where NTis the census size of potentially breeding adults in
the population. This equation also requires the assumption
that all social groups are equally successful in contributing
offspring to the next generation. For additional equations
under a variety of assumptions, see Chesser et al. (1993),
Sugg and Chesser (1994), and Nunney (1999).
Basset et al. (2001) argued that a distinction should
be made between predispersal estimates of gene dynamics,
which include individuals that have not yet dispersed from
their natal site to their first breeding area, and postdisper-
sal estimates of gene dynamics for adults that are capable
of breeding. In many species, offspring disperse away from
the natal site, like the predominant juvenile male disper-
sal common in many mammals (e.g., Greenwood 1980;
Dobson 1982). Such dispersal can generally be expected
to move individuals away from their philopatric kin. In ad-
dition, individuals that disperse to new homes may most
often encounter unrelated individuals. Thus, gene correla-
tions should generally be lower after, as compared to be-
fore, dispersal events, especially gene correlations measured
within the range of dispersal movements. Consequently,
F-statistics at the level of social breeding groups should also
be lower following dispersal. When females become breed-
ing adults in their natal group, and males disperse widely,
greater genetic correlations among females are expected
than among males and between males and females. This
difference in genetic patterns between adult males and fe-
males can influence estimated values of both F-statistics and
the resulting effective population sizes. Predispersal gene
dynamics can also be predicted, and should generally reflect
the genetic patterns of young individuals. Thus, it is criti-
cally important to distinguish pre- and postdispersal esti-
mates and the expected gene dynamics that they reflect.
Empirical Studies of Mammals
The breeding-group model is most appropriately applied
at the level of population structure where matings occur,
such as lineage or family groups within colonies. At larger
geographic scales, such as colonies within a geographic
region, classical F-statistics are appropriate. Evidence of
population-genetic structure within mammals has been
demonstrated at a variety of spatial scales (e.g., Storz 1999).
However, if social groups are not carefully identified, the
level of population substructure at which matings occur
may be missed (Sugg et al. 1996; Dobson 1998). When this
level is not sampled, Chesser’s (1991a, 1991b; Chesser et al.
1993) breeding model and Nunney’s (1999) arbitrary-group
model may lead to misleading inferences. Thus, in look-
ing for evidence of the influence of social groups on genetic
substructuring of populations, knowledge of the behavioral
ecology of a species, especially mating and dispersal pat-
terns, is very important.
Studies of a variety of mammals have quantified the de-
gree of substructuring of populations attributable to social
groups. In a seminal and prescient paper on social groups
in yellow-bellied marmots (Marmota flaviventris), Schwartz
and Armitage (1980) found significant genetic differentia-
tion among small colonies (table 14.1). These social ro-
dents live in kin-based groups, exhibit a polygynous mating
system, and males disperse while females are philopatric
(reviewed by Armitage 1999b). Thus, the marmots are par-
ticularly appropriate for asking whether social groups cre-
ate genetic substructure within local populations. Allozyme
data from primarily postdispersal adults and subadults of
both sexes were used to estimate F-statistics. FLSwas signifi-
cant, indicating genetic differentiation among social groups,
maintained by a polygynous mating system and female-
biased philopatry.FSTvalues were not calculated, since so-
cial groups were studied within a single local subpopulation.
A study of gene dynamics of pocket gophers (Thomomys
bottae) identified genetic structure at the level of local po-
lygynous groups (Patton and Feder 1981). Allozyme data
were collected from the population of breeding adults of
both sexes, and used to calculate postdispersal F-statistics.
This species likely exhibits male-biased dispersal from the
natal site. Significant genetic differentiation was found
among the polygynous groups, as well as among popula-
tions within the region, among regions within broader geo-
graphic ranges, and among geographic ranges with the spe-
cies range (Patton and Smith 1990). FLSwas lower than FST,
but it still indicated significant social genetic structure of
pocket gophers within a local population. F-statistics at the
broader levels of genetic hierarchy likely indicated the influ-
ence of geographic variation due to the limited dispersal
abilities of these fossorial rodents (Daly and Patton 1990;
Steinberg and Patton 2000). Patton’s studies are notable for
examining several levels of population substructure, partic-
ularly at geographic scales.
VanStaaden et al. (1994) examined the gene dynamics of
the moderately social Richardson’s ground squirrel (Sper-
mophilus richardsonii). This species is promiscuous, with
multiple mating by both males and females (Michener and
McLean 1996). As in more highly polygynous species, males
typically exhibit greater reproductive success than females,
due to a strong female bias in the adult sex ratio. Thus, ge-
netic correlations among offspring may be high, on aver-
age, despite possible multiple paternity and dispersal by
both sexes (though females are often philopatric; Michener
and Michener 1977; Davis 1984c). Although matrilines of
closely related and spatially contiguous adult females occur,168 Chapter Fourteen