Why Is Altruistic Behaviour Problematic for the Theory of Natural Selection?

Nice guys finish first, a chapter title in R. Dawkins' revolutionary popular science book the Selfish Gene. Although true altruism can not exist according to the classical theory of natural selection if such an evolutionary protagonist as RD has time for it, then there must be a good reason. In fact, we see much behaviour in nature that appears altruistic: alarm calling, guarding, defence and foraging by non-reproductives and grooming are just a few examples. Since work first began on altruistic behaviours, various mechanisms have emerged that have been able to squeeze them into the conventional model of natural selection. However, by no means can all altruistic behaviour now be explained by these methods.

First, altruism must be defined so we can see how it defies the conventional theory of natural selection. Perhaps more importantly we must then ask why, when inexplicable using Darwin's original model for selection, we still frequently observe altruism in nature. Three answers to this question have been put forward: kin selection, group selection and reciprocation. These will be examined in order.

When an actor's behaviour increases the fitness of the behaviour's receiver at the expense of the actor, then the behaviour is said to be altruistic. This definition is simplistic but provides a good starting point; throughout the discussion it will be sharpened. In contrast to so-called selfish behaviour increases the actor's fitness while reducing the fitness of the receiver. If both the actor and the receiver gain in fitness following a behaviour, then the behaviour is cooperative. We must keep in mind that these costs and benefits are measures of reproductive success, the number of offspring surviving to reproductive age and thus the number of genes passed to the next generation.

It is worth mentioning that much altruistic behaviour may directly benefit those performing the behaviour in a way that may not be immediately obvious to the observer. Guard watches in meerkats may be an example of this. It was always assumed that meerkats guards were giving up valuable foraging time to watch for predators to the benefit of their group. However, it has recently been suggested that in fact they only take turns on guard when they are not hungry and thus they gain a significant head start in the escape if they are the first to spot the predator. They also tend to guard while positioned close to their bolt hole, giving them yet more of an increased chance of escaping the predator relative to the other members of their group. Rhesus monkey (Macaca mulatta) allomothers provide another such example: the behaviour that was originally thought to be purely altruistic is now thought to directly benefit the allomothers, as when it come to their own reproduction they have had experience in offspring care. These behaviours can therefore be classified as selfish.

Why a gene encoding truly altruistic behaviour can not be selected for seems intuitively obvious: by its own action it will decrease its frequency in the population by decreasing the fitness of those who carry it. It will also increase the fitness of other members of the population who may not carry the same gene. Why then do we still see so many animals committing suicide, giving up time, effort and reproduction for the sake of other individuals? So far the most convincing answer and certainly the one with most supporting evidence is kin selection.

Kin selection does not contradict the conventional natural selection but rather applies a filter to its perspective, changing it from a view of selection based on the level of the individual to one based on the level of the gene. It states that individuals gain 'indirect' fitness benefits, coined originally by Hamilton, through the survival and reproduction of nondescendent offspring. These can even be quantified using r, the coefficient of relatedness. r for a brother, for example, is 1/2, as brothers share on average 50% of their genes. Thus many altruistic actions may be explained by the fact that they reproductively benefit the relatives of the altruists and thus indirectly, but still conventionally according to natural selection, themselves.

Of course, the reproductive benefit to an altruist for it to help its brother must be twice the reproductive cost of the helping action. This occurs in the bicoloured wren which practices sibling care as it greatly increases the survival chances of the helped siblings. The system also occurs in reverse: if the direct reproductive benefit of siblicide is twice the indirect reproductive cost it incurs, it will happen, as it does in herons and egrets. If indirect reproductive benefits are included in an animal's fitness it is called inclusive fitness, after Hamilton. Our definition of altruism now needs a restyling: if the behaviour of an actor decreases its own inclusive fitness while increasing the inclusive fitness of the receiver, then it is truly altruistic.

The literature still refers to behaviour that may be explained by kin selection as altruistic, with the added condition that it may increase an altruist's indirect fitness. However, for the purposes of this argument it serves to dismiss these behaviours as non-altruistic as they are not problematic for the conventional theory of natural selection.

A large number of the observed cases of altruism in nature may be explained by kin selection. The haplodiploid hymenoptera are an extreme example. Here sisters have up to a 75% percentage chance of sharing a given gene. Thus actions like suicide in bees armed with barbed stings, ant workers and colonial defence in wasps can be explained by the high genetic relatedness between the actors, who pay the costs and the receivers, who reap the benefits. Eusociality has also been observed in a species of shrimp (Synapheus regalis), the naked mole rat and termites. Here, once again, all the altruistic defence, foraging and care can be explained by kin selection.

An alternative explanation for the occurrence of altruistic behaviour that chronologically preceded kin selection was group selection. This concept in itself is problematic for the theory of natural selection. The individual is the unit upon which natural selection acts. We can skew our perspective to see how the process affects selection on the level of the gene and there is a possibility that genes rather than individuals may be able to control their passage through generations independently of the other genes they share a genome with. However, we can not apply the same logic to group selection. Even if group selection does occur, as it theoretically could, there is no evidence for it and its effects would