Distribution and diversity of aquatic protists: an evolutionary and ecological perspective.

Assessment of the distribution and diversity of free-living protists is currently hampered by a limited taxonomic resolution of major phyla and by neglecting the significance of spatial and temporal scaling for speciation. There is a tremendous physiological and ecological diversity that is hidden at the morphological level and not apparent at the level of conserved genes. A conceptual framework linking the various levels of diversity is lacking. Neutral genetic markers are useful indicators of population structure and gene flow between populations, but do not explain adaptation to local habitat conditions. The correspondence between protein-coding genes, ecophysiological performance, and fitness needs to be explored under natural conditions. The area and the associated typical temporal dimension of active cells (their 'home range') are much smaller, respectively shorter, than the area and time period potentially covered during passive dispersal of protist resting stages. The assumptions that dispersal rates are generally high in free-living protists and that extinction of local populations is, therefore, infinitesimally small wait rigorous testing. Gene flow may be uncoupled largely from dispersal, because local adaptation and numerical effects of residents may strongly reduce or even prevent successful invasion (immigration). The significance of clonal selection depends on the as yet unknown frequency and timing of sexual reproduction, and on the stability of the environment. The extent of local adaptation and the fitness-related ecophysiological divergence are critical for the speciation process and, hence, for defining protist species. [ABSTRACT FROM AUTHOR]