Evolution__3rd_Edition

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We define Hamilton’s rule
Hamilton’s rule has been tested in
the Florida scrub jay
Under what condition will natural selection favor altruism? The altruist still pays a
cost of c for performing the act, and the recipient receives a benefit b. However, the
chance that the altruistic gene is in the recipient is r. When rb exceeds c there will be a
net increase in the average fitness of the altruists. The number of copies of the gene for
altruism will increase because the loss of copies from the excess death of the individuals
who actually perform acts of altruism is more than made up for by the excess survival of
the individuals who receive it (and contain the gene for altruism). The condition for
natural selection to favor altruism among relatives is that it should be performed if:
rb > c
CHAPTER 11 / The Units of Selection 299
This is the theory of kin selection. It states that an individual is selected to behave
altruistically provided that rb > c. The condition itself is called Hamilton’s rule, after
W.D. Hamilton, who mainly invented the theory of kin selection.
Hamilton’s rule is testable. For an act of altruism we can measure the benefit and the
cost, and r can be deduced if the pedigree relationship between the altruists and recipients
is known. The details of how b and c are estimated depends on the example. Here
we shall look at one example: “helpers at the nest” in the Florida scrub jay (Aphelocoma
coerulescens, see Plate 6, between pp. 68 and 69).
The scrub jay is distributed widely across the western USA, and also has an isolated
population that breeds in the shrinking areas of oak scrub in central Florida. It has been
continuously studied there since 1969 by Woolfenden and his colleagues. A breeding
pair of Florida scrub jays may be helped by up to six other birds. Woolfenden knows the
pedigrees of the birds and therefore has been able to show that the majority of these
helpers are either full or half sibs of the young they are helping. r is therefore known,
but how can we estimate b and c?
“Benefit” and “cost” properly refer to the change in lifetime reproductive successes of
the altruist and recipient, relative to if the act had not been performed. The true values
of b and c are therefore unmeasureable, because they refer to a situation that does not
exist (namely, if the act had not been performed). They can, however, be estimated.
Mumme (1992) experimentally removed the helpers from 14 nests in 1987 and 1988,
and measured the reproductive success both in these experimental nests and in 21
untreated control nests in the same area. There were an average of almost 1.8 helpers
at the experimental nest, which we can round up to two helpers per nest in some approximate
calculations below; there were a similar number of helpers at the control nests.
The removal of the helpers significantly reduced the survival of the offspring
(Table 11.2). Mumme found that the contribution of helpers is mainly in defending the
nest against nest predators such as snakes and other birds. A nest with a helper is more
likely to have a sentinel bird present at the nest at any time than is a nest without
helpers, and nests with helpers can “mob” predators more effectively. (“Mobbing” is a
kind of group defense of birds against predators, in which the birds dive at and harrass
the predator. It is most commonly seen in birds such as crows mobbing domestic
cats.) The young in nests with helpers are also fed more and (probably in consequence)
survive better after fledging.
Mumme’s result enables us to estimate the benefit (b) of helping as the difference
between the survival rate of young in nests with and without helpers. In Table 11.2, the