Distinguishing among these hypotheses is problematic
in that only the hypothesis of ‘gene flow’ can be
falsified rigorously with genetic data and deployment of
allele (or genotype) distribution-based statistical tests.
This is especially true if gene flow varies in intensity and
duration and is demographically insignificant in the
short term but sufficient in the long term to preclude
genetic divergence. We sought to evaluate these
hypotheses, especially the latter, by using a nested phylogeographic
(clade) analysis (Templeton 1998) of
mitochondrial (mt)DNA sequences. The nested clade
approach utilizes evolutionary genealogical information
to evaluate the spatial and temporal distribution of genetic
variation and often can detect cryptic geographical
associations or patterns that are not readily identifiable
using traditional population genetic measures. Nested
clade analysis can also discriminate between the effects
of restricted but recurrent gene flow and historical events
such as range expansion and/or population fragmentation
(Templeton et al. 1995; Templeton 1998). In addition,
we employed both mismatch-distribution analysis
and a maximum-likelihood coalescence analysis of
mtDNA sequences to further evaluate historical population
demography of red snapper in the northern Gulf.