Resolving the paradox of evolutionary stasis

研究表明，高个子的人收入更高，对异性也更有吸引力。这种现象并不局限于人类，也是多种生物体的普遍发现：大个体往往比小个体更好。此外，体型大的父母总是能生出体型大的后代。综合起来，这两项发现表明，大多数生物体应该进化出更大的体型。然而，这方面的证据很薄弱，许多物种似乎在数千代中保持相同的大小。目前，我们还没有很好的解释为什么物种在很长一段时间内保持相同的大小。一种可能性是，我们往往忽视了个体的体型对家庭成员的影响。如果体型大对其他家庭成员有害，那么我们可以预期物种会比对个体有益的小。例如，如果父母有贪婪的后代，那么他们一生中可能只能生育很少的后代，而与贪婪的兄弟姐妹一起长大的个体更有可能死于饥饿。虽然我们有一个很好的想法，这两种现象存在，我们不知道他们是否可以解释为什么生物体是他们的大小。此外，我们对父母的照顾或兄弟姐妹之间的竞争的遗传几乎一无所知。这很重要，因为这些现象可以改变正常的遗传模式。例如，在正常遗传的情况下，来自基因编程为快速生长的家庭的个体将比平均值大。然而，当每个人都试图快速成长时，兄弟姐妹之间的竞争可能会更强烈，而年幼的兄弟姐妹最终可能会成长得非常缓慢，尽管他们有试图使他们长大的基因。我们可以通过观察亲属之间的相似度是否高于非亲属之间的相似度来衡量遗传模式。然而，亲戚之间可能会相似，因为他们拥有相同的基因，或者因为他们在相同的环境中长大。为了区分先天和后天的影响，我们可以让父母抚养一些自己的后代和一些寄养后代。如果涉及到基因，那么父母的后代应该彼此相似。然而，如果环境也起作用，那么父母自己的后代也可能类似于寄养的后代。在人类中建立寄养家庭是不道德的，但我们可以在蓝山雀中建立寄养家庭，因为它们似乎没有注意到任何差异。虽然蓝山雀与人类有很大的不同，但它们确实表现出一些相似之处：它们与父母双方的兄弟姐妹一起长大，并且存在大量的竞争，最年轻的通常是最差的。此外，由于它们非常常见，并且会使用人造巢箱，因此可以创建许多正常和寄养后代混合的家庭。只有当我们比较了许多家庭，才有可能准确地衡量基因和环境的相对重要性。此外，当寄养家庭以某种方式建立时，它也让我们看到基因是否在兄弟姐妹竞争中发挥作用，几代人之后，我们也可以看看基因是否也在父母照顾中发挥作用。一旦我们了解了体型、父母的照顾和兄弟姐妹之间的竞争是如何遗传的，一旦我们了解了体型大是否对其他家庭成员有害，我们就会知道家庭的相互作用是否会使物种变小。

Tall people have been shown to earn more and appear more attractive to the opposite sex. This phenomenon is not restricted to humans, and is a general finding across may types of organism: large individuals tend to be better than smaller individuals. In addition large parents are consistently shown to produce large offspring. Taken together, these two findings suggest that most organisms should be evolving larger body sizes. However, the evidence for this is weak, and many species appear to stay the same size for thousands of generations. Currently, we do not have a good explanation of why species stay the same size for large periods of time. One possibility is that we have tended to ignore the impact that an individual's body size has on its family members. If being large is harmful to other family members then we may expect species to be smaller than what is good for the individual. For example, if parents have greedy offspring then they may only be able to produce few offspring over their lifetime, and individuals raised with greedy siblings may be more likely to die from starvation. Although we have a good idea that both these phenomenon exist, we do not know if they can explain why organisms are the size they are. In addition, we know almost nothing about the inheritance of parental care or sibling rivalry. This is important because these phenomena can change the normal inheritance patterns. For example, with normal inheritance individuals from families that are genetically programmed to grow fast will be larger than the average. However, sibling rivalry may be stronger when everyone is trying to grow fast and younger siblings may end up growing very slowly despite having genes that try and make them large. We can measure patterns of inheritance by looking if relatives resemble each other more than non-relatives. However, relatives may resemble each other because they share the same genes or because they have been raised in the same environment. To separate the effects of nature and nurture we can let parents raise some of their own offspring and some foster offspring. If genes are involved, then the parents own offspring should resemble each other. However, if the environment also plays a role then the parents own offspring may also resemble the fostered offspring. It is not ethical to create foster families in human populations, but we can create foster families in blue tits, as they do not appear to notice any difference. Although blue tits are very different from humans they do show some similarities: they are raised with their siblings by both parents, and there is a great deal of competition, with the youngest usually coming out the worst. Also, because they are very common and will use man made nest boxes it is possible to create many families that are a mixture of normal and fostered offspring. Only when we have compared many families is it possible to accurately measure the relative importance of genes and the environment. Also, when foster families are created in certain ways it also allows us to see if genes play a role in sibling rivalry, and after a few generations we can also look to see if genes also play a role in parental care. Once we understand how body-size, parental care and sibling rivalry are inherited, and once we understand whether being large is harmful to other family members, we will know whether family interactions keep species small.