Van Buskirk Josh, McCollum S Andy, Werner Earl E
Department of Biology, University of Michigan, Ann Arbor, Michigan, 48109.
Evolution. 1997 Dec;51(6):1983-1992. doi: 10.1111/j.1558-5646.1997.tb05119.x.
Models suggest that phenotypic plasticity is maintained in situations where the optimal phenotype differs through time or space, so that selection acts in different directions in different environments. Some empirical work supports the general premise of this prediction because phenotypes induced by a particular environment sometimes perform better than other phenotypes when tested in that environment. We have extended these results by estimating the targets of selection in Pseudacris triseriata tadpoles in environments without predators and with larval Anax dragonflies. Tadpoles displayed significant behavioral and morphological plasticity when reared in the presence and absence of nonlethal dragonflies for 32 days in cattle tanks. We measured selection in the absence of free predators by regressing growth and survival in the tanks against activity and several measures of tail and body shape. We measured selection in the presence of predators by exposing groups of 10 tadpoles to Anax in overnight predation trials and regressing the average phenotype of survivors against the number of tadpoles killed. Selection in the two environments acted in opposite directions on both tail and body shape, although the affected fitness components were different. In the presence of Anax, tadpoles with shallow and narrow body, deep tail fin, and wide tail muscle survived best. In the absence of free predators, tadpoles with narrow tail muscle grew significantly faster, and those with shallow tail fin and deep body grew somewhat faster. Activity was unrelated to survival or growth in either environment. Developmental plasticity in tail shape closely paralleled selection, because tail fin depth increased after long-term exposure to Anax and tail muscle width tended to increase. In contrast, there was no plasticity in body shape in spite of strong selection for decreasing body depth. Thus, when confronted with a dragonfly predator, P. triseriata tadpoles adjusted their tail shape (but not body shape) almost exactly in the direction of selection imposed by Anax. These results suggest that phenotypic plasticity in some morphological traits, such as tail depth and tail muscle width, has evolved under intermittent selection by dragonflies. Other traits that undergo selection by dragonflies, such as body morphology, appear developmentally rigid, perhaps because of historically strong opposing selection in nature or other constraints.
模型表明，在最优表型随时间或空间变化的情况下，表型可塑性得以维持，从而使选择在不同环境中朝着不同方向起作用。一些实证研究支持了这一预测的总体前提，因为在特定环境中诱导产生的表型，在该环境中进行测试时，有时比其他表型表现得更好。我们通过估计在没有捕食者和有幼虫期的大蜓（Anax）蜻蜓的环境中，三齿拟蝗蛙（Pseudacris triseriata）蝌蚪的选择目标，扩展了这些结果。当在牛水槽中，在有和没有非致命蜻蜓的情况下饲养32天时，蝌蚪表现出显著的行为和形态可塑性。我们通过将水槽中的生长和存活情况与活动以及几种尾巴和身体形状的测量值进行回归分析，来测量在没有自由捕食者情况下的选择。我们通过在夜间捕食试验中将10只蝌蚪一组暴露于大蜓，并将幸存者的平均表型与被杀死的蝌蚪数量进行回归分析，来测量在有捕食者情况下的选择。尽管受影响的适合度成分不同，但在两种环境中的选择在尾巴和身体形状上都朝着相反方向起作用。在有大蜓的情况下，身体浅而窄、尾鳍深且尾肌宽的蝌蚪存活得最好。在没有自由捕食者的情况下，尾肌窄的蝌蚪生长明显更快，尾鳍浅且身体深的蝌蚪生长也稍快一些。在两种环境中，活动与存活或生长均无关。尾巴形状的发育可塑性与选择密切平行，因为长期暴露于大蜓后尾鳍深度增加，尾肌宽度趋于增加。相比之下，尽管对减小身体深度有强烈选择，但身体形状没有可塑性。因此，当面对蜻蜓捕食者时，三齿拟蝗蛙蝌蚪几乎完全朝着大蜓施加的选择方向调整它们的尾巴形状（但不是身体形状）。这些结果表明，一些形态特征，如尾深和尾肌宽度的表型可塑性，是在蜻蜓的间歇性选择下进化而来的。其他受蜻蜓选择的特征，如身体形态，在发育上似乎是刚性的，这可能是由于在自然环境中历史上存在强烈的反向选择或其他限制因素。
