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BIOLOGY 517: ANIMAL BEHAVIOR

LEARNING UNIT 2.0 — BEHAVIORAL GENETICS AND EVOLUTION

2.3 The Evolution of Behavior. Chapter 6, Drickamer et al. 2002, pp 63-76

How does behavior become an element of natural selection? By what process does this occur?

Discussion Topics can be found in the Discussion Folder, titled as Discussion 2.3.Essays (assignments e-mailed to the instructor) are located in the Assignments Folder, titled “Essays 2.3”.

Behavior and EvolutionBehavior may also influence gene–behavior relations by

altering the frequency and expression of certain genes in a population. This alters gene frequency, with the result of more genes carrying successful behaviors than those that limit reproductive success. Examples include courtship displays, the killing of offspring by dominant males, and so on.

Gene-controlled behavior may vary among a population of animals, some due to environmental differences based on different habitat characteristics, others due to adaptations (successful mutants that are propagated by natural selection). Animals adapt to their environment, with different behavioral suites developing based on those habitat differences.

Most behaviors have some form of genetic basis, and behavior is part of the phenotype that is most likely to change in response to environmental change. Some changes are accumulated, and are thus microevolutionary (gulls). Others are macroevolutionary (where peripheral isolation leads to rapid change).

Domestication and BehaviorBreeding animals to reinforce the inheritance of desired traits is

an element of domestication. Breeding for friendliness and play

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behavior, for instance, has resulted in the retention of juvenile characteristics in some dogs: shorter muzzles, larger eye width/skull width ratio, and so on. The selection of a trait for each breed has led to substantial changes from the ancestral dog.

However, the selection of desired traits can produce unexpected side effects: hip dysplasia in some dog breeds, spinal problems in dachshunds, aggression in hatchery trout. Thus, artificial selection of behavior can result in morphological change as well as behavioral changes. And this is the model Darwin examined when developing the concepts of natural selection.

Field ObservationsField observations also support this selective process, but with

those species undergoing changes due to natural events. Perching preference for melanistic pepper moths was distinctly altered by bird predation—dark moths that continued to perch on light tree trunks were eaten by birds, whereas those that preferred dark trees survived more often. Thus, those dark moths with a light-trunk preference—and the genes that guided that preference—were removed from the pepper moth gene pool, leaving greater and greater amounts of moths with dark-trunk preference.

Another means of studying such changes in behavior would be to study populations of a species under different environmental conditions. The text covers a representative case involving spiders, but similar studies include fishes in different ponds, island populations of rodent species, and the like.

Evidence for the Evolution of BehaviorEvidence includes phylogeny, fossils (few fossils of behavior,

other than tracks; still, much can be derived about foraging patterns and group relations), and adaptive radiation (Darwin's finches).

The change of communicative behaviors is another example. In ritualization, adaptive evolutionary changes in behaviors from noncommunicative functions toward increased signal efficiencies derived from intention movements or redirected behaviors. This is the basis behind displacement activities.

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Homology also supports the evolution of behavior. Segments of behavioral patterns can be dissected from the whole; leading to study of homologous behavioral patterns (shared behaviors by species through descent from a common ancestor). This has best been seen in domestication, where behavioral plasticity is greater in some domesticated animals due to the increase in neoteny of those animals, while in other cases degeneracy occurs. Dogs and breeding for behavior, for instance, have favored the appearance and behavior of puppy traits over those of ancestral adults.

Comparative series refers to the comparative studies of behaviors from closely related species. For example, balloon fly species make a good comparative series, where each species displays different mating behaviors.

1) Conspecific females often eat males.2) Males offer prey to females to prevent being eaten during or

following copulation.3) Prey becomes a stimulus for mating, rather than a bribe alone.4) Prey “adorned” with silk to quiet prey.5) Elaboration of adornments.6) Males consume the prey, wrap the husk, and present the

husk/balloon to the female during mating; she manipulates the balloon during copulation.

7) Prey item becomes very small in size, useless as a meal item; the balloon becomes the main incentive for mating.

8) Prey are dispensed with and only a balloon is presented, with no prey.Constant patterns of behavioral evolution are fixed patterns,

providing evidence for monophyletic groupings (such as those shown by weaver birds).

