CLADISTICS, WOOD ANATOMY AND ANGIOSPERM PHYLOGENY - A CHALLENGE

Abstract

Conclusions reached by Young (1981) concerning the evolutionary status of homoxyly in woody Angiosperms are biassed because the results of his analyses are based upon the silent acceptance of certain postulates and on assumptions emanating from such current but most probably untenable tenets. The phylogenetic history of the Angiosperms, if unravelled by alternative viewpoints and by, e.g., correlation studies yielding an “advancement index” (Sporne 1982), and as substantiated by recent paleobotanical findings, most probably proceeded pleiorheithrically rather than monophyletically. The construction of cladograms starting from the postulation of a monophyletic ascent of the Magnoliophyta does not necessarily reflect the true evolutionary history of these Flowering Plants. In Young's cladistic analysis another difference between his adopted conventional viewpoints and alternative, dissentient ideas is that some or all of the supposedly apomorphic (i.e. newly acquired) character states need not have originated only once (as accepted and emphasised in his paper), but may well have arisen repeatedly and independently in two or in more parallel evolutionary lineages evolved from technically still gymnospermous ancestral forms and each terminating in some present-day angiospermous assembly. The principal conclusion attained by Young, viz., that the incidence of homoxyly in arborescent Angiosperms is attributable to a secondary “loss” of the wood vessels by paedogenesis or otherwise, with corollary is partly based on cladistic analyses said to be leading to absurdities concerning wood vessel evolution, can be challenged because alternative starting points render these allegedly unacceptable evolutionary events highly plausible. At least some of the homoxylous primitive angiospermous taxa are early arrivals in the paleontological record, which is more compatible with an ancientry (i.e., plesiomorphy) of the character state of homoxyly in these taxa. Most if not all other conclusions emanating from Young's viewpoints are likewise to be rejected. Since prior to cladistic and other manipulations certain assumptions had been made and conventional ideas adopted as factual data, the ensuing evaluation of features and of character states is at least questionable and the qualifications of such characteristics as plesio- or as apomorphic is decidedly aprioristic. Parallelisms are not accounted for either, which means that the advent of vessel perforations need not have been a single evolutionary event but may have taken place repeatedly, some taxa never evolving beyond the level of homoxyly. For these and for other reasons one need not amend the generally held views concerning the primitive status of certain vessel-less, arborescent Dicotyledons and the pleiomorphy of the character state of homoxyly in these taxa. This is another example of the inherent handicap in the application of cladistics that the results are to a large extent already aprioristically decided by the acceptance of tenets and “current” ideas and not by the cladistic methodology. One should first attempt to come to agreement as regards the evolutionary polarity of certain conditions and character states (i.e., the direction of phylogenetic advancement) before entering upon any form of cladistic analysis, lest cladistic approaches to Angiosperm phylogeny remain entirely futile because the results may prove to be meaningless.