Download SEXUAL SELECTION

1
EEB 210
Spring 2008
Lecture #4: Kin selection
Kin selection: In the mid- to late-1900s, W.D. Hamilton and others developed theories
of kin selection. These evolutionary biologists recognized that: (a) the results of natural
selection are best understood by focusing on individual genes (alleles) as the units that
are being selected, and (b) the ‘total fitness’ of an individual organism was represented
not only by its success in producing viable offspring, but also included the organism’s
ability to support the reproductive efforts of closely related kin that carried a significant
proportion of the same genes. This concept of ‘total fitness’ was given a special name:
inclusive fitness.
Hamilton initially applied kin selection theory to eusocial insects. Eusociality is a
social system in which each social group contains only a relatively few reproductive
individuals (sometimes just a single reproductive female and one or more reproductive
males). Most of the members of the group do not reproduce, but help to maintain
conditions under which breeders that are genetically closely related to them can be
successful. The Hymenoptera is an order of insects that includes wasps, bees and ants; a
large proportion of these species are eusocial.
Hamilton noted that the Hymenoptera all exhibit an unusual genetic structure
known as haplodiploidy. In haplodiploidy, females are diploid but males are haploid
(males develop from unfertilized eggs, whereas females develop from fertilized eggs). In
bees, the males (drones) do not forage for food or help the colony in other ways, but are
the source of potential mates for the reproductive females (queens).
a. in haplodiploidy system, females are genetically 50% similar to their
daughters (as with the normal diploid system), since half of the genes come
from each parent
b. in haplodiploidy, if the queen has mated with only one male, females are
genetically 75% similar to their sisters---because all share 100% of the genes
received from the father and 50% of the genes received from the mother
(queen)
It is important to understand that a queen bee is not a queen because she is
genetically different from workers; rather, a female bee may become either a worker or a
queen depending on what type of food she receives during larval development.
Hamilton and others developed an explanation for the evolution of eusociality that
is consistent with a Darwinian theory. They suggest that sterile individuals can still
achieve a degree of reproductive fitness if they devote resources to helping the
reproductive efforts of closely related kin that carry many of the same genes.
2
Despite the initial appeal of haplodiploidy as an explanation for evolution of
Eusociality in Hymenoptera, it now appears that this may not be the direct explanation;
rather, it may be more directly related to the fact that a number of primitively social
members of the Hymenopterans have young that stay in the nest for extended periods of
time. Furthermore, not all eusocial organisms are haplodiploid. Termites and naked
mole-rats are eusocial, but they have a typical genetic system in which all individuals are
diploid. And in all cases of eusociality, members of each social group are closely related
to one another---even if not so closely related as in the case of the haplodiploid
Hymenoptera.
Kin recognition: All present day human groups (culture) possess elaborate means to
identify relatives (such as using surnames and maintaining detailed genealogies). In
other organisms, mechanisms for distinguishing kin from non-kin and treating these
categories differently) are common, even among relatively simple organisms, including
plants. Current interest in kin recognition traces back to W.D. Hamilton’s (1964)
development of kin selection theory. Hamilton’s ideas contributed much toward shifting
the emphasis from individuals to genes as targets of natural selection; he coined the term
inclusive fitness (as a sort of variant of reproductive fitness).
Mating with unrelated individuals can have both beneficial and deleterious effects:
a. reduces probability of formation of homozygous recessive deleterious
mutations
b. increases likelihood of breaking up gene combinations that produce favorable
traits
c. optimal outbreeding theory---developed in 1970’s by Bateson and Shields--attempts to analyze these factors and to explain why many organisms prefer to
mate with individuals to whom they are neither too closely nor too distantly
related
How organisms recognize their relatives:
a. by phenotype (for example, by odors or by visual cues)
b. by clues related to time or place (many rodents may recognize individuals
with whom they lived during early life---these would normally be littermates--and avoid mating with them)
Sometimes a combination of techniques is used:
a. bank swallows nest in colonies in burrows on sandbanks
b. pair of parents will feed any nestling that is placed in their burrow
c. flightless young remain in their natal burrow for 3 weeks after hatching, so
during this time parents feed only their own young
3
d. after chicks learn to fly, brood mix extensively, but parents still feed only their
own young; by 20 days of age, chicks are recognized by their parents by their
distinct vocal signatures
4