178 
Fishery Bulletin 95( I ), 1997 
a lengthy pelagic larval stage, there is no reason to 
assume a priori that gene flow occurs via dispersal 
of red grouper larvae. Nonetheless, it cannot be ruled 
out as a contributing factor. 
Historical bottleneck 
In a general sense, genetic homogeneity and absence 
of phylogenetic structure are compatible with lim- 
ited gene flow under models where isolated popula- 
tions (or subpopulations) have recently diverged from 
a panmictic population that possessed low levels of 
genetic variation and where each subpopulation has 
a small effective population size. An example of iso- 
lated subpopulations that are genetically homoge- 
neous and also genetically depauperate are African 
cheetahs, where subspecies in east and south Africa 
are essentially monomorphic for the same alleles at 
numerous genetic loci (O’Brien et al., 1987). To ac- 
count for both genetic homogeneity between, and low 
genetic variability within, subpopulations, O’Brien 
et al. (1987) hypothesized the past occurrence of at 
least two genetic bottlenecks. Their hypothesis was 
based on the premise that genetic homogeneity of 
isolated subpopulations was consistent with a histori- 
cal event; whereas low genetic variability in extant 
populations was consistent with a more recent event. 
Red grouper fit the cheetah model in that the sub- 
populations surveyed are genetically homogeneous 
and each possesses limited genetic variation. We 
suggest the possibility that red grouper from west 
Florida and Mexico are isolated genetically, but that 
recurring genetic bottlenecks continue to generate 
high frequencies of the most common genotype. In 
addition, we suggest that red grouper from these two 
regions were not isolated historically and that the 
historical population underwent a severe bottleneck 
that reduced much of the extant genetic variation. 
These suggestions account for the observed genetic 
data and how isolated populations can be genetically 
homogeneous. A historical bottleneck could have oc- 
curred during late Pleistocene times when environ- 
mental fluctuations impacted the biota of the region 
(Rezak et al., 1985; Graham and Mead, 1987). Our 
suggestions could be tested, in part, by asking 
whether rare haplotypes found in both locals are 
identical by descent (i.e. independently derived). In 
red grouper, two of the three haplotypes shared be- 
tween west Florida and Campeche Banks are the 
result of a site loss from the common haplotype and 
could be the result of a nucleotide substitution at 
any one of six nucleotide positions. Examination of a 
more rapidly evolving nuclear marker in individu- 
als from each locality that share these rare 
haplotypes would address this issue. 
Acknowledgments 
We thank C. Furman and K. Burns for help in pro- 
curing specimens of red grouper from Mexico. Work 
was supported by the Marfin Program of the U.S. 
Department of Commerce Award NA90AA-H-MF755, 
administered by the National Marine Fisheries Ser- 
vice and by the Texas Agricultural Experiment Sta- 
tion under Project H-6703. Part of the work was car- 
ried out in the Center for Biosystematics and 
Biodiversity, a facility funded, in part, by the Na- 
tional Science Foundation under grant DIR-8907006. 
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