Data from: Persistent directional selection on body size and a resolution to the paradox of stasis

Directional selection on size is common but often fails to result in micro-evolution in the wild. Similarly, macro-evolutionary rates in size are low relative to the observed strength of selection in nature. We show that many estimates of selection on size have been measured on juveniles, not adults. Further, parents influence juvenile size by adjusting investment per offspring. In light of these observations, we help resolve this paradox by suggesting that the observed upward selection on size is balanced by selection against investment per offspring, resulting in little or no net selection gradient. We find that trade-offs between fecundity and juvenile size are common, consistent with the notion of selection against investment per offspring. We also find that median directional selection on size is positive for juveniles but no net directional selection exists for adult size. This is expected because parent-offspring conflict exists over size, and juvenile size is more strongly affected by investment per offspring than adult size. These findings provide qualitative support to the hypothesis that upward selection on size is balanced by selection against investment per offspring, where parent-offspring conflict over size is embodied in the opposing signs of the two selection gradients.

针对体型的定向选择（directional selection）虽普遍存在，但在自然环境中往往难以引发微进化（micro-evolution）。同理，相较于自然界中观测到的选择强度，体型相关的宏观进化速率偏低。我们的研究发现，诸多体型选择的评估均以幼体而非成体为研究对象。此外，亲本可通过调整对每枚后代的投入，对幼体体型产生影响。基于上述观测结果，我们提出如下假说以解决这一悖论：观测到的针对体型的正向选择，会被针对单枚后代投入的负向选择所抵消，最终几乎不存在净选择梯度（selection gradient）。我们发现，繁殖力与幼体体型间的权衡（trade-off）普遍存在，这与针对单枚后代投入的负向选择这一观点相符。同时我们还发现，幼体体型的定向选择中位数为正值，但成体体型则不存在净定向选择。这一结果符合预期，因为亲子间在体型方面存在亲子冲突（parent-offspring conflict），且幼体体型受单枚后代投入的影响强于成体体型。上述研究结果为下述假说提供了定性支持：针对体型的正向选择会被针对单枚后代投入的负向选择所抵消，而二者选择梯度符号相反正体现了体型相关的亲子冲突。