Evolution__3rd_Edition

298 PART 3 / Adaptation and Natural Selection
Some organisms show altruistic
behavior
Altruistic behavior between
relatives is explained by kin
selection
11.2.4 Natural selection working on groups of close genetic relatives is
called kin selection
In species in which individuals sometimes meet one another, such as in social groups,
individuals may be able to influence each other’s reproduction. Biologists call a behavior
pattern altruistic if it increases the number of offspring produced by the recipient
and decreases that of the altruist. (Notice that the term in biology, unlike in human
action, implies nothing about the altruist’s intentions: it is a motive-free account of
reproductive consequences.) Can natural selection ever favor altruistic actions that
decrease the reproduction of the actor? If we take a strictly organismic view of natural
selection, it would seem to be impossible. And yet, in a growing list of natural observations,
animals behave in an apparently altruistic manner. The altruism of the sterile
“workers,” in such insects as ants and bees, is one undoubted example; here the
altruism is extreme, as the workers do not reproduce at all in some species.
Altruistic behavior often takes place between genetic relatives, and when it does the
most likely explanation is the theory of kin selection. Let us suppose for simplicity that
there are two types of organism, altruistic and selfish. A hypothetical example might be
that, when someone is drowning, an altruist would jump in and try and save him or her
whereas the selfish individual would not. The altruistic act decreases the altruist’s
chance of survival by some amount, which we can call c (for cost), as the altruist runs
some risk of drowning too. It increases the chance of survival of the recipient by an
amount b (for benefit). If the altruists dispensed their aid indiscriminately to other
individuals it would be received by other altruists and selfish individuals in the same
proportion as they exist in the population. Natural selection would then favor the
selfish types, because they receive the benefits but do not pay the costs.
For altruism to evolve, it has to be directed preferentially to other altruists. Suppose,
to begin with, that acts of altruism were only ever given to other altruists; what would
be the condition for natural selection to favor altruism? The answer is that the altruism
must take place only in circumstances in which the benefit to the recipient exceeds the
cost to the altruist. This will be true if the altruist is a better swimmer than the recipient,
but it does not logically have to be true (if, for instance, the “altruist” were a poor swimmer
and the “recipients” were capable of looking after themselves, the net result of the
altruist’s heroic plunge into the water might merely be that the altruist would drown).
If the recipient’s benefit does exceed the altruist’s cost then the average fitness of the
altruistic types as a whole will increase. This condition is only of theoretical interest. In
practice altruism usually (maybe always) cannot be directed only to other altruists,
because they cannot be recognized with certainty. However, altruism can be directed
only at a class of individuals that contains a disproportionate number of altruists relative
to their frequency in the population. This is true when altruism is directed toward
genetic relatives: if a gene for altruism is in an individual, it is also likely to be in its
relatives. Define r (for relatedness) as the probability that a new rare gene that is in
one individual is also in another individual. The probability is between zero and one,
depending on the other individual concerned. The appropriate r can be deduced from
Mendel’s rules. If the new mutation is in a parent, there is a half chance it will be in its
offspring; and there is likewise a half chance that a gene in an individual is also in its
brother or sister.
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