Resolving the paradox of evolutionary stasis

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.