Download Chapter 12: Family, Society, and Evolution

Chapter 12: Family,
Society, and Evolution
Robert E. Ricklefs
The Economy of Nature, Fifth Edition
(c) 2001 W.H. Freeman and
Company
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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Cooperation or Competition?
All interactions between members of the
same species delicately微妙地 balance
conflicting tendencies of cooperation and
competition, altruism利他主义 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对抗.
Consequences of these interactions for
individuals are substantial, with effects on
individual fitness.
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Territoriality领域性
Any area defended by an individual
against the intrusion入侵 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优势等级
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外围 of a flock集群
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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对抗, 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博弈论
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 individuals.
Living in groups results in benefits and
costs to flocking群集 birds, like the
European goldfinch金翅雀:
benefit is less individual vigilance警戒
cost is the more rapid depletion耗尽 of
resources, forcing the flock to move more
frequently
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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
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A Classification of Behaviors
Four combinations of fitness increments增长 to
donor and recipient lead to the following
classification:
cooperation (benefits donor, selected for)
selfishness (benefits donor, selected for)
Spitefulness恶性 (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盛行
yet altruistic acts
are common in social species
(c) 2001 W.H. Freeman and
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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血统识别
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.
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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
(c) 2001 W.H. Freeman and
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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 (c) 2001 W.H. Freeman and
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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 childproducing pair
a portion of the behavior of non-nuclear
members of the extended family are directed
toward well-being福祉 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
(c) 2001 W.H. Freeman and
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Cooperation Among
Unrelated Individuals
Social groups can form to promote mutual
self-interest互利营私 of unrelated
individuals.
Can groups of unrelated individuals move
toward true cooperation?
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Game Theory and
Cooperation
The paradox悖论:
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:
the hawk strategy is not the best overall because
hawks incur招致 costs of conflict
a dove never competes over resources, sharing
resources with other doves, yielding them to
hawks:
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得到
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持续 of hawks and doves in
this case is an evolutionarily stable mixed
strategy.
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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繁荣,
so do the parents’ genes.
Parents and offspring come into conflict
when accumulation of resources by one
offspring reduces the overall fecundity生
育力 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停止 providing
care
offspring should continue to request additional care
until the benefit to parent of providing that care falls
below twice the (c)
cost
care
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Freeman
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reproduction
Eusocial群居的 Insect
Societies
Social insects exhibit the extreme of
family living, in which most offspring
forego放弃 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真社会性?
Eusociality entails:
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 等翅目(termites白蚁) and
Hymenoptera膜翅目 (ants, bees, wasps), and
to one mammal,
theW.H.naked
mole rat裸耳鼠.
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How did eusociality evolve?
Potential sequence of evolutionary events:
parents have a lengthened延长 period of care for
developing brood 孵蛋(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成年, the way is open to relinquishing放弃
reproductive function to support mother’s
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Organization of Insect
Societies
Insect societies are dominated by one or a few
egg-laying 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白蚁
colonies, which are
headed by a king and queen.
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Coefficients of Genetic
Relationship in Hymenoptera膜翅目
Hymenoptera have a haplodiploid sexdetermining mechanism:
females (workers) develop from fertilized eggs
males (drones雄峰) 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.
(c) 2001 W.H. Freeman and
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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一致.
(c) 2001 W.H. Freeman and
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