NATURAL SELECTION AND BEHAVIOR Evolution is a genetic phenomenon which results when the genetic composition of a population (called a gene pool) changes between successive generations. Natural selection, on the other hand, is a process of differential reproduction among individual variants within a single population, i.e., the environment will select certain individuals over others for survival and reproduction. NATURAL SELECTION WILL ACT AS AN EVOLUTIONARY FORCE ONLY IF THOSE SELECTED DIFFER GENETICALLY FROM THOSE NOT SELECTED. The prerequisite that individual phenotypic variation has a heritable component is essential for natural selection to change the composition of a gene pool which results in evolution. Behavior: Nature vs Nurture Behavior is as much an aspect of an individual's phenotype as is morphology, but this fact has only recently been appreciated. For a long time biologists overlooked behavior because it is not easily observed and is very difficult to measure or quantify. A group of European zoologists, headed by Konrad Lorenz and Niko Tinbergen, demonstrated through careful field observation that animal behavior is not idiosyncratic or individualistic, but rather follows patterns that are largely stereotyped and species-specific, i.e., behavior shows little individual variation within species, although it varies considerably between species. With this demonstration of behavioral pattern, the science of ethology was born. Unfortunately, these early ethologists assumed that behavior, since it appeared to be inflexible and stereo- typed, was innate and under strict genetic control. This assumption brought them into immediate conflict with the behaviorist school of psychology which held that behavior was completely flexible and controlled by learning, not genes. The resulting nature vs. nurture debate ignored the equation Phenotype = Genotype + Environment and did little to advance the study of behavior. Ethologists viewed behavioral phenotypes as due solely to the genotype (genetic determinism) and so ignored the environment. Behaviorists, on the other hand, saw behavioral phenotypes as the product of environmental influences (environmental determinism) and so ignored any genetic influence. Only when behavior was shown to develop as a gene/environment interaction like all other aspects of the phenotype was this controversy resolved. Sociobiology: Darwinism and Behavior At about the same time that the nature-nurture battle was being waged, evolutionary theory was being crystallized around Darwin's concept of natural selection: natural selection came to be viewed as the directing force behind evolution through its production of adaptation. This development in evolutionary theory and the discovery by ethologists that behavior followed genetically-based patterns paved the way for the development of sociobiology, which was named and formally launched in 1975 by E. O. Wilson in his comprehensive survey of animal social behavior entitled Sociobiology: The new synthesis. Unlike ethology, which was descriptive and phylo- genetically oriented, sociobiology took an analytical posture in attempting to explain the ultimate cause of animal social behavior. The focus of sociobiology is on the evolution of social behavior as an adaptive response to the environment resulting from natural selection. The central principle of sociobiology views animals as reproductive strategists attempting to maximize their fitness (a measure of reproductive success) through their behavior. Thus, sociobiologists use modern evolutionary theory to explain why social behavior is adaptive and how it evolved through the process of natural selection. Sociobiologists assume that social behavior has a heritable component sufficient to allow natural selection to operate on individual behavioral variation and that social behavior is adapted to meet environmental dictates. Granted these assumptions, they then search for the adaptive significance of behavior and attempt to identify the selective pressures which produced variation in social behavior both within and between different species. In so doing, they focus on the ultimate cause of behavior rather than on its proximate causes which occupy the attention of ethologists (genetic control), behavioral psychologists (control through learning) and physiologists (neuroendocrine mechanisms behind behavior). Whereas both ethology and psychology lean toward the view that behavior is determined either by genes (ethology) or past experience (psychology), sociobiologists adopt the position that behavior is only influenced (rather than determined) by both; consequently, individuals are never fated to behave in a particular way and genetic predispositions can be modified by learning. This position is consistent with the resolution of the nature-nurture controversy, which preceded the development of sociobiology. Behavior and natural selection Selfishness. Natural selection is a reproductive contest between individuals in the same population and it favors selfish behavior, i.e., those individuals whose genetically influenced behavioral patterns enable them to leave more surviving offspring than others will have a greater impact on the composition of future gene pools. Selection therefore favors the evolution of selfishness when it comes to reproductive competition. Yet, not all behavior is selfish. Many examples of cooperative behavior have been described in a large number of different species, e.g., in food gathering, predator defense, conditioning of the environment to make it more hospitable, and parenting. Can altruism (as cooperative behavior is sometimes called) be a product of natural selection? The answer is yes and no depending on how you define altruism. Cooperation. If the cost to an individual in fitness (a measure of reproductive success) involved in assisting others is less than the benefit in fitness received, then natural selection will favor cooperation. Cooperation will evolve via natural selection when to behave selfishly, rather than cooperating with others, will result in a selective disadvantage. For example, many fish cooperate by swimming in schools and this behavior enables them to be more successful in avoiding predators than if they swam by themselves. The school presents itself to predators as a larger fish and therefore tends to repel attacks. Loners would be more vulnerable to predation; hence, any genetically-mediated tendency for independent swimming rather than joining a school would be strongly selected against. By joining a school, a fish does improve the fitness potential of genetic competitors (a cost) but reaps an even greater benefit in terms of its own chances for surviving and reproducing successfully. Reciprocal altruism or reciprocity. Would it ever pay an individual to incur a cost in personal fitness to assist a genetic competitor? The answer is yes provided the benefit received is greater than the cost. Reciprocal altruism is the same as cooperation except that in cooperative behavior the cost and benefit of the act are experienced at the same time. In reciprocal altruism the cost is paid first with the expectation that a greater benefit will be received in the future. Because of the time lag between cost paid and benefit received, reciprocal altruism requires that individuals are able to identify each other as individuals; hence, it only occurs in species with a high level of intelligence, i.e., humans and some other primate species. Because reciprocal altruism can be explained by a cost-benefit fitness ratio (B > C or the benefit must be greater than the cost), it evolves through the agency of natural selection. There are some situations which cannot be explained by the theory of natural selection because individuals appear to incur a cost in fitness greater than the benefit received from their actions. These exceptions fall into two categories: altruism directed towards relatives (nepotism) and altruism directed toward nonrelatives (who are genetic competitiors). To explain these two types of behavior within the evolutionary paradigm, biologists have extended the concept of selection to include kin selection (unit = gene) and group selection (unit = population). These two evolutionary processes are not forms of natural selection (unit = individual) and so require separate treatment in the next section of the course.