Fitness is normally defined as the number of offspring an individual will produce. Natural selection works to maximize fitness, because traits can only be selected for if they are passed down to progeny. Natural selection therefore seems to favor selfish actions that promote one's own fitness over the fitness of other members of one's species. This seeming truth runs into difficulties, however, when faced with actions of social altruism that are quite prevalent in animal society. For example, in eusocial colonies, some individuals forgo reproduction altogether. How can such an action be explained in light of what we know about natural selection and evolution?
The answer comes when we consider an individual's inclusive fitness, which is the sum of an individual's direct fitness, the number of offspring produced, and indirect fitness, the number of relatives (nieces and nephews) produced multiplied by the degree of relatedness of those individuals.
The degree of relatedness (r) is the probability of being identical by descent through a common ancestor. As we shall see, this value is dependent on the genetic nature of the species involved. The degree of relatedness between diploid animals will defer than that for haplodiploid animals.
The parent to offspring relationship is r=.5 because each parent has half its genes in the offspring. The offspring to parent relationship is also r=.5 because half the offspring's genes come from each parent. Full siblings are related by r=.5 and half siblings are related by r=.25. shows how to calculate relatedness.
Let's now examine the case for haplodiploid organisms, such as ants, in which females inherit a set of genes from both parents, but males are haploid, resulting from unfertilized eggs, and receive only half of their mother's genes.
Now that we have included indirect fitness in our definition of inclusive fitness, we can see that individuals can derive some benefit by helping to raise their siblings or other relatives: through these siblings and relatives the genes of the helping individual are passed on. This is really not "altruism" in the true sense of the word because the individual does indirectly benefit. When and who should an individual help? The answer lies in the relative costs and benefits of the aid. Danger or giving up your own chance to reproduce are costs (C) of altruism. The benefit (B) is the aided individual's reproductive success. Therefore, individuals should only act altruistically when the indirect benefit is greater than the cost, or mathematically, when B > C.
In order to act altruistically preferentially toward relatives, an individual must be able to recognize his or her degree of relatedness to other individuals. This can be accomplished in several ways. Some animals recognize their kin by imprinting on their nestmates. Siblings reared together can recognize each other via vision or scent. Other animals imprint on the smell of their nesting material, and recognize kin nestmates in this fashion. Some animals recognize kin by molecular cues. For example, MHC Class II are gene loci which are highly polymorphic, meaning the probability is low that any two individuals will share the same set of MHCII loci. It is therefore a useful way to discern how closely two individuals are related. There is some evidence that mice shed MHC molecules or their byproducts in their urine, and that they use this clue to recognize kin.
One form of kin altruism in birds is "helpers at the nest." Primary helpers are immediately accepted by a breeding pair, invest as much work as the breeders, and are usually a son of that breeding pair born the previous year. These sons do not mate with females that year. Secondary helpers are accepted into the nest after the eggs have hatched. These helpers are unrelated to the breeding pair, and often do not work nearly as hard as the breeders. If the male breeder should die, the secondary helper will often become the mate of the widow. 90% of birds are monogamous, and so like the human dating scene, most of the good ones are already taken. Helping at the nest allows the male the opportunity to mate the female if her mate dies. Females of some species may remain with their parents for a year and help with the next brood. These females receive an indirect benefit by increasing the survival of siblings, but they also tend to have a higher direct fitness when they do breed than individuals who did not help at the nest. Whether this is due to gained experience during that year, low success for young females, or some other reason is unknown.
Eusocial insect societies involve the cooperative rearing young, usually by females. There is a reproductive division of labor, meaning some females devote their energy to reproducing, and others forgo reproduction in order to raise the young of others. Generations overlap, meaning several generations are present at any given time. Many eusocial insects are haplodiploid. This creates a conflict of interest between the queen and the workers. The queen is equally related to her sons and daughters, where r=.5, and so her ideal sex ratio is 1:1. However the female workers are more related to their sisters than their brothers (r=.75 for sisters and r=.25 for sister/brother) as we saw in . Consequently, the workers would ideally have a 3:1 sex ratio in favor of females since they are 3 times more closely related to their sisters. In a single queen colony, the sex ratio is often 3:1 almost exactly. In multiple queen colonies, the sex ratio is closer to 1:1.