The song "Easy to Be Hard" from the 1967 musical "Hair" posed the question: "How can people be so heartless?" Ever since Darwin, though, biologists have struggled with the opposite question: "How can people be so nice?" If traits arise through intense competition between living organisms in a war of all against all, how does a tendency to altruism arise in a population? What benefit does altruist behavior have that it contributes to the survival and reproductive success of the altruist?
How we define "altruism" is crucial to the form that the explanation will take, although the underlying facts, of course, are not changed by the definition. There is no question but that a heritable, genetic trait, in order to be selected for by natural selection, must provide some differential reproductive advantage to the gene complex coding for that trait. So if we define "altruism" as describing traits that benefit ONLY others, and confer NO adaptive advantage on either the organism bearing or the gene complex, then necessarily genes are selfish, and altruism cannot arise by natural selection. If such "altruistic" behavior in fact is not only neutral, but places the bearer at a selective disadvantage, then it cannot long survive, even if it arises by chance.
But this is a trivial result of the nature of selection, and a particular definition of "altruistic", and leaves a lot of real-life facts unexplained. Nature is full of examples of behavior that SEEMS altruistic, in the sense that it seems to be for the benefit of others, and of immediate cost to the organism exhibiting the behavior. Simply stating that there must be some hidden benefit to the organism, or at least to some of the gene complexes which code for the nature of the organism, does not explain how each particular behavior could develop.
So, rather than sacrifice a perfectly good word to preserve a trivial result (and make do, in consequence, with cumbersome phrases like "seemingly altruistic"), I prefer to define "altruistic behavior" simply as behavior that seems to benefit primarily others, at an immediate net cost to the altruistic individual. This definition of altruistic defines "fuzzier" classes of behaviors and organisms than the alternate definition, but classes that have the decided advantage that the constituent behaviors and organisms actually exist.
Elliot Sober’s book The Nature of Selection makes a strong connection between the evolution of altruistic behavior, and the process of "group selection". Group selection is a controversial concept among biologists, part of what Sober refers to as the "unit of selection" debate. The question is on what entities or "units" does natural selection work? The oldest tradition, starting with Darwin, focus on the organism (or phenotype). Individual organisms vary in traits, and have different rates of survival and reproduction deriving from those traits, causing certain types of organisms, with certain combinations of traits, to be "selected" as being better adapted to their environment, and others to die out.
Other candidates for the unit of selection exist. Many biologists have strongly argued for the individual gene as the only proper unit to be considered. Others extend this to complexes of genes. More lately, some theorists have argued for selection at the level of the group or "deme", or even at the species level. I won’t go into detail on all this. Sober argues strongly for a multi-level view; i.e., that all of these may be, from time to time, the level at which selection operates, and the particulars must be carefully examined in each case. In particular, he argues that the "genes only" school arises as a trivial result of the fact that the mathematics in each case can be reduced to calculations based on the averaged relative fitness of each gene. But to really understand the process, he believes you have to look at causality – at what level are the actual natural forces shaping evolution being applied?
The way Sober defines "group selection", there must be a trait or property definable only at the group level which confers a selective advantage on individual organisms. There is actually some flexibility in whether the "benchmark" of selection is the individual organism, a particular gene or gene complex, or the group, but focusing on the organism will do for now. The crucial feature is that the property that confers the selective advantage must be definable only at the group level. For instance, predators may avoid large individuals, but show even more aversion to groups of large animals; in that case, being a member of group of organisms with a large average size will confer a selective advantage to each organism in the group, irrespective of its own, individual size. There may be other selective effects in operation. For example, if smaller size happens to confer some selective advantage on organisms competing with other organisms within the group, then there will be two countervailing evolutionary forces at work, and the result is indeterminate (in the mathematical sense that one must look to other factors for a resolution), but this does not alter the fact that there is a group selective force at work.
Altruism seems, at least in many cases, to be a situation where group selection and individual selection act at cross purposes. Since altruists help other group members, there is a significant selective advantage to being a member of a group containing a large number of altruists. But this advantage accrues to selfish individuals, as well as to altruistic ones. Since selfish individuals gain the selective advantages of group membership without the personal costs, they may have a higher "fitness" value for intra-group competition. Which one wins out – whether selfish organisms or altruistic ones will predominate or "go to fixation" (one trait driving the other into extinction) – will depend on the relative strength of the two forces. The existence of individual (organism-level) selection as a countervailing force leads to an inherent instability in the fitness of altruism based on group selection, absent other forces.
