Zelditch M L, Sheets H D, Fink W L
Museum of Paleontology, University of Michigan, Ann Arbor 48109, USA.
Evolution. 2000 Aug;54(4):1363-71. doi: 10.1111/j.0014-3820.2000.tb00568.x.
Heterochrony, evolutionary changes in rate or timing of development producing parallelism between ontogeny and phylogeny, is viewed as the most common type of evolutionary change in development. Alternative hypotheses such as heterotopy, evolutionary change in the spatial patterning of development, are rarely entertained. We examine the evidence for heterochrony and heterotopy in the evolution of body shape in two clades of piranhas. One of these is the sole case of heterochrony previously reported in the group; the others were previously interpreted as cases of heterotopy. To compare ontogenies of shape, we computed ontogenetic trajectories of shape by multivariate regression of geometric shape variables (i.e., partial warp scores and shape coordinates) on centroid size. Rates of development relative to developmental age and angles between the trajectories were compared statistically. We found a significant difference in developmental rate between species of Serrasalmus, suggesting that heterochrony is a partial explanation for the evolution of body shape, but we also found a significant difference between their ontogenetic transformations; the direction of the difference between them suggests that heterotopy also plays a role in this group. In Pygocentrus we found no difference in developmental rate among species, but we did find a difference in the ontogenies, suggesting that heterotopy, but not heterochrony, is the developmental basis for shape diversification in this group. The prevalence of heterotopy as a source of evolutionary novelty remains largely unexplored and will not become clear until the search for developmental explanations looks beyond heterochrony.
异时性，即发育速率或时间安排上的进化变化，使得个体发育与系统发育产生平行现象，被视为发育过程中最常见的进化变化类型。诸如异位性（发育空间模式的进化变化）等替代假说则很少被考虑。我们研究了两个食人鱼分支身体形态进化过程中异时性和异位性的证据。其中一个是该类群之前报道过的唯一异时性案例；其他案例之前被解释为异位性案例。为了比较形态的个体发育过程，我们通过对几何形状变量（即局部弯曲分数和形状坐标）与质心大小进行多元回归，计算出形态的个体发育轨迹。对相对于发育年龄的发育速率以及轨迹之间的角度进行了统计比较。我们发现锯脂鲤属物种之间的发育速率存在显著差异，这表明异时性是身体形态进化的部分解释，但我们也发现它们的个体发育转变之间存在显著差异；两者之间差异的方向表明异位性在该类群中也起作用。在臀点脂鲤属中，我们发现物种间发育速率没有差异，但个体发育过程存在差异，这表明异位性而非异时性是该类群形态多样化的发育基础。异位性作为进化新奇性来源的普遍性在很大程度上仍未被探索，直到对发育解释的探索超越异时性，才会变得清晰。
