The Tetonius-Pseudotetonius (T-P) transition is an often-cited example of phyletic gradualism, but rates of evolution and the roles of neutral and adaptive processes across this lineage remain unclear. Linking Tetonius and Pseudotetonius, two omomyid primates, are a series of stratigraphic and morphologic intermediates revealing trends suggestive of a functional and developmental reorganization of the dentition. Notable changes involved the P3, which reduced size and became indistinguishable from the canine and I2, and the P4, which became a robust tall-cusped tooth comparable to a molar. We test two hypotheses: (1) neutral evolution can explain the observed phenotypic differences in the lineage, and (2) P4 lost developmental association with P3 and became integrated with the molars. First, we calculate the rate of evolutionary differentiation, based on the ratio between inter- and intra-species variation in length and width of P3, P4, M1, and M2 teeth, between lineage segments and over the entire lineage. We test the second hypothesis by comparing bivariate correlations between teeth within individual lineage segments. As the lineage evolved, correlations between P3 and the molars diminished, whereas correlations between P4 and the molars increased. We found evidence of varying degrees of stabilizing selection in the lengths and widths of all cheek teeth and evidence of neutral evolution in the width of P4. These results support a trend towards P4 becoming integrated into the molar morphogenetic field, and demonstrates that morphological rates of evolution, and consequently the degree of selective pressures, vary through time and between teeth.

中文翻译：

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Tetonius 进化中的模块和马赛克 – Pseudotetonius 牙列

Tetonius-Pseudotetonius (TP) 过渡是系统渐进主义的一个经常被引用的例子，但进化速度以及跨越该谱系的中性和适应性过程的作用仍不清楚。将 Tetonius 和 Pseudotetonius 联系起来，这两种 omomyid 灵长类动物是一系列地层和形态学中间体，揭示了暗示牙列功能和发育重组的趋势。显着的变化涉及 P3，它减小了尺寸，变得与犬齿和 I2 没有区别，而 P4，它变成了与磨牙相当的坚固的高尖牙。我们测试了两个假设：(1) 中性进化可以解释在谱系中观察到的表型差异，以及 (2) P4 失去了与 P3 的发育关联并与臼齿融为一体。首先，我们计算进化分化率，基于种间和种内 P3、P4、M1 和 M2 牙齿、谱系段之间和整个谱系的长度和宽度变化之间的比率。我们通过比较各个谱系片段内牙齿之间的双变量相关性来测试第二个假设。随着谱系的演变，P3 和磨牙之间的相关性减弱，而 P4 和磨牙之间的相关性增加。我们发现所有颊齿的长度和宽度都有不同程度的稳定选择的证据，以及 P4 宽度中性进化的证据。这些结果支持 P4 整合到磨牙形态发生领域的趋势，并表明进化的形态速率以及因此选择压力的程度随时间和牙齿之间的变化而变化。