Reproductive Success = Survival
If evolutionary outcomes in a Darwinian world are described as natural economies, then individual reproduction is the currency of these economies and of natural selection. Given both naturally occurring genetic variation among individuals and a certain environmental dynamic, it follows that some individuals will be better adapted to locally changing environments than others. Such differential adaptation is expressed as a difference in the frequency with which individual genes pass into future generations. This simple scenario fulfills the genetic definition of evolution—change in allele frequencies in natural populations—by explaining environmental influences on these changes. Note that this argument emphasizes, as its central postulate, the importance of individual reproduction rather than simple survival. Survival of the fittest is therefore more properly viewed as the differential propagation of genes.
A challenge to such a scenario is the paradox of altruism. Altruism is defined as any behavior that benefits another at a cost to the altruist. Charles Darwin
himself suggested that this problem was a “special difficulty . . . which at first appeared . . . insuperable, and actually fatal to [the] whole theory” of natural selection. The individual who pushes siblings from the track as he himself is killed by the rushing locomotive is an altruist; the colony sentinel that issues an alarm call to her cohort to take cover, despite the risk of drawing the attention of an approaching predator, is also acting altruistically. These behaviors make no sense in Darwin’s economy, since they appear to decrease the likelihood of individual reproduction—unless, as W. D. Hamilton suggested in the early 1960s, Darwinian success is not limited to the success of individual bodies harboring particular genes but may be extended to include the reproductive success of relatives who share genes with the altruist. Hamilton defined inclusive fitness as the sum of an individual’s own fitness plus the influence that individual has on the fitness of relatives. According to Hamilton, kin selection is the evolutionary mechanism that selects for behaviors that increase the inclusive fitness of altruists. Even though there are potential costs to altruistic behavior, the evolutionary economy of an altruist operates in the black because actors profit (beyond associated costs) by helping others who share their genes. The bottom line is that altruists increase their inclusive fitness through the reproduction of others.
Argument for Kin Selection
One of the best arguments for kin selection is the social structure of certain groups of insects, including the Hymenoptera (ants, bees, and wasps). A unique system of sex determination (haplodiploidy) in which females are diploid and males are haploid predisposes some group members to behave altruistically. In certain bees, for example, the queen is diploid and fertile. Worker bees are female, diploid, and sterile. Drones are male, developed from unfertilized eggs, and haploid. Such a situation makes for unusual patterns of genetic relationship among hive members. In diploid systems, the genetic relation between parents and offspring and among offspring is symmetrical. Offspring receive half their genetic complement from their mother and half from the father; sons and daughters are related to each parent by one-half and full sibs (siblings) are related to each other by one-half. In the haplodiploid system such genetic relationships are asymmetric. Drones are haploid and receive half of the queen’s genome. Workers are diploid and share 100 percent of their paternal genes and, on average, half of their maternal genes with their sisters. Sisters of the same father are therefore related to each other by three-quarters; however, research conducted after Hamilton published his groundbreaking theory indicates that queens may mate with as many as twenty different drones, resulting in the sister bees being only one-third related. In this economy, it makes sense that workers should act altruistically to assist the queen in the production of sisters. What would appear to be purely altruistic acts, on the part of workers, result in greater inclusive success than if the workers had reproduced themselves. In contrast, drones contribute little to community welfare and serve only to fertilize the queen. Note that in this system there is no conscious decision on the part of workers not to reproduce; their sterility is an inherent part of this unusual system of sex determination.
A Test of Predictions
One prediction made by the kind of kin selection described above is that, assuming the queen produces male and female offspring in equal proportion, female workers should invest three times the energy in caring for sisters than they do for brothers. Because queens are related to both male and female offspring equally, one would predict that eggs are equally divided between the sexes. Because workers are related to their sisters by as much as three-fourths and to their brothers by one-fourth, one would predict that they should invest three times the energy in care of eggs eventually yielding sisters than they do in the care of eggs eventually yielding brothers. Remarkably, it has been shown that certain worker ants are able to identify and then selectively care for eggs containing sisters. Kin recognition has also been studied in the house mouse, Mus musculus domesticus, and in some cases individuals, can distinguish full sibs from half sibs on the basis of their major histocompatibility complexes
(glycoproteins important in immune system function). The specific MHC type is fairly unique for each mouse, but related individuals will have similar patterns and share some specific MHC glycoproteins. MHC glycoproteins are found in mouse urine, and individuals can distinguish these molecules by smell. Consistent with the foregoing hypothesis, the degree of female altruism toward the offspring of close relatives was predicted by the degree of relation based on MHC type and type recognition.
