Transcript Chapter 12: Family, Society, and Evolution

A few points
 Your quizzes:
 Your Exam I
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Still on April 22
Chapters 1, 2, 3, 4, 5, 6, 7, 10, and 11 (9 chapters)
Extra Credit: 8 and 9
Leaves 14 chapters
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Chapter 12: Family, Society,
and Evolution
Robert E. Ricklefs
The Economy of Nature, Fifth Edition
Background
 The behavior and, indirectly, life histories and
ecological relationships of an individual are
under strong selective pressure from:
 the social and family environment
 relationship to members of both sexes
 For example, fitnesses of the male morphs of the
side-blotched lizard are dependent on
frequencies of other male morphs in the
population:
 these morphs interact through complex social
interactions that determine reproductive success
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Background
 Individuals interact with other members of
the same species throughout their lives.
 Each individual must perceive the behaviors
of others and make appropriate responses:
 some interactions pay benefits for cooperative
behaviors because of a common interest:
 interactions with kin (common evolutionary
heritage)
 interactions with mates (common interest in success
of offspring)
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 Insert figure 12.2
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Cooperation or Competition?
 All interactions between members of the
same species delicately balance
conflicting tendencies of cooperation and
competition, altruism and selfishness.
 Such a balance is evident in humans, the
most social of animals:
 society is sustained by role specialization
 social life balances cooperation and conflict
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What is Social Behavior?
 Social behavior includes all interactions among
individuals of the same species.
 These interactions range from cooperation to
antagonism.
 Consequences of these interactions for individuals are
substantial, with effects on individual fitness.
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Territoriality
 Any area defended by an individual against
the intrusion of others may be regarded as a
territory:
 territories vary enormously in size and
permanence
 animals are likely to maintain territories if:
 the resource is defensible
 the rewards outweigh the cost of defense
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Dominance Hierarchies
 Defense of territories may not always be practical.
 In absence of territories, the outcome of conflict may
be establishment of social rank.
 When individuals order themselves by social rank
or status, the result is a dominance hierarchy.
 Social rank and occupancy of space may be directly
related, as low-ranking individuals may be
relegated to the periphery of a flock. (positive
feedback loop)
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To fight or not to fight?
 Establishment of territories or social rank
depends on the outcome of contests between
individuals.
 In any confrontation, participants must weigh:
 costs of fighting and benefits of winning
 likely outcome of the contest
 Determining optimal behavior is complicated
by each individual’s lack of knowledge about
the behavior of the other participant.
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Optimal Behaviors and Game
Theory
 Game theory analyzes the outcomes of behavioral
decisions when these outcomes depend on the
behavior of other players.
 Game theory predicts the individual’s behavior
based the best estimates of:
 the other contestant’s response
 the reward for winning
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Advantages and Disadvantages of
Living in Groups
 True social groups result from a purposeful joining
together of (unrelated) individuals.
 Living in groups results in benefits and costs to
flocking birds, like the European goldfinch:
 benefit is less individual vigilance
 cost is the more rapid depletion of resources, forcing
the flock to move more frequently
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 Insert figure 12.5
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Natural selection balances the costs and
benefits of behaviors.
 Toward a classification of behaviors:
 Most social interactions can be broken into acts performed
by:
 donors - individuals initiating behaviors
 recipients - individuals toward whom behaviors are
directed
 Each act has the potential to affect the reproductive
success of both the donor and the recipient of the
behavior
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A Classification of
Behaviors
 Four combinations of fitness increments to donor
and recipient lead to the following classification:
1.
2.
3.
4.
cooperation (benefits donor, selected for)
selfishness (benefits donor, selected for)
spitefulness (benefits no one, selected against)
altruism (benefits recipient at cost to donor)
 Altruism, among these, is most problematic:


selfish behaviors would be expected to prevail
yet altruistic acts are common in social species
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 Insert figure 12.6
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Kin selection favors
altruistic behaviors.
 When an individual directs a behavior toward a
sibling or other close relative, it influences the
fitness of an individual with whom it shares more
genes than it does with an individual drawn at
random from the population.
 This special outcome of social behavior among
relatives is called kin selection.
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Identity by Descent
 The likelihood that two individuals share copies of any
particular gene is the probability of identity by
descent, which varies by degree of relationship:
 also called the coefficient of relationship
 full sibs have a 50% probability of sharing any gene
 parents and children also have 50% probability of sharing
any gene, etc.
 Two cousins probability of 12.5% (1/8)
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A Model for Assessing
Altruistic Behavior
 Total fitness of a gene responsible for a particular
behavior is its inclusive fitness:
 contribution to fitness of donor plus product of change
in fitness to recipient X, weighted by coefficient of
relationship
 a gene promoting altruistic behavior will have a
positive inclusive fitness if:
C < Br
where:
C = cost to donor
B = benefit to recipient
r = coefficient of relationship
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Implications of the Model
 Genes for altruistic behaviors should increase in the
population when:
 behaviors have low cost to donor
 behaviors are restricted to close relatives
 Opportunities for evolution of altruistic behaviors do
exist:
 individuals often associate in family groups
 individuals can often assess their relatedness
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Is guarding altruistic?