I am mostly concerned with the evolution of altruistic behavior within primate species, and in particular within our own. I think group selection, as Sober defines it, is a large factor in this, but I think other factors can be identified, reinforcing what otherwise might seem to be an inadequately strong effect. One such factor is kin selection. Kin selection is a more powerful force than group selection, because an individual’s genes benefit more directly from the individual’s actions. This effect is most strong when I act for the benefit of my (biological) offspring. Even if I sacrifice my life for my children, this may well preserve my genotype much more effectively than had I failed to take the risk. The force of kin selection is somewhat less strong when I act for the benefit of siblings, who generally share most of my genetic material, and so on with decreasing force through first cousins, second cousins etc. I suspect that altruistic behavior may have originated, very early in the evolution of primate lineages, as a generalization of kin-selective behavior. Once it evolved, the group-selection advantages – the increase in fitness experienced by each individual member of a cooperative group – would have exercised at least a weak selective effect.
I think sex selection (selection by females of certain preferred traits in sexual partners) may also have played a part in the evolution of certain types of altruistic behavior, particularly in hominid lineages. Humans are unique in that our infants require a great deal of care – more care, and for a longer period than other primates, possibly more than any other animal. Stephen J. Gould has connected this to the neotenous nature of our species – we retain as adults, features that are infantile in other apes – for instance we continue to learn at a higher rate through much of our lives. But also we are born at a less developed stage. This may be in part because of our large brain size – a more fully developed brain and skull would be too large to pass through the birth canal.
For whatever reason, the result has been that human infants require much care, over a long period. This has led to other evolutionary changes; for example, it very likely led to the "always on" nature of human female sexuality. Other primate females, unlike human women, are interested in sex, and sexually interesting to males, only during the period in which they are fertile. By making sexuality a constant, hominid females could attract males to serve as ongoing helpmates, if not actually helping much with child rearing tasks (then or now), at least supplementing the female’s efforts in other areas, such as providing food and protection when the female was engaged with the child. This would have led to a tendency on the part of females to select for at least certain types of altruistic behavior, those that could fall under the rubric of being a "good provider". Constant sexuality, and the resulting pair-bonding, also led to another revolution – awareness of paternity, thereby extending the possible scope of kin selection.
Another factor I think played a big effect in the evolution of altruistic behavior, not only in human lineages but in other primates as well, at least those most closely related to us, such as gorillas and chimpanzees, is something we might call police action. Part of primate altruism involves cooperation and sharing – but it is not strictly necessary that these benefits be equally distributed between altruists and selfish individuals. Groups of individuals can choose NOT to share with individuals they don’t feel to be deserving. Groups can also band together to limit the power of otherwise dominant individuals who are perceived as abusing their power, and in extreme cases can even drive a selfish individual from the overall group. Evolution of policing behaviors would strongly support the group-selective benefits deriving from the evolution of altruistic forces, by lessening the selective advantage enjoyed by selfish individuals within the group, and the combination of traits would thus tend to be much more stable than altruism alone.
I’m no population geneticist, and I’m not going to be able to put together mathematical models to demonstrate all this, but it seems to me that these factors: kin selection, group selection, sex selection and evolution of cooperating behavior complexes (policing) together make up a sufficiently powerful set of forces to be adequate to ensure the evolution by natural selection of human altruism. The details I’ve laid out may not be quite right, but something like this history must be have occurred – because, indisputably, human altruism exists.
The lyric from "Hair" teaches us one important lesson: how natural all this seems to us. Because (saving a few who have imbibed too much Libertarian philosophy), we all tend to respond in the way the lyric suggests – we are surprised and startled by "heartlessness". By and large, in our day-to-day activities, we humans are more likely to go out of our way to be nice to each other than to be mean. Granted, exemplary occurrences of altruism are remarkable, and inspire awe and admiration, but it is meanness that shocks us, nags at our consciousness, and leaves us with the conviction that something must be done.