Griffith Oliver W, Blackburn Daniel G, Brandley Matthew C, Van Dyke James U, Whittington Camilla M, Thompson Michael B
School of Biological Sciences, Heydon Laurence Building (A08), University of Sydney, Camperdown, NSW, Australia.
Department of Biology, Trinity College, Hartford, Connecticut.
J Exp Zool B Mol Dev Evol. 2015 Sep;324(6):493-503. doi: 10.1002/jez.b.22614. Epub 2015 Mar 2.
To understand evolutionary transformations it is necessary to identify the character states of extinct ancestors. Ancestral character state reconstruction is inherently difficult because it requires an accurate phylogeny, character state data, and a statistical model of transition rates and is fundamentally constrained by missing data such as extinct taxa. We argue that model based ancestral character state reconstruction should be used to generate hypotheses but should not be considered an analytical endpoint. Using the evolution of viviparity and reversals to oviparity in squamates as a case study, we show how anatomical, physiological, and ecological data can be used to evaluate hypotheses about evolutionary transitions. The evolution of squamate viviparity requires changes to the timing of reproductive events and the successive loss of features responsible for building an eggshell. A reversal to oviparity requires that those lost traits re-evolve. We argue that the re-evolution of oviparity is inherently more difficult than the reverse. We outline how the inviability of intermediate phenotypes might present physiological barriers to reversals from viviparity to oviparity. Finally, we show that ecological data supports an oviparous ancestral state for squamates and multiple transitions to viviparity. In summary, we conclude that the first squamates were oviparous, that frequent transitions to viviparity have occurred, and that reversals to oviparity in viviparous lineages either have not occurred or are exceedingly rare. As this evidence supports conclusions that differ from previous ancestral state reconstructions, our paper highlights the importance of incorporating biological evidence to evaluate model-generated hypotheses.
要理解进化转变，有必要识别已灭绝祖先的性状状态。祖先性状状态重建本质上是困难的，因为它需要准确的系统发育、性状状态数据以及转变速率的统计模型，并且从根本上受到诸如灭绝类群等缺失数据的限制。我们认为基于模型的祖先性状状态重建应用于生成假设，但不应被视为分析的终点。以有鳞目动物的胎生进化以及向卵生的逆转作为案例研究，我们展示了解剖学、生理学和生态学数据如何用于评估关于进化转变的假设。有鳞目动物的胎生进化需要生殖事件时间的改变以及构建蛋壳的特征的相继丧失。向卵生的逆转要求那些丧失的性状重新进化。我们认为卵生的重新进化本质上比相反的情况更困难。我们概述了中间表型的 inviability 如何可能对从胎生到卵生的逆转构成生理障碍。最后，我们表明生态学数据支持有鳞目动物的卵生祖先状态以及向胎生的多次转变。总之，我们得出结论，第一批有鳞目动物是卵生的，向胎生的频繁转变已经发生，并且在胎生谱系中向卵生的逆转要么没有发生，要么极其罕见。由于这一证据支持与先前祖先状态重建不同的结论，我们的论文强调了纳入生物学证据以评估模型生成的假设的重要性。
