Units of Selection
We think that the only way that
adaptations can arise is through natural
selection. The effects of such
adaptation can be seen at many levels:
gene frequencies, frequencies of types
of organisms, even populations.
But…. where is the power of natural
selection focused. On what units does
selection work?
Why is this an important question?
At what level does the benefit lie?
Segregation distortion is an example of
an adaptation at the gene level that may
actually be disadvantageous to the
organism that carries it.
Sometimes, certain cell lines in a body are
favored at the expense of the rest of the
body.
There are many examples of adaptations that
benefit organisms.
Is it possible for natural selection to
operate at the level of the group?
Altruism – a behavior pattern that
increases the number of offspring
produced by recipient while decreasing
that of the donor.
If such behaviors have a genetic basis,
then the elimination of altruists should
cause the frequencies of altruistic genes to
decrease. We would not expect alarm calls
to be an evolutionarily stable strategy.
So, how do these behaviors persist?
If altruists “hand out” altruism
indiscriminately, then selfish individuals will
be favored because they will receive the
benefits without any cost.
Altruism must first be dispensed
preferentially to other altruists. This is the
only way for the behavior to evolve, and
only then if the benefit to the altruist
outweighs the cost.
W.D. Hamilton suggested that we expand
our ideas about fitness and selection. He
defined inclusive fitness as the relative ability
of an organism to get its genes, or copies of
its genes, into the next generation.
Hamilton suggested that it is irrelevant to the
evolutionary process whether an individual
passes on its own genes or enhances the
transmission of copies of them in another
individual.
So, who carries copies of an individual’s
genes? Obviously, relatives are the most
likely. Altruistic behavior often takes place
between genetic relatives, where it is
explained by the concept of kin selection.
Actually, we can calculate a coefficient of
relationship, r, which is the average
proportion of genes shared by two
individuals.
Coefficients of relationship
RELATIONSHIP
Mother/Child
Father/Child
Full siblings
Half siblings
Grandparent/Grandchild
Aunt or uncle/nephew or
niece
First cousins
r
0.50
0.50
0.50
0.25
0.25
0.25
0.10
An altruistic allele can increase in
frequency if it increases the inclusive
fitness of the individual.
According to Hamilton, we can quantify the
probability that an allele will increase in
frequency. It can increase if:
k1
r
Where k is b/c, the ratio of the benefit
to the recipient to the cost to the
altruist.
A test – Aphelocoma coerulescens
Benefit of helping is 35% - 7% or 28%.
Since there were approximately 2
helpers at each nest, divide by 2 to get:
b = 14%
Cost is more difficult. Mumme concluded
that the cost could be zero (assuming
that the helper jays would not have bred
anyway), or could be as high as the
reproductive success of jays without
helpers, in which case:
c = 7%
In this case, the tradeoff is between
helping in the production of siblings, or
producing their own offspring. So we
must evaluate whether:
rsibc > roffc
In this case, regardless which estimate of
cost we use:
(0.5)(14) > (0.5)(0)
or
(0.5)(14) > (0.5)(7)
So, selection favors helping.
Genetically based Cooperation
Honeybees are haplodiploid. Females are
diploid and males are haploid. Unfertilized
eggs develop into males, while fertilized
eggs develop into females. This skews the
typical coefficients of relationship. Sisters
are more closely related to each other than
parents and offspring are.
According to the theory of kin selection, a
sister is more valuable than a daughter.
This explains the ultimate altruism –
forgoing one’s own reproduction to care for
one’s sisters.
Ecologically based cooperation
The social unit of acorn woodpeckers is a
territorial group of up to a dozen members
that gather and store acorns in holes drilled
in a tree (granary). Granary trees are
developed, maintained, and defended by all
members of the group.
The system has three unusual features:
1. Division of labor. Breeders and helpers.
2. Mate sharing. Only one active nest at
any one time. Among breeders, any
female may breed with any male.
3. Infanticide. Females kills the first eggs
laid by other females. When birds enter
a group to fill a vacancy, they kill all the
young.
How do we explain this?
The territory is saturated, with little opportunity
for young to disperse. Groups resist
immigration. Best fitness option for young is to
remain with the group and contribute to its
reproductive success.
Mate sharing brings advantages to the
individual breeder. Woodpeckers that share
mates raise more young than do monogamous
pairs.
Infanticide provides a genetic advantage.
Infanticidal females stop when they can no
longer be sure that the infant she is killing is not
her own.
Beyond the idea of kin selection, which
is not truly altruistic, group selection is
thought to be a weak force at best in
evolution.