A period of rapid diversification occurred among plants during the Silurian-Devonian interval, following the evolution of tracheophytes. Phylogenies involving plants from this period are based on anatomical, morphological, and reproductive features gleaned from fossilized remains. There has been little analysis, however, of the relative contributions of these three data types to producing well-resolved evolutionary trees of ancient species. While reproductive characteristics are traditionally held to be more evolutionarily conservative, it is not known if this is the case for early vascular plants.
In a recent paper published in the American Journal of Botany, authors Karl Niklas and William Crepet set out to evaluate the ability of these three data types, individually and in pairs, to reproduce a well-resolved phylogeny created using all three together. The researchers used 37 well-preserved Paleozoic fossil plants scored with a set of 54 characters comprising 18 reproductive, 14 anatomical, and 22 morphological characters. They then compared the topology of the 54-character tree to topologies produced using only reproductive, anatomical, or morphological characters, as well as the three possible pair-combinations.
The authors found that using either reproductive or anatomical characters alone produced an almost completely unresolved tree, while morphological characters produced a partial replication of the complete tree. Of the pairwise comparisons, the best resolution was obtained by anatomical and morphological characters together. The authors were able to determine that the poor resolution produced by the reproductive characters was not due to the smaller number of this type of characters included in the analysis.
These results indicate that reproductive and anatomical changes played a less significant role than morphological ones during this period of tracheophyte evolution. “Well before the seed plant habit made its first appearance by the end of the Devonian, our analyses show that very few character states distinguish among the reproductive or anatomical characteristics of the vascular land plants,” write the authors.
“Traditionally, reproductive features are thought to be evolutionarily conservative compared to vegetative features — if you play with reproduction, you either come up with something entirely new, or you die. But the extent to which the fossil plants in our analysis were extremely reproductively conservative was surprising,” says Niklas.
The authors describe this result as natural selection ‘seeing’ the phenotype, meaning, as Niklas puts it “that natural selection acts on an organism’s features and not on how these features are achieved. The morphology of a plant is the physical manifestation of its development, and there can be many developmental roads to getting to the same morphology, but with different anatomical configurations.”