Group Selection and Altruism PDF

Sociobiology GROUP SELECTION AND ALTRUISM DR. JAZI AL-ENEZI  Group Selection and Altruism  This chapter is about natural selection at the levels of selection in between the individual and the species. Individual Selection:  Darwin held that natural selection operates at the...

Sociobiology GROUP SELECTION AND ALTRUISM DR. JAZI AL-ENEZI  Group Selection and Altruism  This chapter is about natural selection at the levels of selection in between the individual and the species. Individual Selection:  Darwin held that natural selection operates at the level of the individual.  Adaptive features are acquired by and passed on to individual organisms, not groups or species, and they benefit individual organisms directly and groups or species only incidentally.  The Components of Natural Selection Figure 3.2 The three components of natural selection. Evolution by natural selection occurs when there is variation, inheritance, and differential reproductive success among individuals in a population.  Group Selection and Altruism Group selection :  Operates at the group level, when it affects two or more members of a lineage group as a unit.  Just above the level of the individual we can delimit various of these lineage groups :  a set of sibs, parents, and their offspring; a close-knit tribe of families related by at least the degree of third cousin; and so on.  Group Selection and Altruism  Evolutionary biologists would sometimes attempt to explain certain aspects of animal behavior or physiology as adaptations that had arisen “for the good of the species” or “for the good of the population”—that is, adaptations that would minimize the chances that the species or population as a whole would go extinct.  Nobel prize–winning ethologist Konrad Lorenz (1903–1989) used this type of argument to explain why animal fights are rarely fatal, despite the seemingly lethal armaments that many species carry. Wynne-Edwards was particularly interested in the reproductive restraint that organisms appeared to display, and he viewed this as a group-level adaptation to avoid overexploiting their food supply and other resources.  Group Selection and Altruism  https://youtu.be/xgNttbEJJTQ  Group Selection and Altruism kin selection:  Imagine a network of individuals linked by kinship within a population.  These blood relatives cooperate or bestow altruistic favors on one another in a way that increases the average genetic fitness of the members of the network, even when this behavior reduces the individual fitness of certain members of the group.  Kin selection is the evolutionary strategy that favors the reproductive success of an organism's relatives, even at a cost to the organism's own survival and reproduction.  The members may live together or be scattered throughout the population.  The essential condition is that they jointly behave in a way that benefits the group as a whole, while remaining in relatively close contact with the remainder of the population. Group Selection and Altruism  Group Selection and Altruism  “I confine myself to one special difficulty, which at first appeared to me insuperable, and actually fatal to my whole theory. I allude to the neuters or sterile females in insect-communities: for these neuters often differ widely in instinct and in structure from both the males and fertile females, and yet, from being sterile, they cannot propagate their kind.” Darwin 1859 In nests of social insects, there are always some members (the sterile workers) who devote their lives entirely to the well-being of others. Problem: Sterile female workers have a fitness = 0 and cannot propagate their own kind  Group Selection and Altruism  “This difficulty, … as I believe, disappears, when it is remembered that selection may be applied to the family, as well as to the individual, and may thus gain the desired end.” Darwin 1859 Solution to Problem: Selection applied at a higher level of biological organization: Selection among families or among colonies. Eventually Darwin decided that the nest as a whole could be treated as a kind of super-organism, with the individual members as parts; hence the individual benefiting from adaptation is the nest rather than any particular insect. Selection has been applied to the family, and not to the individual, for the sake of gaining a serviceable end. Darwin 1859  Alleles that are shared because of common ancestry are referred to as identical by descent.  A most recent common ancestor is the most recent individual through which two (or more) organisms can trace gene copies that they share by descent.  Full siblings share the same mother and father, cousins share some subset of the same grandparents, and so on.  An individual can get copies of his genes into the next generation either by producing more offspring himself or by helping his kin in their reproductive endeavors.  This observation forms the basis for the notion of inclusive fitness.  W. D. Hamilton (1936–2000) proposed that an individual’s total fitness can be viewed as the sum of:  (1) its direct fitness, which is the number of viable offspring that it produces, and  (2) its indirect fitness, which is the incremental effect that the individual’s behavior has on the (direct) fitness of its genetic relatives.  