Kristine N. Moody, PhD

1. Local adaptation despite high gene flow.

For organisms like amphidromous fishes that change habitats during their life history, the balance between selection and migration can shift through ontogeny, making the likelihood of local adaptation difficult to predict. In Hawaiian waterfall-climbing gobies, it has been hypothesized that larval mixing during ocean dispersal counters local adaptation to contrasting topographic features of streams, like slope gradient, that can select for predator avoidance or climbing ability in juvenile recruits. To test this hypothesis I used morphological traits and neutral genetic markers to compare phenotypic and genotypic distributions in recruiting juveniles and adult subpopulations of the waterfall-climbing amphidromous goby, Sicyopterus stimpsoni. Morphological divergence between streams was present in both juvenile recruits and adult subpopulations, and was correlated with the steepness of the streams’ slope, a factor that influences the local selective environment. Spatially and temporally neutral genetic divergence was found only in juvenile recruits and was positively correlated with morphological divergence. These results suggest that temporal and spatial variation in recruiting cohorts with strong post-settlement selection on juveniles provides the opportunity for morphological adaptation to local stream environments. This work demonstrates the balance between migration and natural selection, which may impede speciation in the sea and may further explain why marine adaptive radiations are rare in Hawai‘i (Moody et al. 2015; see Wainwright 2015 MEC New and Views Perspective).

 

2. Opportunity for selection: the shape of fitness landscapes.

Natural selection is an important driver of adaptive evolution, but contrasting environmental pressures may lead to trade-offs between phenotypes that confer different performances and thus weaken the strength of selection and/or generate complex fitness surfaces with multiple local optima corresponding to different selection regimes. I evaluated how differences in patterns of phenotypic selection might promote morphological differences between subpopulations of the amphidromous Hawaiian waterfall-climbing goby, Sicyopterus stimpsoni. Collaborators and I conducted laboratory experiments to compare linear and nonlinear selection and the opportunity for selection in fish from Kaua‘i and the Big Island (Hawai‘i) due to the contrasting pressures of predator evasion and waterfall climbing which vary in disparity between the islands. Selection was strongest when individuals were exposed to their primary selective pressures (predator evasion on Kaua‘i and waterfall climbing on the Big Island). However, the opportunity for selection was greater on Kaua‘i for climbing and greater on the Big Island for predation. Canonical rotation analyses demonstrated that individuals from Kaua‘i and the Big Island occupy regions near their local fitness peaks for some traits. Therefore, selection for predation on Kaua‘i and climbing on the Big Island may not be as effective in promoting morphological changes in this endemic 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 the constraints on the opportunities for selection, population differences in phenotypic traits can arise due to differences in selective regimes. (Moody, Kawano et al. 2017)

 

3. Connectivity matters: Linking genomics, oceanography and ecology to understand adaptation.

Deciphering ecological and evolutionary factors that contribute to population divergence has been challenging in marine species, which oftentimes have low levels of population differentiation. However, environmental variables such as oceanic currents and post-settlement habitat differences can reduce gene flow resulting in fine-scale genetic structure, particularly at loci under selection. I analyzed population structure of S. stimpsoni across the Hawaiian archipelago by combing genomics with oceanographic models of passive larval dispersal and individuals-based models of natural selection (Moody et al. in prep) to examine how migration and selection interact to shape population connectivity and local adaptation. 

 

4.Trade-offs between natural and sexual selection shaping adaptation.

Juvenile S. stimpsoni exhibit phenotypic divergence correlated with exposure to different predominant pressures across their range. However, phenotypic divergence is reduced among adults from these localities, potentially because the removal of predators and the onset of sexual selection make selection pressures more similar across streams for adults than for juveniles. To test if ontogenetic erosion of phenotypic divergence is related to temporal changes in selection, collaborators and I will test how body shape affects predator escape response and matting success in S. stimpsoni. Clarifying how morphological variation impacts the performance for age-appropriate functions will elucidate which factors contribute to, and restrict, phenotypic divergence.