EVOLUTION OF BEHAVIOR

One general view in the study of the evolution of behavior is that behaviors can have a genetic basis. This is not to say that all behaviors are genetically based; indeed many behaviors are entirely culturally transmitted or learned and may have little to do with genetics (why are you sitting in the same seat?). For genetically influenced behaviors we can treat them as we would treat any other genetically controlled trait of an organism: 1) if there are genetically based differences in a behavior, and 2) these differences affect fitness then, 3) behaviors can evolve by natural selection.

Two examples of genetically based behaviors: cricket song. Different species of crickets have different calling songs with different characteristics, e.g., inter chirp interval, pulse repetition rate, etc. Hybrids between closely related species often exhibit songs with intermediate characteristics (pulse repetition will be intermediate, inter pulse interval will be intermediate, etc.) a hypothetical example with time on horizontal axis and each chirp = a group of vertical lines:

Another example (on a larger phylogenetic scale) is head scratching with the hind leg in amniotes (reptiles, birds, mammals; those with an amniotic sac). Most reach the hind leg over the fore limb to scratch the head; that birds and mammals do it suggests that this behavior has a genetically programmed basis and has been inherited through much of higher vertebrate evolution.

Behavior is usually dissected into two components for analysis: Proximate causes/questions in which one asks how the behavior is performed and ultimate causes/questions in which one asks why the behavior is performed. Tinbergen has identified four questions to pose when analyzing a behavior 1) what is the cause, 2) what is the development (ontogeny), 3) what is the current function 4) what is the phylogenetic history. A strict course on evolution focuses more on the latter two questions (recall adaptation/preadaptation/exaptation discussion and the identification of current utility vs. historical origin).

Herring gulls breed is large colonies on the ground and defend territories. Two separate calls used for 1) advertising nest site ("choking" call) and 2) as a territorial claim (the "oblique pose" and "long call"). The Kittiwake also breeds in colonies but nests on vertical cliffs and its nest pad is its territory and breeding site. In this species only one behavior serves both functions: "choking" behavior is both defensive and part of mate recognition/pair formation. This is seen as an adaptive behavioral shift wit respect to the nest location (steep cliff).

There are many behaviors that at first appearance do not seem "adaptive". Infanticide in lions was first viewed as "aberrant" behavior by abnormal individuals because it was not "good for the species" (male lions displace other males from groups of females and their offspring, and frequently kill the cubs). It is true that killing infants is not, in the short term, an effective means of increasing population numbers of a species. BUT, we now know (post W.D. Hamilton's 1963, 1964 papers on inclusive fitness and kin selection and G. C. Williams book on Adaptation and Natural Selection) that the more appropriate way to address such problems is to think about them in the context of whether the behavior is good for the individual.

In analyzing infanticide from the perspective of gene thinking it is 1) not adaptive for a male lion to invest reproductive effort in an individual with whom he shares no genes and 2) once the infant is killed it is advantageous for the female to come into estrous and have more offspring with the new male (this will increase her reproductive output over leaving with the displaced male, and not benefiting from other advantages of group living: foraging, avoiding predation on young). Given the situation for both male and female, the observed behaviors make sense in terms of propagating ones genes.

The role of the gene (or genes!) as the unit that is relevant in the evolution play an important part in two influential books in the mid 1970s Sociobiology by E. O. Wilson, and The Selfish Gene by Richard Dawkins. To grossly oversimplify one of their main messages: "an organism is just DNAs way of making more DNA"

If we take the case of bird migration we want to know how the bird navigates to the breeding location (solar and magnetic cues during flight), how the bird knows when to begin migration (internal clocks and changes in day length [physiological changes]). There is usually a high cost associated with migration so we also want to know why birds do it since many die in the process (more time for feeding, more available food). Individuals that do migrate must leave more offspring than those that do not - again gene thinking helps account for why the behavior exists

Population genetic approaches to the evolution of traits rarely tell us why a phenotype affects fitness in a particular way; the models usually look at whether fitness increases or not. The optimality approach to the analysis of behavior attempts to builds models where different behaviors are treated as the traits and asks which one of these behaviors might evolve. The approach generally ignores the mechanics of underlying genetic basis of the behavior (i.e., its mendelian and transmission genetics). Optimal models assume there is a genetic basis and treat each behavior as a haploid (asexual) trait that is inherited intact.

While Gould and Lewontin (and many others) have criticized optimal models, the builders of optimal model (e.g., John Maynard-Smith, Univ. Sussex) argue that the models do not assume that the organisms are optimal (because there are constraints on evolution of traits), but by treating the problem as an optimality issue, it can tell you what kinds of behaviors might evolve.

Two general type of optimal models: frequency independent models are designed independent of what other strategies are doing, and seek to define the conditions which might influence behavior (recall the "optimal foraging" model we described in the adaptation lecture where a bird assess, quality, availability, distance to food items, etc.).

Frequency dependent models are ones where the strategy of one type depends on the strategies and frequencies of other types in the population. The general approach is to look for Evolutionary Stable Strategies (ESS) = a strategy that, if adopted by all, cannot be "invaded" by a mutant strategy. Here a strategy = the behavior of an individual in a certain situation. These types of model apply nicely to ritualized behaviors, distinct display behaviors which take the place of aggressive interactions. Maynard-Smith's approach involves:

H = hawk who fights until the opponent retreats or will continue fighting injured with cost C

D = dove who displays but will retreat if the opponent escalates

V = payoff of winning an encounter

C = cost of losing an encounter

These values can be put into a payoff matrix:

H:H interaction = 1/2(V-C) because each individual hawk will win half of the time and lose half of the time. In the D:D interaction each will win half of the time and retreat half of the time (retreat with no cost). Which strategy is an ESS? Answer by asking if a strategy can invade. Can H invade a population of D's?: Is payoff (D against D) > payoff (H against D)? i.e., is V/2 > V? Answer = NO, so H can invade a population of D's.

Is H an ESS? Is payoff (H against H) > payoff (D against H)? i.e., is 1/2(V-C) > 0? Answer: it depends on the values of V and C: if V > C then payoff to H will be positive and H is an ESS; if V < C then payoff to H will be negative and neither D nor H will be favored (H will always invade a population of D's until H's become so frequent that they encounter each other frequently. D can invade a population of H's because H's tend to damage each other too much. In fact a population of all H's with V<C would go extinct. Thus which behavior evolves depends on the nature of the interactions.

One can imagine many other games and payoff matrices that could be built to model other behaviors. The point of all this is to imagine the following: some species have ritualized displays that appear "civil" in an anthropomorphic sense. Have these behaviors evolved through a stage where hawks killed each other (C was high) to their current state where the cost C to engaging in a behavior is considerably less? This question could be addressed by comparing the behaviors of related species and applying the game theory approach.