Size and development time are universally important traits. Yet evolutionary trends in development time are often viewed as allometric and physiological by-products of initial size, when life-history theory predicts that both traits are targets of selection and evolve adaptively through opposing effects on fitness. Surprisingly, this prediction has rarely been tested by disentangling the direct and indirect effects of size and development time on fitness, as necessary to understand selection on each trait. Here, in a marine external fertiliser that provides novel scope for such tests, we measure directional, quadratic, and correlational selection acting on early size (of embryos and post-hatch larvae) and development time (from fertilisation to hatching) through survival of juveniles in the field. We find little directional selection acting on traits during this selective episode. Rather, selection is primarily correlational, targeting combinations of development time and post-development size in a way that acts against their already-weak positive correlation, and could eventually drive a negative correlation between them if persistent enough. Lack of correlational selection on combinations of embryo size and development time, in contrast, suggests that physiological or allometric constraints more likely explain their positive association. Hence, neither life-history theory nor principles of allometry and physiology alone may predict the evolution of size and development time, warranting greater appreciation of the tension between adaptive and non-adaptive explanations for evolutionary trends in these traits.

中文翻译：

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大小和发育时间的相关选择在生命早期阶段不一致

大小和开发时间是普遍重要的特征。然而，发育时间的进化趋势通常被视为初始大小的异速生长和生理副产品，当时生命史理论预测这两种特征都是选择的目标，并通过对适应度的相反影响自适应地进化。令人惊讶的是，这种预测很少通过解开大小和发育时间对适应度的直接和间接影响来测试，这是了解每个性状选择所必需的。在这里，在为此类测试提供新范围的海洋外部肥料中，我们测量了定向、二次和相关选择，这些选择作用于早期尺寸（胚胎和孵化后幼虫的大小）和发育时间（从受精到孵化）直至幼鱼的存活在该领域。我们发现在这个选择性事件中几乎没有定向选择作用于特征。相反，选择主要是相关的，以开发时间和开发后规模的组合为目标，这种方式与它们已经很弱的正相关相反，如果持续足够，最终可能会导致它们之间的负相关。相反，缺乏对胚胎大小和发育时间组合的相关选择表明，生理或异速生长限制更有可能解释它们的正相关。因此，无论是生活史理论还是异速生长和生理学原理都不能单独预测大小和发育时间的进化，从而保证了对这些特征进化趋势的适应性和非适应性解释之间的紧张关系的更大理解。