The newest issue of Science Magazine includes a lovely demonstration of multilevel selection by Omar Tonsi Eldakar, my former graduate student, who is currently at the University of Arizona’s Center for Insect Science.
Readers who have been following my “Truth and Reconciliation for Group Selection” series will be well prepared to appreciate the import of the Science article. Group selection requires variation among groups. Variation among groups is eroded by dispersal. Therefore, group selection can only take place in groups that are highly isolated from each other. That is part of the reasoning the led to the conclusion that group selection can only take place under highly restrictive conditions.
But wait. This argument assumes that dispersal is random. What if dispersal is conditional? What if individuals stay in groups when they are sufficiently cooperative but leave when they become overrun by selfish individuals? In this case, dispersal might increase variation among groups, improving the conditions for group selection. John Pepper and Athena Aktipis (featured in T&R XII) are two theorists who have studied this “walk away” process in agent-based simulation models.
Omar and his co-authors have provided a lovely empirical demonstration of the walk-away process for sexual conflict in water striders. Male striders differ greatly in the aggressiveness of their mating tactics. Some would qualify as psychopathic sexual predators by human standards, while others are perfect gentlemen. In previous research, Omar and his colleagues created groups in which the composition of males varied from all gentlemen to all psychopaths. Females are far more productive in the company of gentlemen, as one might expect, but the psychopaths have the highest mating success within any group containing both types. Thus, the gentlemen count as altruistic and the psychopaths count as selfish in a classic group selection scenario–what I call “the original problem” in the T&R series.
Six compartments could be closed off or opened to allow dispersal among compartments. In the closed condition, there was a strong positive relationship between male aggressiveness and mating success, as shown by the top graph. In the open condition, females dispersed in response to the presence of psychopathic males. The psychopaths could also move, but the end result was a considerable amount of variation among groups, as predicted by the walk away models. Thanks to the clustering of the females around the gentlemen, the relationship between male aggressiveness and mating success becomes hump-shaped with the peak at a low level of aggressiveness, as shown in the bottom graph. The fact that the same individual striders were used in both conditions makes this a particularly good demonstration of the effect of population structure on the fitness consequences of
Group selection explains how locally disadvantageous traits can nevertheless evolve in the total population by virtue of increasing the productivity of their group. Knowing what evolves in the total population does not provide sufficient information to evaluate whether group selection needs to be invoked. It is the comparison between local fitness differentials and global fitness differentials that provides sufficient information. This study is lovely precisely because it provides the comparison at both the local and global scales and because the variation among groups emerged from the movement of the animals themselves, rather than being imposed by the experimenter.