The first step in the analysis of gene flow in
species' populations is the determination of population genetic structure by
statistical examination of the frequencies of the variants (hereafter called
alleles) of individual traits (genes) in each population. A usual statistical
approach to the description of population genetic structure is the use of
F-statistics [ Wright, 1969], which allow hierarchical analysis of the
distribution of genetic variation
within and between populations. The statistic, 
, reflects the
proportion of
the observed genetic variation that can be explained by partitioning between
populations (values range from 0 to 1.0). 
is calculated by:
where 
is the variance of the frequency of allele p, p is the
mean allele frequency across all samples, and n is the number of alleles
[ Wright, 1969]. Using this value, the number of
individuals exchanged between two populations per generation may be estimated
by:
where N is the number of individuals in a given population and m is the
proportion of those individuals resulting from immigration [ Wright,
1969]. As a benchmark, values of Nm > 1 (corresponding to 
) are typical of ``high gene flow'' species, which will become genetically
homogeneous in the absence of counteracting forces. Counteracting forces,
especially differential selection, may be very strong and may maintain
population differentiation in the presence of gene flow far in excess of
Nm = 1. Some attributes of marine zooplankton suggest that population
differentiation may be possible despite expected high gene flow with ocean
mixing. In particular, the very high mortality rates (
%) that are
characteristic of marine zooplankton may allow strong selection to maintain
genetic differences among populations.
A very simple and straightforward means of assessing population genetic structure is to statistically compare the frequencies of the different alleles of a gene in different populations. Whether genetic variation is measured by assaying allozymes or molecular variation of DNA, the critical parameter is the frequency of each allele (or genotype or haplotype). Statistical differences in the frequencies of each allele in samples collected across the domain is evaluated by a chi-square test by Monte Carlo simulation [ Roff and Bentzen, 1989]. Significant differences in allele frequencies between geographic populations provide evidence of population genetic structure.
There are also a number of new statistical approaches that are currently used in the analysis of gene flow. Among the new approaches, phylogeographic analysis uses the techniques of phylogenetic reconstruction of the evolutionary history of organismal groups to examine intraspecific patterns of genetic and geographic variation [ Avise, 1994]. This approach is based on the reconstruction of a tree showing the molecular relationship among the individuals assayed. The grouping of individuals from the same geographic region on the same branches of the tree provide evidence of population genetic structure. Statistical evaluation of tree topology is by a bootstrap test [ Kumar et al., 1993]. Other tree-based approaches seek to incorporate both differences in genotype frequency and genetic distances between the genotypes in the analysis of population structure [ Slatkin and Maddison, 1990].