Hamilton termed the sum of the two components the inclusive fitness of an individual.  To quantify the relatedness between two individuals, we should calculate the coefficient of relatedness (often denoted r) between two individuals “A” and “B”.  Coefficient of relatedness can be calculated by following these steps:  1. We locate the most recent common ancestor or ancestors of A and B.  This may be a single individual, or it may be a mated pair.  2. For each most recent common ancestor, we calculate the probability that a given allele copy in that ancestor has been passed on to both A and B. Figure 18.2 Pedigrees for calculating relatedness. Squares indicate males; circles indicate females. Individuals A and B may have one or two most recent common ancestors (dark shading). (A) A and B have the same grandmother but different grandfathers. Thus, their grandmother is their sole most recent common ancestor. (B) A and B have the same maternal grandmother and the same maternal grandfather. Thus, both maternal grandparents are the most recent common ancestors.  Group Selection and Altruism Interdemic (or interpopulation) selection:  A cluster of populations belonging to the same species may be called a metapopulation.  Metapopualtion is known as a group of same individual living in different places forming patches, but movement of individual from one population to another occurs regularly.  A type of natural selection which acts upon populations within a species.  At any moment of time a given patch may contain a population or not; empty patches are occasionally colonized by immigrants that form new populations, while old populations occasionally become extinct, leaving an empty patch.  Group Selection and Altruism  Interdemic selection :  Occurs when populations die out or give rise to new populations at different rates, depending on whether they have traits that are beneficial or harmful to the population as a whole.  It is also possible for a trait to be harmful to the individuals within the population which have it but be beneficial to the population as a whole; such a trait would thus be selected for until every individual within the population has the trait (that is, the trait becomes fixed). Figure 5-1 Ascending levels of selection. Group selection in the broadest sense can consist of either kin selection, in which the actions of individuals differentially favor relatives, or interdemic selection (also called interpopulation selection) , in which entire populations are diminished or extinguished at different rate s. Every gradation between these two extremes is possible. The differential tendency to disperse is referred to as migrant selection.  Reciprocity  In 1971, Robert Trivers hypothesized that if individuals benefited from exchanging acts of altruism, then this sort of reciprocal exchange system—which Trivers called reciprocal altruism— might be favored by natural selection. If individual A pays some cost to help individual B, but the cost is recovered at some point in the future (when B helps A), then natural selection might favor behaviors that lead to this type of reciprocity. Reciprocal altruism might be especially likely to occur among individuals that live in stable groups because they are likely to have ongoing interactions with the same set of partners.  The gist of the free- rider dilemma is that it may be hard to establish costly cooperation in groups because each individual has an incentive to “free ride” on the efforts of the others.  The behavior of mobbing a predator provides a good example.  Mobbing behavior is an antipredatory tactic, in which one or more individual's approach, chase, and sometimes even attack a potential predator that may be much larger than individuals of the mobbing species.  This sort of behavior is common among birds, where mobbing behavior often causes a Figure 18.8 Crows mobbing an potential predator to leave an area as a result owl. Evidence from some species of continual harassment. suggests that this sort of antipredator behavior may involve reciprocity among the mobbers.  Mobbing behavior can be costly, both in terms of the time and energy invested, and because mobbing individuals are occasionally caught by the predator they are trying to mob.  But once a predator is driven away, all of the prey individuals in that area benefit, not just those that were involved in mobbing. So, why do individual birds join a mobbing group?  Indrikis Krams and his colleagues designed an experiment to examine whether reciprocity played a role in the mobbing behavior of the pied flycatcher. Figure 18.9 The experimental design for examining reciprocal mobbing in pied flycatchers. Three nest boxes were placed on a triangular grid spaced roughly 50 m apart. (A) Phase one: A stuffed predator (owl) was placed near nest box 1. Birds from nest boxes 1 and 3 mobbed the predator at nest box 1, but birds in pair 2 could not join this mob. (B) Phase two (conducted 1 hour after phase one): A stuffed predator was placed at nest boxes 2 and 3. Pair 1 joined the mob at nest box 3, but not at nest box 2.