My new book, This View of Life: Completing the Darwinian Revolution, claims that the Darwinian revolution won’t be complete until it makes sense of everything associated with the words “human”, “culture”, and “policy” in addition to the word “biology”. I rely heavily on Multilevel Selection (MLS) theory, and since this theory is famously controversial, it might seem to be a weak link in my argument. But this inference is mistaken. Instead, the major claims of my book are supported by all theories of social evolution based on what they share in common, as I will now show.

The first thing that all theories of social evolution share in common can be called the fundamental problem of social life. Think of any behavior that can be called prosocial, which means oriented toward the welfare of others or one’s group as a whole, and that behavior is very likely to be vulnerable to exploitation by behaviors that would be regarding as self-serving in human terms. Altruism is vulnerable to selfishness. Honesty is vulnerable to deceit. Bravery is vulnerable to cowardice, Generosity is vulnerable to hoarding one’s own wealth. Pitching in is vulnerable to free-riding. There might be some cases where maximizing one’s relative advantage within a group also benefits others and one’s group as a whole, but there can be no doubt about the rule.

This basic fact about social life posed a dilemma for Darwin. If prosocial behaviors are vulnerable to more self-serving behaviors in every group where both types of behaviors occur, then how can they evolve by natural selection? This dilemma only gradually dawned upon Darwin and is reflected in the revisions that he made in successive editions of On The Origin Of Species (go here for more). It was a problem of the first rank. If his theory of natural selection could not explain the evolution of prosocial behaviors, then it could not explain all aspects of design that had been attributed to a creator.

The dilemma faced by Darwin is inescapable and therefore faced by all modern theories of social evolution. It is simply a fact of life that most prosocial behaviors are not selectively advantageous within the groups where the social interactions are taking place. Darwin’s solution was that the evolutionary advantage of prosocial behaviors takes place at a larger scale. If the evolving population consists of many groups of socially interacting individuals, then more prosocial groups will robustly outcompete less prosocial groups, even if more prosocial individuals lose to less prosocial individuals within groups. Competition between groups can be direct or indirect, just as for competition between individuals. Darwin became increasingly clear about the need for between-group selection to explain the evolution of prosocial behaviors in the Descent of Man and later editions of On the Origin of Species. In short: Group-level selection was an amendment that he needed to add to complete his theory of natural selection.

All theories of social evolution must also reach the same conclusion. If a given prosocial trait is selectively disadvantageous within every group of a multi-group population, then how else can it evolve except with a positive selective differential at a larger scale?

Remarkably, all theories of social evolution must also reach the same conclusion about what counts as a group. For non-social traits, the fitness of an individual can be calculated based on the properties of that individual. For social traits, calculating the fitness of an individual requires knowing its properties and the properties of the other individuals with whom it interacts. This is not arbitrary and must be determined on a case by case basis. If individuals interact in groups of 5 individuals, that will result in different outcomes than if individuals interact in groups of ten individuals. There are other details that must be determined on a case by case basis, such as how the groups form and dissolve, which are loosely termed the “population structure” for the evolving trait. For example, if members of a group disperse as individuals, that will result in a different outcome than if they bud off from the group as subgroups. Any model of social evolution must get these biological details right on a case by case basis, just to calculate the fitness of individuals (and their genes in a genetic model).

What I just described clearly deserves the label “Two-level selection”, which can be generalized to Multilevel selection by frameshifting both downward (e.g., selection among genes within individual organisms) and upward (e.g., selection among groups-of-groups or multispecies ecosystems such as microbiomes). Its logic is represented in Darwin’s writing, the three pioneers of population genetics theory (Ronald Fisher, J.B.S. Haldane, and Sewall Wright), and G.C. Williams, who wrote in his 1966 book Adaptation and Natural Selection (p 92): “It is universally conceded by those who have seriously concerned themselves with this problem that…group related adaptations must be attributed to the natural selection of groups of individuals and that the natural selection of alternative alleles within populations will be opposed to this development.” The great puzzle that historians of science will need to answer is why the central logic appeared to be rejected in the 1960’s and replaced by a parade of other constructs, such as inclusive fitness theory, selfish gene theory, evolutionary game theory, and social selection theory, which claimed to explain “seemingly” prosocial behaviors in more individualistic terms (go here for more).

In the interest of keeping this essay short, suffice it to say that all of these constructs are like the old song Let’s Call the Whole Thing Off with its lyric “You say Tom-A-to, I say Tom-AH-to”. Look under the hood of any theory of social evolution and you will find the central logic of MLS theory. The evolving population consists of multiple groups. The groups are defined in terms of the individuals who are influencing each other’s fitness with their social interactions. The behaviors labeled “altruistic” or “cooperative” are selectively disadvantageous within the socially interacting groups. And a positive fitness differential at the group level is required for the prosocial behaviors to evolve. If a theory of social evolution does not include the central logic of MLS theory, it isn’t representing the biological facts of life and can’t even get started. See my previous book Does Altruism Exist? for a longer but still concise defense of this claim.

The bottom line is that my new book does not rely on a controversial theory in its claim that an evolutionary worldview can make sense of everything associated with the words “human”, “culture”, and “policy”. It relies on assumptions that are as robust as the assumptions underlying the theory of natural selection, which all theories of social evolution share in common.