It is of fundamental importance for the field of evolutionary biology to understand when and why major evolutionary transitions occur. Live-bearing young (viviparity) is a major evolutionary change and has evolved from egg-laying (oviparity) independently in many vertebrate lineages and most abundantly in lizards and snakes. Although contemporary viviparous squamate species generally occupy cold climatic regions across the globe, it is not known whether viviparity evolved as a response to cold climate in the first place. Here, we used available published time-calibrated squamate phylogenies and parity data on 3,498 taxa. We compared the accumulation of transitions from oviparity to viviparity relative to background diversification and a simulated binary trait. Extracting the date of each transition in the phylogenies and informed by 65 my of global palaeoclimatic data, we tested the nonexclusive hypotheses that viviparity evolved under the following: (a) cold, (b) long-term stable climatic conditions and (c) with background diversification rate. We show that stable and long-lasting cold climatic conditions are correlated with transitions to viviparity across squamates. This correlation of parity mode and palaeoclimate is mirrored by background diversification in squamates, and simulations of a binary trait also showed a similar association with palaeoclimate, meaning that trait evolution cannot be separated from squamate lineage diversification. We suggest that parity mode transitions depend on environmental and intrinsic effects and that background diversification rate may be a factor in trait diversification more generally.

中文翻译：

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胎生的进化起源与古气候和谱系多样化一致。

对于进化生物学领域来说，了解主要进化转变发生的时间和原因至关重要。活产幼体（胎生）是一种主要的进化变化，在许多脊椎动物谱系中独立地从产卵（卵生）进化而来，在蜥蜴和蛇中最为丰富。尽管当代胎生有鳞物种通常占据全球寒冷的气候区域，但尚不清楚胎生是否最初是作为对寒冷气候的反应而进化的。在这里，我们使用了 3,498 个分类群的可用已发布时间校准有鳞类系统发育和奇偶校验数据。我们比较了相对于背景多样化和模拟二元性状从卵生到胎生过渡的积累。提取系统发育中每次转变的日期并根据 65 米的全球古气候数据，我们测试了胎生在以下条件下进化的非排他性假设：(a) 寒冷，(b) 长期稳定的气候条件和 (c)背景多样化率。我们表明，稳定和持久的寒冷气候条件与有鳞动物向胎生的转变有关。这种奇偶模式和古气候的相关性反映在有鳞类的背景多样化中，二元性状的模拟也显示出与古气候类似的关联，这意味着性状进化不能与有鳞类的谱系多样化分开。