ALTRUISM: KIN SELECTION AND GROUP SELECTION Behavior which cannot be explained by natural selection theory is self-sacrificing behavior whereby an individual incurs a fitness cost in assisting others which appears to be greater than the benefit received. This is altruism in its strictest sense. The extreme case would be when an individual sacrifices its life for others. How could such altruistic behavior evolve, assuming that it does exist and is heritable? If the benefit received is less than the cost in terms of fitness (B < C), then altruism could not be the product of natural selection. [Note that this is a more restricted definition of altruism which excludes cooperation and reciprocal altruism]. To resolve this problem sociobiologists have posited different mechanisms to explain the evolution of altruism depending on whether or not the recipient of the behavior is related to the actor (the one who performs the behavior). Altruism directed towards relatives: nepotism. According to natural selection theory, individual behavior acts to maximize personal (Darwinian) fitness which is measured by the number of grandchildren produced. Males and females will cooperate in producing and caring for offspring because it is in their best genetic interests to do so. But why should siblings assist one another, or family members experience a fitness cost greater than the benefit received? That such behavior occurs is amply documented, and the tendency to assist relatives more than unrelated individuals is so pervasive that it has received a name: nepotism. Darwin was well acquainted with this problem and suggested that perhaps it involved some form of family selection. Sociobiologists, however, took a different view and suggested that it was the result of genic selection. This concept, called kin selection, was formalized by W. D. Hamilton who introduced the idea of inclusive fitness as opposed to personal or Darwinian fitness. Inclusive fitness is a measure of an individual's genic representation in the gene pool of the next generation resulting from its own reproduction and that of close relatives (devalued by the degree of relationship). The currency of inclusive fitness is not grandchildren, the measure of Darwinian or personal fitness, but rather the number of genes passed on to the next gene pool. Obviously, an individual's genes can be transmitted to the next generation through its own reproduction, but they can also be transmitted by the reproductive success of that individual's relatives who share, by reason of their relationship, some of the same genes. What matters is how many genes are passed on and it does not matter whether these genes are transmitted by the successful reproduction of the individual or its relatives. Thus, inclusive fitness has two components: (1) the individual's own reproduction, i.e., its personal fitness, and (2) the increment in genetic representation in the next generation due to the reproduction of relatives. Two points regarding inclusive fitness must be emphasized. First, in order for an individual to benefit from the reproductive success of relatives, it must assist a nondescendent relative to survive and reproduce and in so doing experience a cost in personal fitness. Secondly, the inclusive fitness benefit received by an actor from the reproduction of relatives depends on how closely these relatives are related genetically to the actor assisting them. This degree of relationship is measured by the parameter, r, the coefficient of relatedness. For siblings, r will be the same as the parent-offspring relationship, i.e., 0.5 which means that they share on average half of their genes. For more distantly related individuals, r will be less, e.g., between aunts/uncles and nephews/nieces r = 0.25, and between cousins r = 0.125. Kin selection, a type of genic selection, is a form of selection which favors the evolution of altruism directed towards nondescendent relatives. Kin selection is measured by inclusive fitness and constrained by the following cost-benefit relationship: B/C > 1/r, i.e., the benefit to the recipient divided by the cost to the altruist must be greater than the reciprocal of the degree of relationship (r). Given this constraint, would an individual maximize its fitness by giving up its life to save a brother or sister who is about to enter reproductive age? The answer is no, because B/C is not greater than 1/r. On a scale from 0.0 to 1.0, the benefit to the recipient whose life is saved equals 1.0, while the cost to the altruist in terms of its personal fitness also equals 1.0. Therefore B/C = 1/1 = 1.0. But the reciprocal of the degree of relationship (1/r) equals 2.0 (1/0.5). Since 1 is not greater than 2, kin selection would not favor an individual risking its life to save only one sibling. It would, however, favor the evolution of any genetically influenced behavioral pattern which resulted in an individual sacrificing its life to save three or more siblings. Why? Since the benefit to each recipient equals 1.0, then the benefit to the three saved would be 3.0. Thus 3.0/1.0 = 3 which is greater than 2.0. The concept of kin selection, which explains altruism directed towards relatives, has been the major contribution so far of sociobiology