Contemporary Evolutionary Divergence for a Protected Species following Assisted Colonization

BackgroundContemporary evolution following assisted colonization may increase the probability of persistence for refuge populations established as a bet-hedge for protected species. Such refuge populations are considered “genetic replicates” that might be used for future re-colonization in the event of a catastrophe in the native site. Although maladaptive evolutionary divergence of captive populations is well recognized, evolutionary divergence of wild refuge populations may also occur on contemporary time scales. Thus, refuge populations may lose their “value” as true genetic replicates of the native population. Here, we show contemporary evolutionary divergence in body shape in an approximately 30-year old refuge population of the protected White Sands pupfish (Cyprinodon tularosa) resulting in a body-shape mismatch with its native environment.
Methodology/Principal FindingsGeometric morphometic data were collected from C. tularosa cultures raised in experimental mesocosms. Cultures were initiated with fish from the two native populations, plus hybrids, in high or low salinity treatments representing the salinities of the two native habitats. We found that body shape was heritable and that shape variation due to phenotypic plasticity was small compared to shape variation due to population source. C. tularosa from the high salinity population retained slender body shapes and fish from the low salinity population retained deep body shapes, irrespective of mesocosm salinity. These data suggest that the observed divergence of a recently established pupfish population was not explained by plasticity. An analysis of microsatellite variation indicated that no significant genetic drift occurred in the refuge population, further supporting the adaptive nature of changes in body shape. These lines of evidence suggest that body shape divergence of the refuge population reflects a case of contemporary evolution (over a 30-year period).
Conclusions/SignificanceThese results suggest assisted colonization can introduce novel, and/or relaxed selection, and lead to unintended evolutionary divergence.

研究背景：辅助定植（assisted colonization）后的当代演化，或可提升作为保护物种风险对冲策略所建立的避难所种群的存续概率。此类避难所种群被视为“遗传重复株”，可在原生栖息地遭遇灾变时，用于未来的重新定殖。尽管圈养种群的适应不良演化分化已得到广泛认知，但野生避难所种群的演化分化同样可能在当代时间尺度上发生。因此，避难所种群可能会丧失其作为原生种群真实遗传重复株的价值。本研究针对建立约30年的受保护白沙滩软鳍鲷（Cyprinodon tularosa）避难所种群展开分析，发现其体型出现当代演化分化，导致其体型与原生栖息地环境不匹配。

研究方法与主要发现：几何形态测量学（geometric morphometrics）数据采集自中型实验生态系统（mesocosms）中饲养的图拉罗萨软鳍鲷养殖群体。实验养殖群体分别由两种原生种群的个体及其杂交后代建立，设置高盐、低盐两种盐度处理，分别对应两种原生栖息地的盐度水平。研究发现，体型具有可遗传性，且由表型可塑性（phenotypic plasticity）导致的体型变异，远小于种群来源所带来的体型变异。无论中型实验生态系统的盐度如何，源自高盐种群的软鳍鲷始终保持修长体型，而源自低盐种群的个体则始终保持宽厚体型。上述数据表明，近期建立的软鳍鲷种群所观测到的体型分化，并非由表型可塑性所致。微卫星（microsatellite）变异分析显示，该避难所种群未发生显著的遗传漂变，进一步证实了体型变化的适应性本质。上述一系列证据表明，该避难所种群的体型分化，实为一场历时约30年的当代演化事件。

结论与意义：本研究结果表明，辅助定植可引入全新的选择压力或缓解原有选择压力，进而引发非预期的演化分化。