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

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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.