PLANT PALEOECOLOGY AND EVOLUTIONARY INFERENCE - 2 EXAMPLES FROM THE PALEOZOIC

Paleobotany can contribute much to evolutionary scenario-building, Her e, we use two case studies - the Devono-Carboniferous vascular plant r adiation and the largely coeval evolution of heterosporous from homosp orous life histories - to examine the interface between phylogeny and ecology. Our observations challenge some tenets of the neo-Darwinian o rthodoxy, notably the assumed role of competition mediated selection a s an active driving force, rather than a passive filter, of evolution. The Devono-Carboniferous class-level radiation of vascular plants was prompted by attainment of a complexity threshold and delimited the mo rphological envelope that enclosed an apparently fractal pattern of su bsequent, lower level radiations. The contrast of low speciation rates with exceptionally high rates of phenotypic divergence in the Devonia n suggests a non-adaptive ''novelty'' radiation, perhaps reflecting sa ltational evolution via ''hopeful monsters''. Successive lower level r adiations were more constrained by the ecological hierarchy that resul ted from progressive niche differentiation and saturation. This in tur n reflected increased speciation rates, thereby completing a well defi ned negative feedback loop in the coevolution of phenotypic and ecolog ical differentiation. Heterosporous life histories evolved independent ly in at least ten lineages. Heterospory allows the sporophyte to impo se, via differential development, a single fixed gender on each gameto phyte prior to spore release. Although the resulting life history is l ess flexible than homospory, which on recent evidence includes a range of subtle and sophisticated strategies, it promotes the sporophyte as the primary target for selection. Gametophytes effectively perform th e role of gametes and are released into the environment prior to ferti lization, thus favoring aquatic-amphibious habitats resistant to occup ation by homosporous pteridophytes; terrestrial heterospory requires a pomixis. Although the profound iteration of heterospory implies a stro ng adaptive advantage, repeated gradual evolution via inferior interme diates exhibiting exosporous heterospory seems unlikely. Seed-plant su ccess reflects economic efficiency and the subsequent evolution of eff ective pollination syndromes, rather than integumentation of the ovule . Major radiations of heterosporous lineages and subsequently of seed- plants required perturbation of pre-existing communities by extrinsic environmental changes rather than genuinely competitive displacement. This typical manifestation of ''home-field advantage'' further emphasi zes the intimate relationship between phylogeny and ecology, and allow s us to make predictions that can be tested by further paleobotanical research.