Department of Ecology and Evolutionary Biology, University of Tennessee Knoxville, Knoxville, TN, 37996, USA.
The ByWater Institute, Tulane University, New Orleans, LA, 70118, USA.
BMC Evol Biol. 2019 Apr 11;19(1):88. doi: 10.1186/s12862-019-1413-4.
Local adaptation of marine and diadromous species is thought to be a product of larval dispersal, settlement mortality, and differential reproductive success, particularly in heterogeneous post-settlement habitats. We evaluated this premise with an oceanographic passive larval dispersal model coupled with individual-based models of post-settlement selection and reproduction to infer conditions that underlie local adaptation in Sicyopterus stimpsoni, an amphidromous Hawaiian goby known for its ability to climb waterfalls.
Our model results demonstrated that larval dispersal is spatio-temporally asymmetric, with more larvae dispersed from the southeast (the Big Island) to northwest (Kaua'i) along the archipelago, reflecting prevailing conditions such as El Niño/La Niña oscillations. Yet connectivity is nonetheless sufficient to result in homogenous populations across the archipelago. We also found, however, that ontogenetic shifts in habitat can give rise to adaptive morphological divergence when the strength of predation-driven post-settlement selection crosses a critical threshold. Notably, our simulations showed that larval dispersal is not the only factor determining the likelihood of morphological divergence. We found adaptive potential and evolutionary trajectories of S. stimpsoni were greater on islands with stronger environmental gradients and greater variance in larval cohort morphology due to fluctuating immigration.
Contrary to expectation, these findings indicate that immigration can act in concert with selection to favor local adaptation and divergence in species with marine larval dispersal. Further development of model simulations, parameterized to reflect additional empirical estimates of abiotic and biotic factors, will help advance our understanding of the proximate and ultimate mechanisms driving adaptive evolution, population resilience, and speciation in marine-associated species.
海洋和洄游物种的局部适应被认为是幼虫扩散、定居死亡率和生殖成功率差异的产物，特别是在异质的定居后栖息地中。我们通过海洋学被动幼虫扩散模型和定居后选择和生殖的个体基础模型来评估这一前提，以推断导致 Sicyopterus stimpsoni （一种具有爬瀑布能力的洄游夏威夷虾虎鱼）局部适应的条件。
我们的模型结果表明，幼虫扩散在时空上是不对称的，更多的幼虫从东南（大岛）向西北（考艾岛）沿着群岛扩散，反映了厄尔尼诺/拉尼娜振荡等流行条件。然而，连通性仍然足以导致群岛上的种群同质。然而，我们还发现，当捕食驱动的定居后选择的强度跨越一个关键阈值时，生境的个体发育变化可以导致适应性形态分歧。值得注意的是，我们的模拟表明，幼虫扩散并不是决定形态分歧可能性的唯一因素。我们发现，由于移民的波动，具有更强环境梯度和幼虫群体形态变化更大的岛屿上，S. stimpsoni 的适应潜力和进化轨迹更大。
与预期相反，这些发现表明，在具有海洋幼虫扩散的物种中，移民可以与选择协同作用，有利于局部适应和分歧。进一步开发模拟模型，参数化以反映对非生物和生物因素的额外实证估计，将有助于推进我们对驱动适应性进化、种群恢复力和海洋相关物种形成的近因和终极机制的理解。
