Multilevel Selection Theory

Multilevel selection theory (MLS), also known as group selection, posits that natural selection can act not only on individual organisms but also on groups of organisms, and even on genes within individuals. This perspective offers an alternative or complementary explanation to individual-level selection for the evolution of traits that appear costly to the individual but beneficial to the group, such as altruism and cooperation. The concept has a long and contentious history in evolutionary biology, with significant implications for understanding the origins of human sociality.

Historical Context and Revival

The idea that selection could operate at levels above the individual has roots in Darwin's own writings, particularly concerning the evolution of morality and self-sacrifice in human groups. However, during the mid-20th century, the concept of group selection fell largely out of favor. Influential figures like George C. Williams (1966) and Richard Dawkins (1976) argued forcefully that selection primarily operates at the level of the gene, or at most, the individual. They contended that traits beneficial to the group but costly to the individual would be outcompeted by 'cheaters' within the group, leading to the collapse of such group-beneficial traits.

This individual-centric view was largely supplanted by the rise of kin selection theory, pioneered by William D. Hamilton (1964), and reciprocal altruism, developed by Robert Trivers (1971). These theories provided robust explanations for cooperation and altruism by demonstrating how such behaviors could evolve through individual-level selection if they benefited genetic relatives (inclusive fitness) or if they were reciprocated. For decades, kin selection was widely considered the primary, if not sole, mechanism for the evolution of social traits.

The modern revival of multilevel selection theory is largely attributed to David Sloan Wilson, who, beginning in the 1970s and continuing with works like Unto Others (Wilson & Sober, 1998), developed a rigorous framework for how group selection could operate. Wilson argued that if groups with a higher proportion of altruists out-reproduce other groups, then altruism could evolve, even if altruists are at a disadvantage within their own group. This 'selection among groups' could counteract 'selection within groups.' Edward O. Wilson later became a prominent proponent of MLS, particularly in his later works such as The Social Conquest of Earth (2012), where he argued for the primacy of group selection in the evolution of eusociality and complex human societies.

The Debate: MLS vs. Inclusive Fitness

A central and enduring debate in evolutionary biology concerns the relationship between multilevel selection theory and inclusive fitness theory. Proponents of inclusive fitness, such as Hamilton, argued that all instances of altruism could be explained by individuals acting to maximize the propagation of their genes, either directly or through relatives. From this perspective, group selection is often seen as a special case of kin selection, or as a less general and more complex way of describing phenomena that inclusive fitness theory already explains more parsimoniously.

David Sloan Wilson and others, however, maintain that MLS provides a distinct and often more intuitive framework, particularly for understanding selection in groups that are not necessarily composed of close relatives. They argue that the two frameworks are often mathematically equivalent in certain contexts, meaning they can predict the same evolutionary outcomes, but that MLS offers a different, and sometimes more illuminating, causal perspective. For example, in a population structured into groups, the fitness of an individual depends on its own genotype and the genotypes of its group members, and the fitness of a group depends on the average fitness of its members. MLS explicitly models these hierarchical levels of selection.

This debate intensified with the publication of a paper by Martin Nowak, Corina Tarnita, and Edward O. Wilson in Nature in 2010, titled "The evolution of eusociality." This paper controversially argued that inclusive fitness theory was a limited and unnecessary framework, and that a simpler, more general population genetics approach (which they aligned with MLS) was sufficient to explain the evolution of eusociality. They claimed that inclusive fitness theory had led to a 'quagmire' and was not predictive.

The 137-Author Response

The 2010 Nowak, Tarnita, and Wilson paper provoked a strong reaction from the evolutionary biology community. In 2011, Nature published a response signed by 137 leading evolutionary biologists, including many prominent figures in kin selection and social evolution theory. This collective response, led by Patrick Abbot, Andy Gardner, and Stuart West, vigorously defended inclusive fitness theory, arguing that it remains a powerful and general framework for understanding social evolution. They contended that Nowak, Tarnita, and Wilson had fundamentally misunderstood and misrepresented inclusive fitness theory, and that their mathematical models, while valid, did not invalidate inclusive fitness but rather demonstrated its equivalence in specific scenarios.

The 137-author response emphasized that inclusive fitness theory provides a clear causal understanding of how genes for social behaviors spread, by tracing fitness effects through genetic relatedness. They also pointed out that the mathematical equivalence between MLS and inclusive fitness in many models means that choosing one framework over the other often comes down to heuristic utility and explanatory power, rather than one being fundamentally 'right' and the other 'wrong.'

Current Status and Open Questions

Despite the heated debates, a consensus has emerged among many researchers that multilevel selection and inclusive fitness are often two different ways of modeling the same underlying evolutionary process. The 'equivalence theorem' suggests that under certain conditions, models based on MLS can be translated into models based on inclusive fitness, and vice-versa. The choice of framework often depends on the specific question being asked and the structure of the population under study. Some argue that MLS offers a more natural way to think about group-level adaptations, while inclusive fitness provides a gene-centric view that is powerful for understanding the spread of specific alleles.

Ongoing research in evolutionary psychology and behavioral ecology continues to explore the conditions under which group selection might be a significant force, particularly in understanding the unique aspects of human cooperation, culture, and the formation of large-scale societies. The debate has pushed researchers to refine their models and clarify the assumptions underlying both frameworks, leading to a deeper understanding of the complex mechanisms driving social evolution.