Development Of The Concept

The intellectual roots of ecosystem self-regulation lie in Darwin's (1859) recognition that some adaptations apparently benefit a group of organisms more than the individual, leading to selection for population stability. The concept of altruism and selection for homeostasis at supraorganismal levels has remained an important issue, despite recurring challenges and alternative models (e.g., Axelrod and Hamilton 1981, Schowalter 1981, E.Wilson 1973,1997).

Behavioral ecologists have been challenged to explain the evolution of altruistic behaviors that are fundamental to social organization. Even sexual reproduction could be considered a form of self-restraint because individuals contribute only half the genotype of their progeny through sexual reproduction, compared to the entire genotype of their progeny through asexual reproduction (Pianka 1974). Cooperative interactions, such as mutualism, and self-sacrificing behavior, such as suppression of reproduction and suicidal defense by workers of social insects, have been more difficult to explain in terms of individual selection. Haldane (1932) proposed a model in which altruism would have a selective advantage if the starting gene frequency were high enough and the benefits to the group outweighed individual disadvantage. This model raised obvious questions about the origin of altruist genes and the relative advantages and disadvantages that would be necessary for increased frequency of altruist genes.

Group selection theory was advanced during the early 1960s by Wynne-Edwards (1963,1965), who proposed that social behavior arose as individuals evolved to curtail their own individual fitnesses to enhance survival of the group. Populations that do not restrain combat among their members or that overex-ploit their resources have a higher probability of extinction than do populations that regulate combat or resource use. Selection thus should favor demes with traits to regulate their densities (i.e., maintain homeostasis in group size). Behaviors such as territoriality, restraint in conflict, and suppressed reproduction by subordinate individuals (including workers in social insect colonies) thereby reflect selection (feedback) for traits that prevent destructive interactions or oscillations in group size.

This hypothesis was challenged for lack of explicit evolutionary models or experimental tests that could explain the progressive evolution of homeostasis at the group level (i.e., demonstration of an individual advantage to altruistic individuals over selfish individuals). Furthermore, Wynne-Edwards' proposed devices by which individuals curtail their individual fitnesses, and communicate their density and the degree to which each individual should decrease its individual fitness, were inconsistent with available evidence or could be explained better by models of individual fitness (E.Wilson 1973). Nevertheless, the concept of group selection was recognized as an important aspect of social evolution (E. Wilson 1973). Hamilton (1964) and J. M. Smith (1964) developed an evolutionary model, based on kin selection, whereby individual fitness is increased by behaviors that favor survival of relatives with similar genotypes. They introduced a new term, inclusive fitness, to describe the contributions of both personal reproduction and reproduction by near kin to individual fitness. For example, care for offspring of one's siblings increases an individual's fitness to the extent that it contributes to the survival of related genotypes. Failure to provide sufficient care for offspring of siblings reduces survival of family members.

This concept explained evolution of altruistic behaviors, such as maternal care; shared rearing of offspring among related individuals; alarm calls (that may draw attention of predators to the caller); and voluntary suppression of reproduction and suicidal defense by workers in colonies of social insects, which usually benefit close relatives. For social Hymenoptera, Hamilton (1964) noted that males are produced from unfertilized eggs and have unpaired chromosomes. Accordingly, all the daughters in the colony inherit only one type of gamete from their father and thereby share 50% of their genes through this source. In addition, they share another 25%, on average, of their genes in common from their mother. Overall, the daughters share 75% of their genes with each other compared to only 50% of their genes with their mother. Therefore, workers maximize their fitness by helping to rear siblings, rather than by having their own offspring.

This model does not apply to termites. Husseneder et al. (1999) and Thorne (1997) suggested that developmental and ecological factors, such as slow development, iteroparity, overlap of generations, food-rich environment, high risk of dispersal, and group defense, may be more important than genetics in the maintenance of termite eusociality, whatever factors may have favored its original development.

Levins (1970) and Boorman and Levitt (1972) proposed interdemic selection models to account for differential extinction rates among demes of metapopula-tions that differ in altruistic traits. In the Levins model, colonists from small populations found other small populations in habitable sites. Increasing frequency of altruist genes decreases the probability of extinction of these small populations (i.e., cooperation elevates and maintains each deme above the extinction threshold; see Chapters 6 and 7). In the Boorman-Levitt model, colonists from a large, stable population found small, marginal populations in satellite habitats. Altruist genes do not influence extinction rates until marginal populations reach demographic carrying capacity (i.e., altruism prevents destructive population increase above carrying capacity; see Chapters 6 and 7). Both models require restrictive conditions for evolution of altruist genes. Matthews and Matthews (1978) noted that group selection requires that an allele become established by selection at the individual level. Thereafter, selection could favor demes with altruist genes that reduce extinction rates, relative to demes without these genes. Interdemic selection has become a central theme in developing concepts of metapopulation dynamics (Chapter 7).

Meanwhile, the concept of group selection was implicit in early models of ecological succession and community development. The facilitation model of succession proposed by Clements (1916) and elaborated by E. Odum (1953,1969) emphasized the apparently progressive development of a stable, "climax," ecosystem through succession. Each successional stage altered conditions in ways that benefited the replacing species more than itself. However, such facilitation contradicted individual self-interest that was fundamental to the theory of natural selection. Furthermore, identification of alternative models of succession, including the inhibition model (Chapter 10), made succession appear to be more consistent with evolutionary theory.

