands. We found directional and nonlinear selection were strongest when individuals were exposed to their primary selective pressures (predator evasion on Kaua‘i, waterfall climbing on the Big Island). However, the opportunity for selection was greater for the non-primary pressure: climbing on Kaua‘i, predator evasion on the Big Island. Canonical rotation of the nonlinear gamma matrix demonstrated that individuals from Kaua‘i and the Big Island occupy regions near their local fitness peaks for some traits. Therefore, selection for predator evasion on Kaua‘i and climbing on the Big Island may be less effective in promoting morphological changes in this species, because variation of functionally important traits in their respective environments may have been reduced by directional or stabilizing selection. These results demonstrate that despite constraints on the opportunities for selection, population differences in phenotypic traits can arise due to differences in selective regimes. For S. stimpsoni, sufficient variation exists in other locomotor traits, allowing for necessary levels of performance in the contrasting selective regime (i.e., climbing on Kaua‘i and predator evasion on the Big Island) through many-to-one-mapping, which may be essential for the survival of local populations in an evanescent island environment.