Biology Reference
In-Depth Information
Table 11.1
Coefficients of relatedness (
r
) for descendant
and non-descendant kin, calculated as the probability
that a gene in one individual is an identical copy, by
descent, of a gene in another individual (assuming
outbreeding).
r
Descendant kin
Non-descendant kin
0.5
Offspring
Full siblings
0.25
Grandchildren
Half-siblings
Nephews and nieces
0.125
Great-grandchildren
Cousins
BOX 11.3
MEASURING RELATEDNESS WITH MOLECULAR MARKERS
The simplest way to estimate relatedness between individuals is if pedigree
relationships are known. In this case, relatedness values can be assigned such as
r
0.25 for a pair of half-siblings and so
on. However, detailed pedigrees would be impossible to assign for many natural
populations. The reasons for this are that parents, especially fathers, often
cannot be assigned with complete confidence, and that we usually lack detailed
breeding histories of populations over past generations. One way around this
problem is to measure relatedness by the extent to which individuals share
common alleles in molecular markers such as microsatellites (microsatellites are
explained in Box 9.1). Queller and Goodnight (1989) provided a method of
estimating relatedness with genetic data, which can be implemented with an
easy to use computer program. This method measures the genetic similarity
between two individuals, relative to that between random individuals in the
population as a whole (Grafen, 1985). We shall return to this statistical
definition of relatedness in Box 11.5.
Direct molecular methods for measuring relatedness offer a number of
advantages. Firstly, they allow a detailed picture to be built up of the relatedness
structure within social groups. For example, in meerkats it has been shown that
natal subordinates tend to be half-siblings of the juveniles that they help raise,
but that immigrant male subordinates can also be related to the juveniles that
they help raise, because they can be related to the dominant male or father the
offspring of subordinate females (Fig. B11.3a; Griffin
et al
., 2003). Secondly,
they have overturned previous assumptions about how individuals within
groups are related. For example, in the social wasp,
Polistes dominulus
, it was
shown that about 35% of helpers at the nest are unrelated to the dominant
queen, so are not helping a close relative reproduce, as is usually assumed in the
social insects (Fig. B11.3b; Queller
et al
., 2000). Thirdly, they allow relatedness
to be assessed in cases where it would not even be possible with pedigrees. For
example, it has been shown that when individuals of the slime mould
Dictyostellium discoideum
come together to form fruiting bodies in the wild, the
mean relatedness is extremely high,
r
=
0.5 between a pair of full siblings,
r
=
0.98, suggesting that most fruiting
bodies are composed of clone-mates (Fig. B11.3c; Gilbert
et al
., 2007).
=
