Selection on many levels....

I'm rereading Unto Others, David Wilson and Elliott Sober's argument for Multilevel Selection. One of the core planks in the book is that supra-individual levels of selection are necessary for the evolution of altruism, and much of the book details what Wilson & Sober perceive are the stretched and implausible explanations that scientists who are straight-jacketed into individual selection need to concoct. So how can selection between groups lead to the emergence of altruism? Consider two groups: Mixed & Selfish Assume that both groups begin in generation 1 with 100 individuals. Mixed: 50 altruistic individuals 50 selfish individuals Selfish: 100 selfish individuals Now, assume that the two groups are distinct & separate. Selfish grows at a rate of 1% per generation. After 30 generations Selfish will have ~135 individuals. Mixed is more complex. The altruists provide a benefit to the group so that the selfish individuals reproduce at a rate of 2% per generation. In contrast, the altruists have a cost so they only grow at 1.25% per generation (imagine that in pairwise interactions they sometimes have to deal with selfish free riders). After 30 generations you have ~163 individuals. Clearly the mixed group is "fitter" in terms of increase. But, where in generation 1 the altruists were 50% of the Mixed deme, they are now 44% of the deme. Though they increase the fitness of their group, they are selected against within group. Additionally, if you add all selfish individuals together (between groups) then you note that they've more than doubled. In contrast, the altruists have only nearly increased their numbers half again. In the far off future of generation 300 in the mixed deme the altruists are 8% of the population and the selfish 92%. But, there is an interesting development, here are the absolute numbers: ~3015 altruists in Mixed ~34437 selfish in Mixed ~2667 selfish in Selfish Though the altruists grow far slower in number then non-altruists, over the long term they've managed get ahead of the total size of the Selfish deme, which in absolute numbers if you will recall consisted of 100 individuals vs. the 50 altruists in the Mixed deme in the initial generation. Simply counting across demes and lumping the altruists and selfish together as if they were an amorphous mass removes information about the dynamics of population expansion and growth. This is a form of what Wilson & Sober term the "averaging fallacy," just averaging fitness across groups of individuals with distinct phenotypes and eliding the substructure of arrangment. But, note that the percentage of altruists keeps declining. In the real world exponential growth eventually is restrained by resource constraints of some sort, and at that point one assumes that the altruists will slowly converge upon extinction as the selection coefficients favor the selfish. But this is where step two comes into play: dispersal. Consider that at generation 300 the two groups disperse into sub-demes of approximately ~100, as in the initial scenario. Also imagine that of the 8% who are altruists they assortatively cluster with each other so that they are distributed in a 50/50 fashion with the selfish, while the vast majority of demes are exclusively selfish. What will occur is simple: the demes with the altruists will supersede those which are exclusively selfish...until once more we are at the point where the altruists are a benighted minority amongst their selfish peers. And so on. The above was a "toy" example, real interdemic selection models are generally more complicated (e.g., imagine for example that the fitness of the selfish within the Mixed demes was dynamically dependent upon the proportion of the unselfish). The great evolutionary biologist John Maynard Smith did not accept the relevance of group level selective effects primarily because of the complexities of the dynamic interactions, in particular the necessity of fissioning in the manner alluded to above. Workers such as Smith implied that the "cheating" of selfish individuals, the power of within group/individual level selection always seems like it should dominate the between group variation (which relied on longer "generations"). Additionally, in most populations between group genetic variance is far less than within group variance, and evolutionary response to selection is proportionate to variation. These are some of the reasons that theories such s kin selection and reciprocal altruism emerged, they are fully reduced down to the level of the individual and so exhibit more apparent parsimony. A more technical elucidation of these topics can be found at this post Defining Group Selection: Price's Equation. I will be posting on this topic more in the near future....