The Unit and Level of Selection

Elliott Sober reviews Samir Okasha’s  A Philosopher Looks at the Units of Selection Evolution and the Levels of Selection in RedOrbit:

Samir Okasha’s wonderful new book, Evolution and the Levels of Selection, is a philosophical examination of the conceptual framework that MLS [multilevel selection] theory deploys. Lewontin’s early formalism may give the impression that the idea of selection occurring at different levels of organization is straightforward and that the difference between group and individual selection is transparent The complexities that have become visible since the 1970s show otherwise. One complication arises in connection with the Price equation. Consider this simple example: There are two groups of zebras, one composed entirely of fast zebras, the other entirely of slow ones. Suppose the fast group is less likely to go extinct. According to the Price equation, in this situation there is group selection and no individual selection, because all the variance in fitness is between groups. But surely it is possible that the groups differ in fitness just because there is individual selection for running fast. Selection at the individual level can create a fortuitous benefit for the group (as George Williams put it). The Price equation is unable to recognize this. Biologists have coped with this problem in different ways-for example, by invoking the statistical techniques of contextual analysis and by employing a methodology called neighborhood analysis. Okasha skillfully analyzes the Price equation’s strengths and limitations and these more recent attempts to do better.

Another complication that arose as MLS theory developed was that there really are two types of MLS. In discussions of the evolution of altruism, a group’s fitness is usually defined as the number of offspring organisms the group produces. But one can also conceive of group fitness in terms of the number of daughter groups (regardless of size) the group produces. This second type of MLS has been important in discussions of species selection and of major evolutionary transitions. Both concepts raise questions about what heritability at the group level means, and here again Okasha does much to clarify what is at stake.