Transcript 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
(c) 2001 W.H. Freeman and
Company
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)
(c) 2001 W.H. Freeman and
Company
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
(c) 2001 W.H. Freeman and
Company
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.
(c) 2001 W.H. Freeman and
Company
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
(c) 2001 W.H. Freeman and
Company
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
(c) 2001 W.H. Freeman and
.
Company
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.
(c) 2001 W.H. Freeman and
Company
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
(c) 2001 W.H. Freeman and
Company
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
(c) 2001 W.H. Freeman and
Company
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
(c) 2001 W.H. Freeman and
Company
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
Company
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.
(c) 2001 W.H. Freeman and
Company
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.
(c) 2001 W.H. Freeman and
Company
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
Company
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
Company
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
(c) 2001 W.H. Freeman and
Company
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
Company
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?
(c) 2001 W.H. Freeman and
Company
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
(c) 2001 W.H. Freeman and
Company
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
(c) 2001 W.H. Freeman and
Company
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
(c) 2001 W.H. Freeman and
Company
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.
(c) 2001 W.H. Freeman and
Company
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.
(c) 2001 W.H. Freeman and
Company
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
(c) 2001 W.H. Freeman and
Company
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
2001 of
W.H.this
Freeman
and for future
Company
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?
(c) 2001 W.H. Freeman and
Company
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.
(c) 2001
Freeman and
Company
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
(c) 2001 W.H. Freeman and
Company
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.
(c) 2001 W.H. Freeman and
Company
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
Company
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
Company