This study will result in rigorous inferences of phylogeny for the marine diatom genus Skeletonema using molecular and morphometric methods in a cladistic framework. It is proposed that this genus can serve as a model system for exploring methods and controversies in the phylogeny reconstruction of primarily asexual organisms at lower phylogenetic levels. The following issues will be explored: 1. Levels of divergence among populations and species measured using chloroplast DNA (cpDNA), nuclear rDNA, and a single copy gene nucDNA gene; 2. Relationships of gene trees and species trees at lower phylogenetic levels; 3. Concordance of molecular and morphological data analyzed using phylogenetic systematic methods. It is hypothesized that the most primitive members of the genus are heavily silicified forms endemic to high-light, oligotrophic warm marine environments and that the most derived members are lightly silicified forms inhabiting turbid, eutrophic temperate freshwater and brackish water environments. The ecological importance and the wealth of background data available for Skeletonema make it an ideal system in which to test hypotheses of adaptation in an historical context.
