Encyclopedia of Evolution.pdf - Online Reading Center

chromosomes (they are diploid) while males have unpaired
chromosomes (they are haploid). Females in these species
produce eggs by meiosis, which reduces the chromosome
number by half, while males do not need to do this (see Mendelian
genetics).
Social insects are famous for the tendency of worker
females to sacrifice themselves to protect the hive. Consider
a beehive in which all of the workers have the same mother
(the queen) and the same father (a lucky drone). Since drones
are haploid, the worker offspring receive not half but all of
his alleles. Since the workers receive only half of the queen’s
alleles, they are more closely related to their fathers than to
their mothers. Their coefficient of relatedness to one another
is (1 × ½) + (½ × ½) = ¾, in which the first term is their relatedness
through their father and the second is their relatedness
through the queen. Because siblings in most other species are
related to one another only by r = ½, a relatedness of r = ¾
indicates that sibling worker ants, bees, and wasps should be
much more altruistic toward one another than siblings usually
are. (A queen typically mates with several drones and
stores their sperm. Many of the worker daughters have different
fathers. Some workers have a coefficient of relatedness r =
½ while others have r = ¾. The average relatedness among
workers in most social insect colonies is therefore somewhere
between one-half and three-fourths, which is still high enough
to allow strong altruism to evolve.) Parents and offspring are
usually related to one another by r = ½. Worker insects are
more altruistic toward one another even than parents and
offspring. Anyone who has experienced an attack by a swarm
of bees or wasps can attest to the way the worker sisters sacrifice
themselves for their common welfare.
Since the worker insects are more closely related to one
another than any of them are to their queen, the workers are
in control of the nest. Even though the term queen implies
rulership, queens in social insect colonies are mere egg-laying
machines. It would be in the best interest of the queen to
produce equal numbers of male and female offspring, but the
workers will not allow this to happen: The workers kill most
of the drone larvae. It is also the workers that decide which
female larvae should receive the “royal jelly,” which, unlike
regular larval food, causes a female to develop into a queen.
Workers may destroy some queens if there are too many.
Haplodiploidy is not the only evolutionary precondition
for the altruism of social insects. All ants, bees, and wasps
have haplodiploidy, yet the only ones that have evolved sociality
are those that have also evolved a life cycle in which the
larvae are helpless grubs (see life history, evolution of)
and in which nesting behavior has evolved. Solitary bees have
haplodiploid genetics but do not sacrifice themselves for one
another.
Kin selection may also explain why animals, including
humans, tend to behave more altruistically toward their true
biological offspring than toward their stepchildren. In many
mammal species, such as lions, a newly arrived dominant
male will kill the offspring of the previous male, as shown in
the photo above. These juveniles, while perfectly good for the
prosperity of the species, have a zero percent genetic relatedness
to the new dominant male. If these offspring represent
altruism
This male lion has just killed a lion cub. When male lions take a new
mate, they often kill the female’s previous offspring, thus making the
resources of the female lion available for raising the new male’s
offspring. (Courtesy of George Schaller)
any cost at all to these males, Hamilton’s rule would predict
that there would be no altruism at all. The unrelated juveniles
do represent a cost, because while the females are feeding and
protecting them they cannot produce offspring for the new
dominant male.
Kin selection helps evolutionary scientists to understand
why humans are less solicitous toward stepchildren than
toward biological offspring. This behavior pattern is a nearly
universal feature among human societies: biologists Martin
Daly and Margo Wilson call it “the truth about Cinderella.”
Crime data from Canada show that, while men very seldom
kill children in their families, they are 70 times as likely to
kill stepchildren as biological children. Stepchildren also have
higher levels of blood cortisol (an indicator of stress) than do
biological children. This indicates that the fathers and stepchildren
both behave as though altruism is often missing
from the father-stepchild relationship. In blended families
with both biological and adopted offspring, fathers spend
more time with their biological offspring than with their stepchildren.
But is this due to kin selection, or simply due to the
fact that stepchildren are older before their stepfather first
becomes acquainted with them? Researchers have found that
fathers were less solicitous of stepchildren than of biological
children even if the stepchildren were born after the stepfather
and the mother had begun living together.
Reciprocal Altruism
Evolutionary biologist Robert Trivers pointed out another
way in which altruism could be favored by natural selection.
An animal might perform some costly act of help to another
animal if the recipient was likely to return the favor at some
time in the future. Because the recipient may reciprocate in
the future, this behavior is called reciprocal altruism. Reciprocal
altruism helps to explain altruism toward individuals to