These examples illustrate that even in the most familiar of marine organisms---the large and conspicuous whales or the ubiquitous coastal seagrasses---current understanding both of the extent of intraspecific genetic diversity and of how such diversity is created and maintained is just cresting the tip of the genetic iceberg. Assumptions about the homogeneous nature of a species' gene pool, which are based largely upon the presumed mobility of the organisms or upon the presumed role of a reproductive mechanism, have sometimes proven wrong. Clearly, the marine world is more diverse genetically than was previously suspected.
The knowledge that species may contain a remarkably high degree of undetected genetic diversity provides the basis for asking critical questions about the importance and role of genetic diversity in maintaining individual species populations and in sustaining the species as a whole. New techniques in molecular genetics, including the ability to examine the structure of DNA in a species by expanding tiny fragments of the genome by methods such as the polymerase chain reaction, now permit relatively rapid insight into the genetic structure of diverse populations [ DeLong et al., 1989; Giovannoni and Cary, 1993]. Studies of the preserved tissues of species in museum specimens of extinct populations---tissues from which DNA can now be extracted---provide valuable opportunities for determining the potential loss of diversity in a given species, and how this may affect the survivability of the species [ Carlton, 1993; Geller et al., 1994]. In addition, oligonucleotide (DNA or RNA, ribonucleic acid) and antibody probes to mark species or genotypes now permit molecular tracking of planktonic organisms for population-genetic studies within an oceanographic setting [ Powers et al., 1990; Ward, 1990; Bucklin et al., 1989]. Important research directions thus include understanding the role of gene flow between populations of a species (and how such gene flow determines the ability of the species to reproduce, grow, adapt, and spread) and how the natural or human-caused interruption of gene flow creates, maintains, and influences populations and subpopulations of a species. Such research will provide the basis for management strategies for exploited, threatened, and endangered species.