The disciplinary roots for the analysis of dispersal across a species' distributional range lie in a central dogma of population ecology:
where N is the number of individuals at a given time (t) and after some interval (t+1), B is the number of births, D is the number of deaths, and I and E are the numbers of individuals immigrating and emigrating, respectively. Dispersal is quantified by assessing the immigration and emigration of individuals among conspecific populations.
Although dispersal is relatively easy to model, it is very difficult to measure. One much-used approach is to infer dispersal from the population genetic structure of a species. The reasoning is that dispersal is the exchange of individuals among conspecific populations, which acts to genetically homogenize the population (assuming the transported individuals eventually reproduce in the destination population, which is the proper definition of dispersal). Gene flow acts to decrease population genetic structure (the genetic differentiation of conspecific populations), making it possible to infer one from the other. Population genetic structure is determined by several processes in addition to gene flow. In particular, differential selection (heterogeneity of selective forces), may maintain population structure in the presence of high gene flow. To a lesser extent, especially for zooplankton, drift (random loss of genes due to the magnitude and variation of population size) and mutational changes will affect population structure. In turn, the population genetic structure of a species will determine: responses to environmental change, likelihood of raciation or speciation events, probability of extinction, and rates of evolution.
The dispersal of marine zooplankton can be inferred from the population genetic structure of a species by statistical analyses developed by population genetic theory (see Wright [1969] for a comprehensive treatment). To do this, the frequencies of individual traits are determined for geographic populations of a species, and analyzed by the statistical approaches of population genetics. The use of genetic characters to examine population structure and patterns of gene flow has been an active area of research for many years (see reviews by Avise [1974] and Wilson et al. [1985]). Genetic approaches may help to resolve long-standing questions about the dispersal of zooplankton in the ocean. Genetic characters have the advantage that they are unambiguous identifiers of an individual or lineage, but a population genetic approach will yield statistical rather than deterministic conclusions about the dynamics of zooplankton populations in the ocean. Population genetics, despite the term, is a study of individual characteristics and cannot be done using groups (or pooled individuals). It is unlikely that it will be possible to predict an individual zooplankton's destination, but it may be possible to determine the proportion of immigrant individuals in a given region on an oceanographically relevant time scale.