The occupation of new environments by evolutionary lineages is frequently associated with morphological changes. This covariation of ecotype and phenotype is expected due to the process of natural selection, whereby environmental pressures lead to the proliferation of morphological variants that are a better fit for the prevailing abiotic conditions. One primary mechanism by which phenotypic variants are known to arise is through changes in the timing or duration of organismal development resulting in alterations to adult morphology, a process known as heterochrony. While numerous studies have demonstrated heterochronic trends in association with environmental gradients, few have done so within a phylogenetic context. Understanding species interrelationships is necessary to determine whether morphological change is due to heterochronic processes; however, research is hampered by the lack of a quantitative metric with which to assess the degree of heterochronic traits expressed within and among species. Here I present a new metric for quantifying heterochronic change, expressed as a heterochronic weighting, and apply it to xiphosuran chelicerates within a phylogenetic context to reveal concerted independent heterochronic trends. These trends correlate with shifts in environmental occupation from marine to nonmarine habitats, resulting in a macroevolutionary ratchet. Critically, the distribution of heterochronic weightings among species shows evidence of being influenced by both historical, phylogenetic processes and external ecological pressures. Heterochronic weighting proves to be an effective method to quantify heterochronic trends within a phylogenetic framework and is readily applicable to any group of organisms that have well-defined morphological characteristics, ontogenetic information, and resolved internal relationships.

中文翻译：

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一种量化进化谱系异时性的新方法

进化谱系对新环境的占领通常与形态变化有关。由于自然选择的过程，预计生态型和表型的这种共变是由于环境压力导致更适合主要非生物条件的形态变异的增殖。已知表型变异出现的一个主要机制是通过改变生物体发育的时间或持续时间导致成人形态发生改变，这一过程称为异时性。虽然许多研究已经证明了与环境梯度相关的异时趋势，但很少有人在系统发育背景下这样做。了解物种的相互关系对于确定形态变化是否是由于异时过程是必要的；然而，由于缺乏量化指标来评估物种内部和物种之间表达的异时性状程度，研究受到了阻碍。在这里，我提出了一种用于量化异慢性变化的新指标，表示为异慢性加权，并将其应用于系统发育背景下的 xiphosuran chelicerates，以揭示协调一致的独立异慢性趋势。这些趋势与环境占领从海洋到非海洋栖息地的转变相关，从而导致宏观进化棘轮。至关重要的是，物种间异时权重的分布显示出受到历史、系统发育过程和外部生态压力影响的证据。