Genetic variation within a species includes genetic variation within natural populations and genetic differentiation between populations. The combination of gene flow, natural selection and genetic drift cause intraspecific genetic variation. The signi- ficance of dispersal and gene flow is expressed for example in the usually larger genetic differentiation among populations of terrestrial species than of marine species. The degree to which genetic differentiation between natural populations is determined by chance than by natural selection is controversial. It is exactly in this field that the research community is active. The central questions include : 1. The patterns of genetic biodiversity in a number of model organisms. 2. The mechanisms which cause these patterns.