The Evolution of Sociality
Terms in this set (38)
occurs when two or more individuals
each receive a net benefit from their joint actions. Each cooperator may pay an
immediate cost for its action, but the overall effect on fitness is positive. Even
when everyone benefits from cooperating, however, it is not obvious that natural
selection will favor cooperative behavior. The reason is that free riding—receiving
benefits but not generating them for others—may be possible, and may be even more beneficial than cooperation to an individual free rider
to understand how natural selection favours cooperation we need to understand how natural selection has solved the following two problems
The altruism problem. Why would natural selection favor an individual
who performs an action that has the immediate consequence of reducing its
own fitness while increasing the fitness of another?
the free rider problem
groups of individuals cooperate,
each investing time, energy, and other resources in activities that benefit
the entire group. Why are individuals selected to do so, when they could
instead free ride on the efforts of others, receiving the public benefits while
shirking their own duties?
path 1 kinship and cooperation
The basic reason
is that genetic relatives are likely to share common genes that they have inherited
from common ancestors—parents, grandparents, and so on
common ancestry and shared alleles
Alleles that are shared because of common ancestry are referred to as identical
a most recent common ancestor
is the most recent individual
through which two (or more) organisms can trace gene copies that they share by
descent. Full siblings share the same mother and father, cousins share some subset
of the same grandparents, and so on.
inclusive fitness and genetic relatedness
He proposed that an individual's total fitness can be viewed as the sum of
(1) its direct fitness, which is the number of viable offspring that it produces,
and (2) its indirect fitness, which is the incremental effect that the individual's
behavior has on the (direct) fitness of its genetic relatives. The latter quantity
reflects the fact that when an individual increases the number of its genetic kin
that survive and reproduce, it is indirectly getting copies of some of its own genes
into the next generation. Hamilton termed the sum of the two components the
inclusive fitness of an individual.
coefficient of relatedness (r)
to calculate the coefficient of relatedness we locate the most common recent ancestor of A and B for each we calculate the probability that a given allele copy in that ancestor has been passed to both a A and B
if there are k meiotic divisions spearating A and B the probability that they share an allele through a single most recent common ancestor is 0.5^k
has played a prominent role in understanding the transition from solitary to group living.
eusociality, it is most
often defined as a social system with the following properties
1. Reproductive division of labor. Only a fraction of the population is actively
breeding at a given time; others are infertile as a result of some form of
Cooperative rearing. Multiple individuals, beyond the immediate parents,
work together to feed and care for the young.
3. Overlapping generations. Not only do the generations of a eusocial species
overlap (unlike annual plants or many annual insect species), but the members
of different generations also live together and work together in a single group.
hypothesized that if individuals benefited from exchanging
acts of altruism, then this sort of reciprocal exchange system—which Trivers called
reciprocal altruism—might be favored by natural selection
Reciprocal altruism reasons.
Reciprocal altruism might be especially likely
to occur among individuals that live in stable groups because they are likely to
have ongoing interactions with the same set of partners.
the prisoner's dilemma
Trivers used a theoretical framework known as game theory.
Game theory allows us to analyze decision
making in a social context. It is useful when dealing with strategic situations, in
which the results of one participant's actions depend on the behaviors that other
participants adopt. I
the prisonner's dilemma
two criminals are caught by the police. if you testify you walk away a free man and the other guy will go to prison for 5 years
if both testify they will both go for 3 yrs and they are aware.
modern group selection models—sometimes called trait-group
selection models—of cooperation are conceptually straightforward.
using genetic relatedness to predict whether an allele for helping one's relatives is favored by natural selection. an allele for helping a relative increases in frequency when
rb − c > 0
where b is the benefit that the genetic relative receives from traits associated with
allele X, c is the cost accrued to the individual expressing the trait, and r is the
coefficient of relatedness
when relatedness is high
benefit to the recipient is high and cost to the actor is low then natural selection should strongly favor individuals who help their kin.
extreme form of sociality is seen here ants bees wasps.
inclusive fitness theory to understand why eusociality evolves so often in ants wasps and bees
thei nest contains hundreds of individuals composed of genetic relatives so the altruistic acts benfits many genetic relatives.
all males are haploid all females are diploid sisters 0.75 r more related to sisters than their own offspring.
multilevel selection is theoretically possible but between group selection is weak to individual selection. group selection as a significant means of evolution is implausible. there is no evidence that group selection can be separated from individual selection
without any mechanism for the evolution of cooperation
natural selection favors defectors. they have a higher payoff than cooperators therefore natural selection reduces the abundance of cooperators until they are extinct
darwinian theory (individual selection) indicates altruism should lead to a decrease in direct fitness of individuals and should be elimated by selection. social cooperation can promote group fitness but a cooperative group is vulnerable to an increase in selfish free riders.
