CONTRASTING MORPHOLOGICAL RATES OF SEED EVOLUTION

Although evolutionary radiations are typically recognized on the basis of taxonomic diversification, competitive success at local scales usually results from some ecological novelty affecting an organism’s interaction with habitat and community. Quantifying rates of morphological evolution is key to understanding the interplay between environmental change and ecophenotypic variation on geologic time scales. Seed size is an intriguing system because of the trait’s correlates with biotic (e.g., dispersal vector, growth form) and abiotic (e.g., latitude, climate) factors. Devonian seed plants had small (ca. 70 mm³) seeds but maximum extant sizes were reached by the Pennsylvanian (ca. 800,000 mm³). Parametric and non-parametric tests of central tendency indicate that the Paleozoic seed size distribution is significantly different from both modern angiosperms and modern gymnosperms. However, tests for homogeneity of variance reveal that, although the Paleozoic cohort is significantly different from modern gymnosperms, it is not distinguishable from modern angiosperms (p = 0.819). The expansion of the modern seed size envelope resulted from multiple Cretaceous angiosperm lineages independent evolving small seeds, a trait correlated with dormancy, habitat tolerances, and dispersal. Rates of seed size evolution (i.e., change in variance among taxa over millions of years) were estimated from (1) stratigraphic occurrences of Paleozoic seed plants and (2) fossil-calibrated molecular phylogenies of extant taxa. Paleontological results indicate that variance expanded irregularly through the Paleozoic with positive Mississippian rates but both positive and negative Pennsylvanian rates. Phylogeny-based estimates suggest that modern angiosperms have higher morphological rates than gymnosperm lineages. Because rates based on molecular phylogenies measured only positive net change, they may be comparable to each other but do not capture a significant portion of the true temporal variation documented by the paleontological analyses. Results suggest that rates of morphological variance in seed size (a trait with clear ecological significance) were higher during the early radiation of Paleozoic seed plants and possibly the early radiation of Cretaceous angiosperms.