 Insert figure 12.10
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Cooperation among Individuals in
Extended Families
 Complex relationships among extended human
families are familiar to us:
 often such families include only one child-producing
pair
 a portion of the behavior of non-nuclear members of
the extended family are directed toward well-being of
these related children
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Cooperation in Bee-Eaters
 Extended families of bee-eaters exhibit cooperative
and competitive behaviors:
 selfish and selfless acts are directed toward others in
direct accordance with the degree of relationship
 inclusive fitness is the appropriate measure of selection
on social behavior:
 altruistic behaviors can evolve among close relatives by
kin selection
 Figure 12.13
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Cooperation Among
Unrelated Individuals
 Social groups can form to promote mutual selfinterest of unrelated individuals.
 Can groups of unrelated individuals move
toward true cooperation?
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Game Theory and
Cooperation
 The paradox:
 conflict can reduce the fitness of selfish
individuals below that of cooperative individuals,
so cooperative behaviors should evolve among
unrelated individuals
 but, when most of a social group consists of
cooperative individuals, a selfish individual can
achieve high fitness by cheating
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The Hawk-Dove Game
 The hawk-dove game (prisoner’s dilemma):
 a hawk always competes over resources, taking all the rewards
when it wins:
 2 hawks always fight. Result – on average – one gets half the reward
so the reward is ½ the average benefit minus the cost of fighting (1/2
B-C)
 Hawk vs dove: hawk gets it all
 the hawk strategy is not the best overall because hawks incur costs of
conflict
 a dove never competes over resources, sharing resources with other
doves, yielding them to hawks:
 When 2 doves meet – they share w/o cost: 1/2B
 the dove strategy is the best overall because resources are shared
without costs of conflict
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Hawks invade societies of
doves.
 Dove behavior is not an evolutionarily stable
strategy:
 a population of doves is easily invaded (from an
evolutionary perspective) by hawkish behavior:
 a hawk in a population of doves reaps twice the rewards
of doves
 a population of hawks is resistant to invasion by dove
behavior, however
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Can hawks and doves
coexist?
 When the benefit is less than twice the cost of
conflict, dove behavior can invade a population of
hawks.
 In this situation the proportion of hawks is one-half
the ratio of the benefit to cost.
 Persistence of hawks and doves in this case is an
evolutionarily stable mixed strategy.
 Each type of behavior can increase in frequency
when it is rare – thereby keeping both in the game
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Parents and offspring may
come into conflict.
 Offspring consume parental resources, but this is
desirable from the perspective of the parents: when
progeny thrive, so do the parents’ genes.
 Parents and offspring come into conflict when
accumulation of resources by one offspring reduces
the overall fecundity of its parents.
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Parents and offspring have
different goals.
 Offspring try to resolve conflicts over
resources in favor of their own reproductive
success.
 For parents, a balanced approach to current
and future reproduction is favored:
 resources allocated to one offspring cannot be
allocated to another
 resources allocated to current offspring reduce
those that can be allocated to future offspring
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When does parentoffspring conflict occur?
 As young mature, the benefit to them of
parental care declines.
 Because of coefficients of relationship among
parents, an offspring, and that offspring’s sibs:
 when the benefit to parent of providing additional
care falls below the cost of this care for future
reproduction, the parent should cease providing care
 offspring should continue to request additional care
until the benefit to parent of providing that care falls
below twice the cost of this care for future
reproduction
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Eusocial Insect Societies
 Social insects exhibit the extreme of family living, in
which most offspring forego reproduction and help
their parents raise siblings.
 This situation raises evolutionary questions:
 how did such societies evolve?
 how can natural selection produce individuals with no
individual fitness?
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What is eusociality?
 Eusociality entails:
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several adults living together in groups
overlapping generations
cooperation in nest building and brood care
reproductive dominance by one or a few individuals,
including the presence of sterile castes
 Eusociality is limited among insects to Isoptera
(termites) and Hymenoptera (ants, bees, wasps),
and to one mammal, the naked mole rat.
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How did eusociality
evolve?
 Potential sequence of evolutionary events:
 parents have a lengthened period of care for
developing brood (parents guard brood or
provision larvae)
 parents live and continue to produce eggs after
first progeny emerge
 offspring are in a position to help raise
subsequent broods
 when progeny remain with their mother after
adulthood, the way is open to relinquishing
reproductive function to support mother’s
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Organization of Insect
Societies
 Insect societies are dominated by one or a few egglaying females, queens:
 queens of ants, bees, and wasps mate once and store
sufficient sperm to produce a lifetime of offspring
 Nonreproductive progeny of the queen:
 gather food and care for their developing brothers and
sisters, some of which become sexually mature and leave
the nest to mate
 Specific details vary somewhat for termite colonies,
which are headed by a king and queen.
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Coefficients of Genetic Relationship
in Hymenoptera
 Hymenoptera have a haplodiploid sexdetermining mechanism:
 females (workers) develop from fertilized eggs
 males (drones) develop from unfertilized eggs
 Coefficients of genetic relationship are skewed:
 female worker to female sibling is 0.75
 female worker to male sibling is 0.25
 queen to son or daughter is 0.5
 Sex ratios are female-biased, 3:1.
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Summary
 All behaviors have costs and benefits to the
individual and to others affected by the behavior,
with special consequences for close relatives.
 Behavior is influenced by genetic factors and is
thus subject to evolutionary modification by
natural selection.
 Interactions within a social setting lead to
important evolutionary consequences when
interests of individuals conflict or coincide.
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A few things
extra
- Do the quiz for chapter 12.
Due Friday (you get a bit
extra time)
- Spend some time reading
the ‘more on the web’
section in this chapter
- Homework: what can an
understanding of ant
genetics and behavior
provide for us humans?
- Email me your thoughts
by Friday midnight.
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