It is a well-known faet, that both the geographie distribution and the speeiation of plankton are produets of the geologieal history of the oeeans, the eontinental barriers, the eurrent patterns and the limitation of survival of individual speeies and populations by both biotie and abiotie environmental eonditions. The geographie distribution is also the produet of the mobility of populations, the seleetive pressure and the time during whieh a taxon has existed. A taxon eonsists of a population, or a group of populations, suffieiently distinet to be provided with a name, to be ranked in a definite eategory and to establish a geographie distribution. Eaeh taxon forms part of biological evolution, and is therefore never sharply delimited in time. Aneestors always influenee the dispersal oftheir deseendants. Environmental eonditons exert a natural seleetive influenee on eaeh population, whieh results in the survival of the most adequately adapted pheno- and genotypes at a partieular plaee and during a partieular period. Consequently, seleetion determines the presenee of a phenotype, or taxon, in an area at a eertain moment.
In this way, the environment determines the type of taxa, their abundanees, their seasonal ehanges and survival in time. In a broad sense, mobility of populations does not merely depend on migration and transport; reproduetion, mortality and population dynamies also influenee the mobility. In that sense, mobility eomprises all phenomena of movement, including the ehanges in number of speeimens present at a loeality.