Biogenic law provides for possible derivation from early forms to later forms during development of juveniles. However, does ontogeny recapitulate phylogeny? Biogenic law is unlikely and discounted.

Study of AdaptationThese studies include adaptive stories, where traits are

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constrained by phyletic heritage, developmental pathways, and general architecture and are not just the result of current selective forces. Other methods include observing natural selection in the field (melanism and the industrial moths, Galápagos finches).

Mate Recognition Systems and SpeciationBehavioral changes may result that restrict the exchange of

genes between populations (should they come into contact). This is most important in mate selection and signals associated with mating. Why? Mate selection leads to nonrandom mating within a population (isolating mechanisms).

Are they isolating? Not really, this is a misnomer. Actually, they operate to increase mating with only members of that particular behavior, and do not actively isolate other members.

Specific-Mate Recognition System (SMRS) is derived through survival—members of a population that have adapted to local conditions tend to produce offspring with higher fitness if those members mate only with other members of that population. Reproductive fitness becomes an incidental effect of SMRS. This has been studied in lizards and gulls. Natural selection leads to greater differences of populations where they overlap than those in areas where they do not; character displacement.

TraditionNatural selection produces both plasticity of expression and

tendency to transmit behavior to offspring through nongenetic means. Thus, learning, where tradition-transmitted information (behaviors) can be rapidly spread through a population, occurs much faster than with genetic information.

How did social patterns evolve? A society is a group of individuals of the same species that is organized in a cooperative manner extending beyond sexual and parental behavior.

These include several species of colonial invertebrates (but there are some problems in determining colonies or individuals). Cnidarian colonies are an example of extreme colonial lifestyle, with formation of cooperative zooids specializing in singular tasks. Zooids

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are cloned from one individual, which is itself derived from one fertilized egg. The advantages include safety from the environment in numbers, to modify the environment when one alone could not, forming larger structures, can outcompete other, smaller forms, and defense.

Social insects include the eusocial insects, which employ cooperative care for the young, produce reproductive castes cared for by nonreproductive castes, and overlap between generations such that offspring assist parents in raising siblings. The basis behind the social structure is chemical communication and population regulation.

In vertebrates, fish exhibit complex schooling behaviors for defense, increased ability to find patchy food, conservation of energy (heat generation, reduction of drag, finding mates). Amphibians and reptiles show some complex social interactions (particularly among crocodilians and chorus frogs), including hibernacula and mating choruses. Bird behavior demonstrates flocking and cooperative breeding with helpers. Nesting behavior shows remarkable social development.

Communal nesting refers to birds that nest or roost together. Some cuckoos nest communally, with several mothers taking care of the entire brood, derived from the protection offered by flock defense, elevated from parental defense. This is necessary in a patchy environment.

Helpers-at-the-nest is more common. Scrub jays offer no helpers, while piñon jays nest in colonies (with little defense of neighboring nests). Others have pair territories with assistance from other birds than the parents (Florida scrub jays), with sibling assistance. Mexican jays take this further, with helper flocks for communal nests.

In mammals most social systems are arranged matrilineally, with adult males as unrelated sires. Grouping exists as a defense, causing patchiness of females that leads to polygyny by males. An extreme form of this is the case of naked mole rats, with matriarch control of subordinates by pheromones.

Why Live in Groups?Benefits:

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1) Protection from physical factors.2) Protection against predators.3) Assembly of sexual species for mate location (swarming).4) Location and procurement of food.5) Resource defense against conspecifics or competing

species (territoriality).6) Division of labor among specialists.7) Richer learning environment.8) Population regulation.

Costs:1) Increased competition for resources.2) Increased chance of spread of diseases and parasites.3) Interference with reproduction.

Early Studies of Social Behavior: Cooperation through Group Advantage

“Living organisms reacted to the environment and so modified it that other species could not exist there….”

“…altruistic or cooperative forces were somehow stronger [than natural selection] … leading to natural cooperation….”

