The Place of Ancestor-Descendant Relationships in Phylogeny Reconstruction

Engelmann, G. F., and E. 0. Wiley (Department of Vertebrate Paleontology, The American Museum of Natural History, New York, NY 10024, and Division of Fishes, Museum of Natural History, The University of Kansas, Lawrence, KS 66045) 1977. The place of ancestordescendant relationships in phylogeny reconstruction. Syst. Zool. 26:1-11.-Three concepts of ancestry are distinguished, the individual organism as ancestor, the population or species as ancestor, and the supraspecific taxon as ancestor. Of these, only the first two can be objectively applied, the third is a taxonomic artifact. Of the first two, only the population or species as ancestor is used extensively in phylogeny reconstruction. We adopt the view that empirical or objective statements must be potentially falsifiable. The introduction of initial assumptions concerning the historical interpretation of empirical relationships can make phylogeny objective in this sense. We suggest that our outlook precludes identification of ancestors as self-evident. But, does it preclude hypotheses of ancestry? The strongest case for incorporating hypotheses of populational or species ancestors into phylogenetic reconstructions rests on the conclusion that the incorporation of such hypotheses seems to add to the information content of a purely "cladistic" statement. On morphological grounds, a statement of ancestor-descendant relationships would seem falsifiable if the hypothesis can be refuted by finding autapomorphies and can be corroborated by finding plesiomorphies in the supposed ancestor. We explore the nature of phylogenetic testing to assess this conjecture. In testing the relationships of a limited group of organisms, conflicts between alternate phylogenetic hypotheses may be resolved without introducing bias if those sets of characters which provide valid tests (the apomorphies) can be sorted out from those sets of characters which do not (the non-homologies and plesiomorphies). The weighting of characters as apomorphous, plesiomorphous, or non-homologous is an appeal to parsimony which represents an effort to sort out which set of characters provide tests of relationship within the problem at hand, given a higher level phylogeny. The resultant estimate will be the most parsimonious of the alternatives within the context of the higher level phylogeny. This estimate may or may not prove most parsimonious in an unweighted character analysis of the problem at hand carried out in isolation from the higher level phylogeny. It is the adoption of the higher level phylogeny which allows the investigator to objectively assess a problem of realistic proportions without ignoring relevant information. Plesiomorphies can not be used to test phylogenetic hypotheses because they are ad hoc statements required by the acceptance of the most parsimonious solution. Therefore, they can not provide corroboration for ancestor-descendant relationships. Autapomorphies are identified because they supposedly differ from the hypothetical ancestor. But, the ancestral morphotype is not a scientific statement, rather, it is a simple summary of characters derived from a cladistic hypothesis. An autapomorphy can only be identified by rejecting the alternate character as synapomorphous. We conclude that autapomorphies can not refute ancestor-descendant relationships. Therefore, given our initial assumptions, ancestor-descendant relationships based on morphology are not objective statements when applied to fossil populations or species. With the additional assumption that a particular cladistic relationship is true, ancestor-descendant relationships might be testable, but, this would render the cladistic relationships untestable. Another argument states that when fossil samples are close in time and space, it is more parsimonious to postulate that the ancestor is represented in the lower stratigraphic sample than to postulate it is missing. The apparent success of this argument rests on the requirement that the investigator answer a question that can not be answered objectively because its structure limits the alternate answers to non-objective statements. [Phylogeny reconstruction; ancestor-descendant relationships; hypothesis testing.] The concept of ancestry is an integral part of any theory of organic evolution because ancestor-descendant relationships are implied by genealogical descent. The term "ancestor" has been used in three ways, (1) the individual organism as ancestor, (2) the species or population as ancestor, and (3) the supraspecific taxon as ancestor.