D. S. Wilson (1976, 1997) developed a model that specifically applied the concept of group selection to the community level. Wilson recognized that individuals and species affect their own fitness through effects on their environment, including the fitness of other individuals. For example, earthworm effects on soil development stimulate plant growth, herbivory, and litter production (see Chapter 14) and thereby increase the detrital resources exploited by the worms, a positive feedback. Furthermore, spatial heterogeneity, from large geographic to microsite scales, in population distribution results in intrademic variation in effects of organisms on their community. Given sufficient iterations of Wilson's model, every effect of a species on its community eventually affects that species, positively or negatively, through all possible feedback pathways. Intrademic variation in effects on the environment is subject to selection for adaptive traits of individuals.

The models described earlier in this section help explain the increased frequency of altruist genes, but what selective factors can maintain altruist genes in the face of evolutionary pressure to "cheat" among nonrelated individuals? Trivers (1971) and Axelrod and Hamilton (1981) developed a model of reciprocal altruism based on the Prisoner's Dilemma (Fig. 15.1), in which each of two players can cooperate or defect. Each player can choose to cooperate or defect if the other player chooses to cooperate or defect. If the first player acts cooperatively, the benefit/cost for cooperation by the second player (reward for mutual cooperation) is less than that for defection (temptation for the first player

Player B

Player A C

Cooperation

Defection

Cooperation

Defection

Cooperation

Defection

Player A C

Cooperation

Defection

R = 3 Reward for mutual cooperation

S = 0 Sucker's payoff

T = 5 Temptation to defect

P = 1 Punishment for mutual defection

Prisoner's Dilemma, defined by T > R > P > S and R > (S + T)/2, with payoff to player A shown using illustrative values. From Axelrod and Hamilton (1981) with permission from the American Association for the Advancement of Science. Please see extended permission list pg 573.

Prisoner's Dilemma, defined by T > R > P > S and R > (S + T)/2, with payoff to player A shown using illustrative values. From Axelrod and Hamilton (1981) with permission from the American Association for the Advancement of Science. Please see extended permission list pg 573.

to defect in the future); if the first player defects, the benefit/cost for cooperation by the second player (sucker's payoff) is less than that for defection (punishment for mutual defection). Therefore, if the interaction occurs only once, defection (noncooperation) is always the optimal strategy, despite both individuals doing worse than they would if they both cooperate. However, Axelrod and Hamilton (1981) recognized the probability of repeated interaction between pairs of unrelated individuals and addressed the initial viability (as well as final stability) of cooperative strategies in environments dominated by noncooperat-ing individuals or more heterogeneous environments composed of other individuals using a variety of strategies. After numerous computer simulations with a variety of strategies, they concluded that the most robust strategy in an environment of multiple strategies also was the simplest, Tit-for-Tat. This strategy involves cooperation based on reciprocity and a memory extending only one move back (i.e., never being the first to defect but retaliating after a defection by the other and forgiving after just one act of retaliation). They also found that once Tit-for-Tat was established, it resisted invasion by possible mutant strategies as long as the interacting individuals had a sufficiently large probability of meeting again.

Axelrod and Hamilton emphasized that Tit-for-Tat is not the only strategy that can be evolutionarily stable. The Always Defect Strategy also is evolutionarily stable, no matter what the probability of future interaction. They postulated that altruism could appear between close relatives, when each individual has part interest in the partner's gain (i.e., rewards in terms of inclusive fitness), whether or not the partner cooperated. Once the altruist gene exists, selection would favor strategies that base cooperative behavior on recognition of cues, such as relat-edness or previous reciprocal cooperation. Therefore, individuals in relatively stable environments are more likely to experience repeated interaction and selection for reciprocal cooperation than are individuals in unstable environments that provide low probabilities of future interaction.

These models demonstrate that selection at supraorganismal levels must be viewed as contributing to the inclusive fitness of individuals. Cooperating individuals have demonstrated greater ability in finding or exploiting uncommon or aggregated resources, defending shared resources, and mutual protection (Hamilton 1964). Cooperating predators (e.g., wolves and ants) have higher capture efficiency and can acquire larger prey compared to solitary predators. The mass attack behavior of bark beetles is critical to successful colonization of living trees. Co-existing caddisfly larvae can modify substrate conditions and near-surface water velocity, thereby enhancing food delivery (Cardinale et al. 2002). Animals in groups are more difficult for predators to attack.

Reciprocal cooperation reflects selection via feedback from individual effects on their environment. The strength of individual effects on the environment is greatest among directly interacting individuals and declines from the population to community levels (Fig. 1.2) (e.g., Lewinsohn and Price 1996). Reciprocal cooperation can explain the evolution of sexual reproduction and social behavior as the net result of tradeoffs between maximizing the contribution of an individual's own genes to its progeny and maximizing the contribution of genes represented in the individual to progeny of its relatives. Similarly, species interactions represent tradeoffs among positive and negative effects (see Chapter 8).

Population distribution in time and space (i.e., metapopulation dynamics; see Chapter 7) is a major factor affecting interaction strengths. Individuals dispersed in a regular pattern (Chapter 5) over an area will affect a large proportion of the total habitat and interact widely with co-occurring populations, whereas the same total number of individuals dispersed in an aggregated pattern will affect a smaller proportion of the total habitat but may have a higher frequency of interactions with co-occurring populations in areas of local abundance. Consistency of population dispersion through time affects the long-term frequency of interactions and reinforcement of selection from generation to generation. Metapopulation dynamics interacting with disturbance dynamics provide the template for selection of species assemblages best adapted to local environmental variation.

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