1) Invertebrate coloniality. Involuntary and primitive with respect to other forms of cooperation

2) Aggregation. Random.3) Orientation to stimuli. Supposed conspecific tolerance.4) Locomotion to favorable locations. Happenstance by

patches.5) Clumping in the absence of substrate. Protection?

Defense?6) Sleeping group. Protection from predators, but increased

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threat of predation.7) Complex social life. Zenith of social systems; “unconscious”

drive that predisposes complex behavioral development.

Recent Studies: Cooperation through Selfishness“…The progressive modification of structure or function [occurs]

only insofar as variations in these are of advantage to the individual.”The “force of cooperation” and group selection is rejected.

Evolution of behavioral traits occurs by means of increased or decreased fitness of offspring.

The Selfish HerdThe “buddy” system—by being in a group, you increase your

survival by making yourself less of a target (mob psychology). Hopefully your buddy gets it and not you.

Kin SelectionIf a gene that causes some kind of altruistic behavior appears,

the gene's success depends ultimately not on whether it benefits the individual carrying the gene, but on the gene's benefit to itself. Altruism serves to increase the number of altruistic genes in the population and self-sacrifice is viable if the sacrifice perpetuates more altruistic genes than the individual itself carries. The more distant the relative, the less likely that that individual will carry the gene, and that many more relatives will have to be saved in order to justify sacrifice. Examples: alarm calls, helpers, social systems (with single breeding pairs).

Kin selection in social insects is odd—eusocial insects are more closely related to siblings than offspring. This is explained by altruism: any genes promoting sibling care would increase faster than genes promoting care for own offspring. Indeed, in most ants females invest care to female siblings 75% and only 25% for sibling males. In slaver ants, where workers are slaves, care is given equally to both (50%) (slaves have no investment to either gender – different species). Problem: many eusocial insects mate with more than one male, producing daughters that are not nearly identical.

Reciprocal altruism is another form, much like mutualism with a

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time delay: “If you scratch my back,” etc.Which type of altruism and whether altruism occurs at all that

occurs is based on a number of factors, such as length of lifetime, dispersal rate, mutual dependence, and so on. This leads to an evolutionary strategy called the Prisoner’s Dilemma.

In the Prisoner's Dilemma, it's better for the individual to defect (get but not give) in the short run; in the long run, cooperation escapes the accumulated penalties of mutual defection and is the better long-term strategy.

A\B Cooperation Defection

Cooperation R = 3; reward for mutual cooperation

S = 0; sucker’s payoff

Defection T = 5; temptation to defect

P = 1; punishment for mutual defection

The Prisoner’s Dilemma explains much of non-kin social groupings. For reciprocity, for instance, among shared feedings (by regurgitation) among vampire bats:

1) pairs must persist long enough to permit reciprocation;2) benefit to receiver must exceed the cost to the donor;3) And donors must recognize cheaters.This exists also among non-kin groups, groups of different

species (cleaner fish), and eusocial insect origins (and loosely-related kin).

For mutualism there are no time delays. Individuals among the group have a higher fitness than when alone. Huddling for warmth, for instance, may occur at one extreme with extreme mutualistic symbiosis at the other. Note, however, there is no cost to the participants, only gain.

Parental Manipulation of OffspringGenes that favor the lifelong reproductive success of the parent

will be more advantageous (more young and more genes in a

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population) than selfish and strongly competitive genes in an offspring that limit parental reproductive output. Parents thus regulate the offspring to the parent's advantage by limiting the amount of parental care per offspring, restricting or eliminating care when resources are insufficient, killing or feeding offspring to the remainder of the brood, forced assistance in temporary brood care (with induced sterility), or forced assistance in permanent brood care (with induced sterility).

Human Mating SystemsSexual dimorphism is related to polygyny by the number of

females that a male can monopolize. Thus, humans may have evolved from mildly polygynous ancestors, retaining those traits today (though this is no excuse…). Culturally, some polygyny does persist (harems, polygamy). In cultures with polygyny, though, males take longer to mature, suffer higher mortality, and senesce more rapidly. High-ranking females in harems should invest more in sons than daughters (sons carry genes to other populations). Low-ranking females should invest in daughters (sons risky, with higher mortality prior to reproduction).

See the Moriss’ Human Animal and related video series.