Abstract
The effect of the environment on fitness in natural populations is a fundamental question in evolutionary biology. However, most empirical field studies of fitness do not experimentally manipulate phenotypes or environmental conditions and rarely investigate more than a single species or population. Thus, the relative importance of the competitive environment versus intrinsic organismal performance in shaping the location, height, and fluidity of fitness peaks on the adaptive landscape remains largely unknown. We experimentally tested the effect of competitive environment on a multi-peak fitness landscape driving a microendemic adaptive radiation of generalist and trophic specialist pupfishes on San Salvador Island, Bahamas. We manipulated phenotypes, by generating lab-reared hybrid crosses, and competitive environment, by altering the frequency of rare phenotypes within field enclosures in their natural hypersaline lake environments on San Salvador. We tracked the growth and survival of 2,611 F4/F5 hybrids for 3 to 11 months in high- and low-frequency treatments replicated across two lake populations. We found strong evidence for frequency-dependent growth rates within and between enclosures, but no evidence for frequency-dependent survival differences. However, both fitness proxies supported a complex fitness landscape isolating generalist phenotypes on a local fitness peak separated by a small fitness valley from a higher fitness peak for molluscivores and a large fitness valley isolating the scale-eating phenotype in all trait dimensions. The striking consistency of this multi-peak fitness landscape across competitive environments, multivariate trait axes, and a previous field experiment provides experimental evidence for stasis, possibly due to fixed biomechanical constraints on organismal performance. These results challenge existing theory and highlight the interplay of organism and environment underlying the static and dynamic features of fitness landscapes.