The evolution of senescence is often explained by arguing that, in nature, few individuals survive to be old and hence it is evolutionarily unimportant what happens to organisms when they are old. A corollary to this idea is that extrinsically imposed mortality, because it reduces the chance of surviving to be old, favors the evolution of senescence. We show that these ideas, although widespread, are incorrect. Selection leading to senescence does not depend directly on survival to old age, but on the shape of the stable age distribution, and we discuss the implications of this important distinction. We show that the selection gradient on mortality declines with age even in the hypothetical case of zero mortality, when survivorship does not decline. Changing the survivorship function by imposing age independent mortality has no affect on the selection gradients. A similar result exists for optimization models: age independent mortality does not change the optimal result. We propose an alternative, brief explanation for the decline of selection gradients, and hence the evolution of senescence.

中文翻译：

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生存到老年的稀有性并不能推动衰老的演变。

衰老的进化通常通过以下论点来解释：在自然界中，很少有人能活到老，因此生物体在衰老时会发生什么在进化上并不重要。这个想法的一个推论是，外在强加的死亡率，因为它减少了活到老的机会，有利于衰老的演变。我们表明，这些想法虽然很普遍，但却是不正确的。导致衰老的选择并不直接取决于生存到老年，而是取决于稳定年龄分布的形状，我们讨论了这一重要区别的含义。我们表明，即使在死亡率为零的假设情况下，当存活率不下降时，死亡率的选择梯度也会随着年龄的增长而下降。通过施加与年龄无关的死亡率来改变生存函数对选择梯度没有影响。优化模型也存在类似的结果：与年龄无关的死亡率不会改变最佳结果。我们对选择梯度的下降以及衰老的演变提出了另一种简短的解释。