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. Moreover, van Veelen et al. (2012) in their article "Group selection and inclusive fitness are not equivalent; the Price equation vs. models and statistics," suggest that the use of the Price equation to support group selection is based on certain mathematical assumptions that are invalid.
Experimental results starting in the late 1970s demonstrated that group selection was far more effective than the then-current theoretical models had predicted. 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.). As a result a many scientists are beginning to recognize that group selection, or more appropriately multilevel selection, is potentially an important force in evolution.
Nevertheless, there is a core of scientists that recognize the gene-centered view, championed by Richard Dawkins . The gene-centered view has long taken into account an enduring code of DNA (the selected 'gene') can facilitate its spread via multiple interactions with higher level entities, named as vehicles. For example, using Hamilton's broad based inclusive fitness model (Hamilton 1964 a, b), Robert Trivers' in Social Evolution (Trivers 1985) has outlined how a selfish gene can maximize its inclusive fitness, spreading copies of itself via interactions that lead solely to selfishness, or to mutualism with unrelated individuals, or to delayed reciprocity, a highly specialized reciprocity that can involve both kin and non-kin that is subject to cheating and remembering cheaters, or to facultative altruism and obligate altruism with kin. Note that any of these multi-level interactions can enhance a gene's fitness. Of course these interactions are done with multiple vehicles (chromosomes, cells, individuals, family, extended family, communities, and so forth). The spread of a gene and the strength of selection, hence, is greatest when a gene has interactions with vehicles that are stable. It is perplexing that Multi-Level Selection theory has been so named and set aside as novel and new, given that a gene is long recognized to interact with various vehicles to maximize its spread. Perhaps the distinction here is that Trivers and Hamilton are focusing exclusively on the gene and its interactions with multiple vehicles for its successful replication, tracing a gene's replication success from the bottom up. By contrast, MLS theory attempts a top-down approach, working from the group down to the gene to attempt to explain a group's formation. The gene centered view (a bottom up view) and a MLS view (a top down view) do lead to different findings, especially if multiple selfish interests are involved in the formation of a higher level vehicle such as a non-kin group, like a herd or a flock. Are we interested in tracing how and why a gene can spread through to the next generation or how and why a group can satisfy multiple genes that come together to form a group? Are we interested in a group of adaptive antelope or in trying to make the group adaptive because each antelope congregate for selfish benefits in a herd? There is reference is to Hamilton's development of kin selection via Price's equation and how kin selection, by itself has weak explanatory power in explaining the formation of non-kin groups. Hamilton did much more that merely develop a kin component to explain the spread of a gene shared through identical descent, i.e. kin selection theory (although that in itself is rich in its implications, since chromosome architectures do indeed differ among kin groups). He developed the broad and encompassing inclusive fitness theory that can explain a gene's spread in any number of ways, e.g., through purely Selfish behavior (actor benefits, the recipient does not), through reciprocity or mutualism (actor and recipient benefit, even if benefits are selfish in gain), through delayed reciprocity (a uniquely dominant human behavior), as well as altruistic social behavior among kin (benefits to actor and recipient are realized to be similar as they share the same gene by common descent that is being replicated). Hamilton's Rule is broad reaching in its explanatory power. History cannot be re-written (see Abbott et al. Nature 471, E1–E4 (24 March 2011). Yet the some revisionists continue to try and often use the path of obscure mathematics is used to throw students into a state of doubt.
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 and through social communication, 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, 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. 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."
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