Kin selection and altruism
A gene that favors its own transmission will
spread in the population
- Can favor its own transmission in other
individuals with that gene
- Genetic similarity more likely in relatives, or
Gene will spread if:
Br - C > 0
• W. D. Hamilton developed theory of kin
selection and inclusive fitness
• Inclusive fitness is the sum of:
direct fitness = own reproduction
Indirect fitness = reproduction of genetic kin
made possible by own assistance, weighted by
calculating r index of relatedness
• r reflects 50% reduction in genetic identity
with each sexual generation
because 50% of genes from mom and 50% from
some familial r's diplolids
Some familial r's, diploids
• Full siblings r = 0.5
• Parent - offspring r = 0.5
• Self r = 1
• Identical twin r = 1
• Uncle or aunt r = 1/4
- Genetic ones not married in
• Grandparent r = 1/4
• Cousin r = 1/8
How else might altruism evolve?
'one good turn deserves another'
• Reciprocity, or Reciprocal altruism
• Requires two conditions be met:
Cost to actor less than benefit to recipient
Cheaters must be punished somehow
requires repeated interactions over time
- Cooperative brood care
- Specialized non-reproductive castes
• E.g., workers or soldiers
- Naked mole rats (Bathyergidae)
• Snapping shrimp
• Diverse groups of insects
How did eusociality evolve?
Two things of major importance
- High degree of relatedness among individuals
benefit is high cost is low
- Ecological constraints on other options
Females produced from fertilized eggs
• Males produced from unfertilized eggs
• Females related to:
Brother = 1/4
Mother = 1/2
Sister = 3/4
Offspring = 1/2
Females prefer to rear (reproductive) sisters
(r = 0.75) over own offspring (r = 0.5)
• Worker females prefer to rear 3 times as
many sisters (r = 0.75) as brothers (r = 0.25)
• Queen females prefer equal numbers of
daughters (r = 0.5 to queen) and sons (r = 0.5
• Predicted conflict between queens and
workers over sex allocation well documented
does haplodiploidy explain eusociality
Not by itself.
• Many species have sisters with lower r than
0.75 because of multiple mating by queen
or multiple queens (1/2 sisters or unrelated)
• Many haploidiploid hymenoptera are
solitary, NOT eusocial
Members of a group do not necessarily have to share genes, but the group lives or dies as a function of how their genes (and individuals) interact. Group selection is a more difficult concept to grasp, and is also a subject of an ongoing debate in evolutionary biology. Group selection is a process entirely analogous to individual selection, but acting at the level of groups. Again the traits underlying group selection must be heritable if the species is to evolve by group selection.
acts at yet a higher level. Members of a species tend to share more genes than two individuals from different species. Those members of a clade (e.g. group of closely-related species) that produce more species, or perhaps have greater longevity in the earth's history (e.g. lower extinction probability) are going to become overly represented on the planet in much the same way that one allele at a particular genetic locus spreads through a population of individuals and increases in frequency at the expense of alternative alleles.
typically acts more on the individual level traits than on group level traits,
modern trait group models specify the circumstances in which selection can favor group beneficial traits even when such traits impose individual level costs
a trait group
a group in which all individuals affect one anothers fitness natural selection operates at two levels within group selection and between group selection,
within group selection
acts against cooperators who pay some cost that others do not. selfish free riders are always favored by within group selection because they reveive any benefits that accrue throuht he ac tions of cooperators but they pay none of the costs
between grou p selection
Favors cooperation if groups with more cooperators outproduce other groups. alarm callers pay a cost within groups but thei sacrifice may benefit the group as a hole. this group may now be able to outpoduce groups with fewer alarm collors.
trait grou model
A. In the "trait group" model, small demes (subpopulations) segregate into small patches after random mating (e.g., the pattern of patching may even be related to the trait itself)
B. In this model, Random sampling stochastically results in some demes with higher frequency of an altruistic trait (or allele specifying such a trait)
C. Because "trait groups" with this trait (or allele) have a higher chance of non-extinction (due to the "altruistic" nature of the trait), they contribute relatively more to the next generation than would groups that have a greater chance of extinction (because of a lower frequency of the trait)
D. The difference between this model of group selection and kin selection is that members of "trait groups" are not related