Analyses by theoretical biologists Martin A. Nowak, Corina E. Tarniţă, and Edward O. Wilson, published in a 2010 Nature article, showed no mathematical difference between kin selection theory and classic natural selection theory and suggested that natural selection could itself account for eusocial behaviors (living in groups where only a select few mate) when those behaviors confer evolutionary advantages. Their work ignited controversy within the scientific community, with many defending kin selection and attacking Nowak, Tarnita, and Wilson's methods. Wilson has long espoused the theory of group selection, in which evolutionary demands apply not only to the individual but also to the social group, not merely the individual. According to this theory, altruism is based on association rather than genetic relatedness. Debate continues over which theoretical model best fits the evidentiary reality.
Maternal Altruism
Altruism may be observed in a variety of natural systems in which groups comprise individuals who share a high degree of genetic relatedness. A classic example of this sort occurs with Belding’s ground squirrels. Males tend to disperse from colonies, while females remain to create highly related maternal groups. Members of such maternal groups demonstrate altruistic behaviors such as alarm calling to warn relatives of danger. Although truly altruistic in the sense that alarm callers may incur risk of personal injury or death, they can be reasonably assured of breaking even in this economy as long as their genes live on in the bodies of those they have saved by their actions.
Reciprocal Altruism
It would seem that altruism based on inclusive fitness would be precluded by human social organization. Scientists have predicted, however, that reciprocal altruism should exist in systems characterized by a high frequency of interaction among member individuals and life spans long enough to allow the recipients of altruistic acts to repay altruists. Note that the theoretical basis for the existence of reciprocal altruism differs from that for kin selection and that any system in which evidence for reciprocity is found must necessarily include the development of a complex web of sophisticated social interaction. Such systems would be expected to foster traits expressing the panoply of human emotion and the development of certain moral architectures and group cohesion.
Likewise, some social science researchers propose that emotional reward is the underlying motivation for altruism. They argue that empathy and compassion based on emotional attachment trigger altruistic behavior when one individual witnesses another's needs. This hypothesis thus relates more closely to Wilson's group selection theory of association-based altruism than to Hamilton's inclusive fitness.
The Altruistic Gene
Research suggests that multiple genes may play a role in altruistic behaviors. A 2011 Social Cognitive and Affective Neuroscience study indicated an association between the Val/Val and Val/Met variants of the COMT gene and prosocial behavior such as donating earnings to charity. According to the authors, the prevalence of these variations differs among ethnic groups and that 75 percent of Caucasians carry one or the other of them. In 2012, a study published in Molecular Psychiatry found that variations in the dopamine receptor gene DRD4 are associated with self-reported altruism. Investigations into the influence of these and other genes on prosocial behavior remain ongoing.
Key Terms
altruism
:
behavior that benefits others at the evolutionary (reproductive) cost of the altruist
evolution
:
a change in the frequency of alleles resulting from the differential reproduction of individuals
haplodiploidy
:
a system of sex determination in which males are haploid (developing from unfertilized eggs) and females are diploid
inclusive fitness
:
an individual’s total genetic contribution to future generations, comprising both direct fitness, which results from individual reproduction, and indirect fitness, which results from the reproduction of close relatives
kin selection
:
an evolutionary mechanism manifest in selection for behaviors that increase the inclusive fitness of altruists
maternal altruism
:
altruism on the part of mothers toward offspring as well as between and among members of groups comprising closely related females
natural selection
:
a process whereby environmental factors influence the survival and reproductive success of individuals; natural selection leads to genetic changes in populations over time
reciprocal altruism
:
mutual exchange of altruistic acts typically associated with highly cohesive social groups
Bibliography
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