CLUSTERS, SPECIES CONCEPTS, AND MORPHOLOGICAL EVOLUTION OF DIATOMS

Empirical comparison of species concepts can be made by taking advantage of a common operational exigency: the use of phenotypically irreducible clusters of individuals as first order estimates of species. A phylogenetic analysis of diatoms was undertaken using such clusters as terminal taxa. Cladistic analysis and biogeographic distributions suggest that different populations of the widespread, plesiomorphic Stephanodiscus niagarae (at least 2 million years old) are ancestral to three autapomorphic taxa that emerged during the Holocene in North America. This phylogenetic hypothesis is supported in one case by a time-transgressive morphological continuity between fossils of the plesiomorphic species (S. niagarae) and of one of the derived endemics (S. yellowstonensis) found only within the Yellowstone Lake basin. Given the phylogenetic hypothesis, the clusters diagnosed by autapomorphy each evolved as a unit, but the cluster identified solely by a unique combination of plesiomorphic characters did not (S. niagarae). Each of the autapomorphic species is found in close proximity to lakes containing S. niagarae, whereas lakes containing S. niagarae populations are widely dispersed. Thus, it would be difficult to simultaneously explain evolution of autapomorphic endemics by breeding isolation and maintenance of the plesiomorphic similarity of S. niagarae populations by interbreeding. Autapomorphic taxa were found on the ecological periphery of the species complex but S. niagarae occurs both in the ecological center and periphery of the entire complex; therefore, the plesiomorphic cluster is not an ecological unit. The results caution against the acceptance of smallest phenetically recognizable clusters of organisms as either units or products of evolution. Cladistic analysis and the autapomorphic species concept should be a routine part of studies of speciation. Stratigraphy and parsimony analysis agreed on the ancestral morphology of S. yellowstonensis.