Biology 517 1
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
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 Evolution
Behavior 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 Behavior
Breeding animals to reinforce the inheritance of desired traits is
an element of domestication. Breeding for friendliness and play
behavior, for instance, has resulted in the retention of juvenile
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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 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 Behavior
Evidence 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.
Homology also supports the evolution of behavior. Segments of
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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
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
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 Adaptation
These studies include adaptive stories, where traits are
constrained by phyletic heritage, developmental pathways, and
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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 Speciation
Behavioral 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
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.
Natural 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
are cloned from one individual, which is itself derived from one
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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
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
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
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
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?
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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
6) Division of labor among specialists.
7) Richer learning environment.
8) Population regulation.
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
“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
5) Clumping in the absence of substrate. Protection?
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 Herd
The “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.
If 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
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.
AB Cooperation Defection
Cooperation R = 3; reward for S = 0; sucker’s payoff
Defection T = 5; temptation to P = 1; punishment for
defect 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
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 Offspring
Genes 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
Human Mating Systems
Sexual 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.