Input data from: Mammalian forelimb evolution is driven by uneven proximal-to-distal morphological diversity

Vertebrate limb morphology often reflects the environment due to variation
in locomotor requirements. However, proximal and distal limb segments may
evolve differently from one another, reflecting an anatomical gradient of
functional specialization that has been suggested to be impacted by the
timing of development. Here we explore whether the temporal sequence of
bone condensation predicts variation in the capacity of evolution to
generate morphological diversity in proximal and distal forelimb segments
across more than 600 species of mammals. Distal elements not only exhibit
greater shape diversity, but also show stronger within-element integration
and, on average, faster evolutionary responses than intermediate and upper
limb segments. Results are consistent with the hypothesis that
late-developing distal bones display greater morphological variation than
more proximal limb elements. However, the higher integration observed
within the autopod deviates from such developmental predictions,
suggesting that functional specialization plays an important role in
driving within-element covariation. Proximal and distal limb segments also
show different macroevolutionary patterns, albeit not showing a perfect
proximo-distal gradient. The high disparity of the mammalian autopod,
reported here, is consistent with the higher potential of development to
generate variation in more distal limb structures, as well as functional
specialization of the distal elements.

脊椎动物附肢的形态通常会因运动需求的差异而反映其生存环境。然而，附肢的近段与远段结构各自的演化路径往往存在分化，这体现了功能特化的解剖学梯度——此前已有研究提出该类梯度可能受发育时序的调控。

本研究以超过600种哺乳动物的前肢近段与远段附肢节段为研究对象，旨在探究骨骼原基凝聚的时间序列是否能够预测演化产生形态多样性的能力差异。研究结果显示，远段附肢结构不仅具有更高的形态多样性，其内部各组成部分间的整合度也更强，且平均演化响应速率显著快于中段与上段附肢。

本研究结果与“发育时序更靠后的远段骨骼比近段附肢骨骼具有更高形态变异度”的假说相符。然而，远侧附肢段（autopod）内部观测到的更高整合度却与前述发育学预测相悖，这提示功能特化在驱动附肢节段内部的协变关系中发挥了关键作用。

研究同时发现，附肢的近段与远段节段呈现出不同的宏观演化模式，尽管二者并未呈现出完美的近-远端梯度分布。本研究所报道的哺乳动物远侧附肢段的高度形态分异度，既契合了“发育时序靠后的远侧附肢结构具备更高变异产生潜力”的假说，也印证了远侧附肢节段所具有的功能特化特征。