Overview

Specific syndromes of selective factors can create situations where groups are selected because they display group properties that are selected-for. Some mosquito-transmitted rabbit viruses, for instance, are only transmitted to uninfected rabbits from infected rabbits that are still alive. This creates a selective pressure on every group of viruses already infecting a rabbit not to become too virulent and kill their host rabbit before enough mosquitoes have bitten it. In natural systems such viruses display much lower virulence levels than do mutants of the same viruses that in laboratory culture readily out-compete non-virulent variants (or than do tick-transmitted viruses—ticks, unlike mosquitoes, bite dead rabbits).

However, theoretical models of the 1960s seemed to imply that the effect of group selection was negligible. Alleles are likely to be held on a population-wide level, leaving nothing for group selection to select for. Additionally, generation time is much longer for groups than it is for individuals. Assuming conflicting selection pressures, individual selection will occur much faster, swamping any changes potentially favored by group selection. The Price equation can partition variance caused by natural selection at the individual level and the group level, and individual level selection generally causes greater effects.

Experimental results starting in the late 1970s demonstrated that group selection was far more effective than the then-current theoretical models had predicted^[8]. A review of this experimental work has shown that the early group selection models were flawed because they assumed that genes acted independently, whereas in the experimental work it was apparent that gene interaction, and more importantly, genetically based interactions among individuals, were an important source of the response to group selection (e.g. ^[9]). As a result many are beginning to recognize that group selection, or more appropriately multilevel selection, is potentially an important force in evolution.

More recently, Yaneer Bar-Yam has claimed that the gene-centered view (and thus Ronald Fisher's treatment of evolution) relies upon a mathematical approximation that is not generally valid. Bar-Yam argues that the approximation is a dynamic form of the Mean Field approximation frequently used in physics and whose limitations are recognized there. In biology, the approximation breaks down when there are spatial populations resulting in inhomogeneous genetic types (called symmetry breaking in physics). Such symmetry breaking may also correspond to speciation.

Spatial populations of predators and prey have also been shown to show restraint of reproduction at equilibrium, both individually^[10] and through social communication^[11], as originally proposed by Wynne-Edwards. While these spatial populations do not have well-defined groups for group selection, the local spatial interactions of organisms in transient groups are sufficient to lead to a kind of multi-level selection. There is however as yet no evidence that these processes operate in the situations where Wynne-Edwards posited them; Rauch et al.'s analysis^[10], for example, is of a host-parasite situation, which was recognised as one where group selection was possible even by E. O. Wilson (1975), in a treatise broadly hostile to the whole idea of group selection. ^[12] Specifically, the parasites do not individually moderate their transmission; rather, more transmissible variants "continually arise and grow rapidly for many generations but eventually go extinct before dominating the system."