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Tamarin: Principles of Genetics, Seventh Edition
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IV. Quantitative and Evolutionary Genetics
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21. Evolution and Speciation
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The McGraw Hill Companies, 2001
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Sociobiology
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M llerian mimicry. (a) Monarch butter y and (b) viceroy butter y. Both have similar colors (orange and black) and a generally similar color pattern. ([a] Robert
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Finke/Photo Researchers, Inc. [b] Richard Parker/Photo Researchers, Inc.)
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Batesian mimicry seen in West African butter ies that live in the same places. Those on the left are species belonging to the genus Bematistes. Those on the right that mimic them are different species belonging to the genus Pseudacraea. ( J. A. L. Cooke/Oxford Scien-
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forsake reproduction in order to work for the colony Sociobiology, the study of the evolution of social behavior, attempts to answer these questions.
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Kin Selection and Inclusive Fitness
In 1964, W. D. Hamilton developed concepts that explained altruistic acts without resorting to group
selection. Starting with the known fact that relatives have alleles in common, Hamilton suggested that natural selection would favor an allele that promoted altruistic behavior toward relatives because the result might be an increase in copies of that allele in the next generation. The proportion of alleles shared by two individuals can be de ned as a coef cient of relationship, r. If an individual has a certain allele, the probability that a particular
Tamarin: Principles of Genetics, Seventh Edition
IV. Quantitative and Evolutionary Genetics
21. Evolution and Speciation
The McGraw Hill Companies, 2001
Twenty-One
Evolution and Speciation
relative also has that allele is r. Siblings have an r 1/2. A squirrel is likely to have virtually all its alleles still viable if it sacri ces itself for two or more siblings. In fact, natural selection should de nitely favor altruism of an individual toward three siblings because, in a sense, natural selection is weighing 1 copy of an individual s alleles (the individual itself) versus 1.5 copies (three siblings). This sort of reasoning has been termed the calculus of the genes. It does not imply that individuals actually think these things out; rather, natural selection has favored the individuals that behave this way. Hamilton referred to the sum of an individual s tness plus the tness effects of alleles that relatives share as inclusive tness. He referred to the way natural selection acts on inclusive tness as kin selection. Hamilton applied his ideas of inclusive tness and kin selection to explain sterile castes in the eusocial (truly social) hymenoptera (bees, ants, and wasps). The workers in these colonies are sterile females. Why do they forsake their ability to reproduce in order to help maintain the hive or colony The answer seems to come from haplodiploidy, the unusual sex-determining mechanism of these species. In the eusocial hymenoptera with sterile castes, fertilized eggs produce diploid females, whereas unfertilized eggs produce haploid males (drones). The difference between a reproductive queen and a sterile worker in bees is larval nutrition: larvae fed royal jelly can become queens. Hamilton showed that since a worker is more closely related to her sisters than to her own potential offspring, kin selection could favor a worker who helps her sisters at the expense of her own reproduction. Figure 21.12 shows a queen (female) with alleles A1 and A2 at the A locus and a haploid drone (male) with the A3 allele. A daughter will have either the A1A3 or A2A3 genotype. If we compare one of these daughters with her sisters, we see that the average r 0.75 half of the time, r 1.0, and the other half of the time, r 0.5. A queen and her daughters have an r 0.5. Thus, we see that workers (females) are more closely related to their sisters, and hence are at a reproductive advantage by raising them rather than their own young. Wilson has pointed out that sterile caste systems have evolved among insects in only one other group beside the eusocial hymenoptera, the termites. Although eusocial hymenoptera make up only 6% of insects, sterile castes have independently evolved at least eleven times.This is compelling evidence for the validity of Hamilton s analysis. Only one noninsect example of a caste has been discovered: the naked mole rat, a small subterranean rodent living in Africa, has this type of social system. Many studies concerned with apparently altruistic acts have provided a large body of support for Hamilton s theory of kin selection and inclusive tness. P. Sherman, working with ground squirrels, for example, has observed that the individuals that make the alarm calls have the most to gain from the standpoint of inclusive tness; these individuals are resident females surrounded by kin.
One other explanation for altruism is also consistent with bene ts to individual tness. It is that many apparently altruistic acts are in reality sel sh they just look altruistic. To be altruistic, an individual must risk reducing its tness to potentially bene t the tness of others. We may, in fact, misinterpret some acts as altruistic that simply are not. This turnaround in thought, from group selection to individual selection, has been an intellectual revolution in modern evolutionary biology. Before this revolution, many of the behaviors in nature that involved apparent altruism were dif cult to explain. Now sociobiological reasoning provides an explanation. The reason so much controversy has sprung up over the theory of genetic control of social behavior is because of the implications the theory has for human social, political, and legal issues. Human husband-wife, parent-child, and child-child con icts, for example, may be built into the genes. Altruism, our highest form of nobility, may be mere sel shness. Many critics fear that sociobiological concepts can be used to support sexism and racism. For human beings, the alternative to the theory of sociobiology is the theory that most human behavior, including cultural learning, is determined by the environment. At present, although much evidence remains to be gathered, the sociobiology concept is compelling to many evolutionists.
Figure 21.12 Haplodiploidy in eusocial hymenoptera produces sisters with an average r of 0.75. Because drones (males) are haploid, queens produce daughters of only two genotypes at any locus. A given daughter has an r of 1.0 with sisters of identical genotype and an r of 0.5 with sisters of the other genotype, for an average r of 0.75. In other words, females have a 75% genetic similarity with their sisters but only a 50% similarity with their own offspring.
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