to the development of evolutionary theory. Kin selection is a modification of natural selection theory which replaces the individual with the gene as the unit of selection and replaces Darwinian fitness with inclusive fitness. As such, the central principle of sociobiology has been restated to include the concept of kin selection: animals behave to maximize their inclusive fitness. Note that in most instances this means that animals are maximizing their personal fitness, since this is included in the concept of inclusive fitness. Only when individuals behave altruistically towards relatives will "maximization of inclusive fitness" refer to both personal fitness and the increment in fitness gained by the reproduction of relatives. Altruism directed towards non-relatives: true altruism If an individual assists a genetic competitor and receives no fitness benefit, then this behavior, called true altruism, cannot be explained either by natural selection or kin selection. If true altruism exists at all, it must be explained by group selection, i.e., populations of altruists outlive and reproduce (through migration) populations of selfish individuals. The unit of group selection is the population but the concept is the same as natural selection acting at the level of the individual, i.e., selection is still a process of differential reproduction. Only the unit selected distinguishes natural selection (unit = individual) from group selection (unit = population). The concept of group selection was emphasized by those ethologists who interpreted behavior as benefiting the population rather than the individual. They considered many aspects of behavior as evolving for the good of the population or species to ensure its continued existence. This concept is quite opposed to that of sociobiologists who interpret behavior as benefiting the individual and evolving as a consequence of fitness maximization. Most examples of behavior cited by ethologists as benefiting the group (either population or species) can be more parsimoniously explained from the reductionist position of natural selection operating for individual benefit: that which benefits individuals also benefits the population. True altruism, however, is an exception because in this form of altruism, the altruist is sacrificing inclusive fitness to benefit the population; hence, it can only evolve through group selection. Group selection theory assumes (1) no migration between populations, otherwise selfish individuals would eventually enter an altruist population and eliminate the altruists via natural selection, and (2) that the rate of population extinction is high - at least on a time scale equivalent to that needed for natural selection to eliminate a trait. While group selection might be theoretically possible, as has been demonstrated in computer simulations, most evolutionary biologists relegate it to minor significance because of its very restrictive assumptions. Selection and Modern Evolutionary Theory The problem of altruism points out a major limitation of the theory of natural selection: its inability to explain the evolution of traits which do not improve individual fitness. The usual way of explaining neutral or maladaptive traits within the theory of natural selection has been to suggest that genes are pleiotropic in their effects and so influence more than one aspect of the phenotype. Thus, in selecting a trait which has a positive adaptive value for the individual, natural selection is indirectly selecting the gene controlling that trait. Since this gene also influences other traits, these traits will tag along even if they confer no adaptive advantage or are even slightly maladaptive. Only when the negative effects of a gene outweigh its positive ones will selection act against it. Darwin realized that natural selection was not the only factor acting to promote evolution but argued that natural selection was the prominent force due to its greater frequency in directing evolution. The term "Darwinism" or "neoDarwinism" (Darwinism considered in the light of modern population genetics) is often used to designate the exclusivity of natural selection as an evolutionary force rather than its relative frequency. Consequently, Darwinism is incapable of explaining those aspects of the real world which act against an individual's best interests (other than the by-product of selection or pleiotropy idea mentioned above). The concepts of (1) genetic drift to explain the evolution of neutral traits and (2) alternative forms of selection, e.g., kin and group selection, to explain maladaptive (for the individual) traits demonstrate that modern evolutionary theory consists of a family of different hypotheses, each of which can be falsified. Modern evolutionary theory is not simply Darwinism. The real problem is to assess for any given case which of the competing hypotheses within the overall structure of modern evolutionary theory provides the best explanation. Consequently, modern evolutionary theory cannot strictly speaking be falsified, because it is not a single theory; but it certainly can be refined as evolutionary biologists test its component hypotheses and determine which are most prevalent in explaining the natural world.