The Genetic Basis of Family Effects and the Evolutionary Limits to Large Body-Size.

在有父母照顾的生物体中，一个人成功的主要决定因素是父母的决定，这决定了这个人应该得到多少资源。蓝山雀的父母能够为后代提供多少资源，差异很大。其中一些变异会有遗传基础，但目前我们不知道基因是起主要作用还是起次要作用。通过强迫蓝山雀抚养非自己的后代，我们可以问，两个有血缘关系的母亲(例如姐妹)的寄养后代是否比没有血缘关系的母亲的寄养后代的生长速度更相似。我们可以用相似程度来说明生长速度的变化在多大程度上是由于父母体内表达的基因的作用。我们也不知道那些拥有让他们为后代提供更多资源的基因的个体是否更成功。为后代提供更多的资源有助于他们的后代存活下来，并将他们的基因传递给后代，因此人们经常认为这些个体拥有成功的基因。然而，提供较少资源的父母可能有更好的机会存活到第二年并再次繁殖，所以也许正是这些个体拥有更成功的基因？通过观察哪些个体一年比一年存活下来，并计算他们有多少后代，我们将能够说出这两种策略中哪一种更受自然选择的青睐，或者实际上，最好的策略是否是两者之间的妥协。一个个体从父母那里获得的资源数量并不是固定的。个体可以操纵父母提供更多食物(通过乞讨等行为)，但我们不知道个体是否有控制自己操纵能力的基因。然而，通过将亲戚(例如兄弟)安置在不同的巢中，我们可以看到它们的两组养父母是否以相似的比率提供食物。如果他们这样做了，这将意味着这两兄弟拥有共同的基因，正在操纵他们的养父母。使兄弟俩非常善于操纵的基因对兄弟俩有利(这会增加他们得到的食物数量)，而对父母不利(他们必须更加努力地工作)。然而，目前还不清楚这些基因是否会对兄弟俩的伴侣产生不利影响。如果一只兄弟强迫父母带更多的食物来，这些食物是由巢中所有成员分享的，还是兄弟征用了所有的食物，留给巢中伙伴的食物更少？如果是前者，操控基因将有利于个体筑巢；如果是后者，操控基因将对个体筑巢不利。我们可以通过比较操纵性兄弟的巢友和非操纵性兄弟的巢友来区分这两个假说。如果爱操控的兄弟的伴侣平均比不爱操控的兄弟的伴侣轻，我们就知道操控的基因对伴侣不利。这些问题的答案可能会帮助我们解决一个长期存在的问题：体型较大的个体往往比体型较小的个体存活得更好，后代更多，由于体型是可遗传的，因此我们预计大多数物种会随着时间的推移进化成更大的物种。这不是我们所看到的。然而，如果使个体变大的基因也对父母不利(因为他们不得不更加努力地工作)或对兄弟姐妹有害(因为他们被剥夺了食物)，那么也许大型不会进化。这是因为使个体变大的基因的相同拷贝也存在于个体的父母和兄弟姐妹中。使个体变大的基因直接使自己受益，而使个体变小的基因则间接帮助个体的亲属复制其自身。通过这种方式，大体型和小体型的基因可能会做得同样好，然后我们就不会指望大体型进化。

In organisms with parental care, a major determinant of an individual's success are the parental decisions that determine how much resource that individual should receive. Blue tit parents vary widely in how much resource they are able to provide to their offspring. Some of this variation will have a genetic basis, but currently we do not know whether genes play a major or minor role. By forcing blue tits to raise offspring that are not their own we can ask if the foster-offspring of two related mothers (e.g. sisters) grow at more similar rates compared to the foster-offspring of unrelated mothers. We can use the degree of similarity to say how much of the variation in growth rate is due to the action of genes expressed in parents. We also don't know whether individuals that have genes that make them provide more resources to their offspring are more successful. Providing more resources to their offspring helps their offspring to survive and transmit their genes into future generations, and so it has often been thought that these individuals have successful genes. However, a parent that provides fewer resources may have a better chance of surviving to the following year and reproducing again, and so maybe it is these individuals that have more successful genes? By looking to see which individuals survive from year to year, and counting how many offspring they have, we will be able to say which of these two strategies is favoured by natural selection, or indeed if the best strategy is a compromise between the two.The amount of resources an individual receives from its parents is not fixed. Individuals can manipulate their parents into providing more food (via behaviours such as begging) but we do not know whether individuals have genes that control how manipulative they are. However, by placing relatives (e.g. brothers) in different nests we can see whether their two sets of foster parents provide food at similar rates. If they did, this would imply that there are genes shared by the two brothers that are manipulating their foster parents. Genes that make the brothers very manipulative would be good for the brothers (it would increase the amount of food they receive) and bad for the parents (they would have to work harder). However, it is unclear whether these genes would have detrimental effects on the brothers' nest mates. If a brother forces the parent to bring more food, is this food shared by all members of the nest, or does the brother commandeer all of the food, leaving less for its nest mates? If the former, manipulative genes would be beneficial for an individuals nest mates, but if the latter, manipulative genes would be detrimental to an individuals nest mates. We can distinguish between these two hypotheses by comparing the nest mates of manipulative brothers, and nest mates of non-manipulative brothers. If the nest mates of manipulative brothers are on average lighter than the nest mates of non-manipulative brothers we know that the genes for manipulation are bad for nest-mates. The answers to these questions may help us to address a long-standing problem: large individuals tend to survive better and have more offspring than small individuals, and because size is heritable we would therefore expect most species to be evolving to be larger over time. This is not what we see. However, if the genes that make an individual large are also bad for parents (because they have had to work harder) or bad for siblings (because they are deprived of food) then perhaps large size will not evolve. This is because identical copies of the genes that make an individual large are also present in that individual's parents and siblings. A gene that makes an individual large directly benefits itself, but a gene that makes an individual small indirectly helps copies of its self in the individual's relatives. In this way genes for large size and small size may do equally well, and then we would not expect large size to evolve.