ANCESTORS AND HOMOLOGY - (THE ORIGIN OF THE TETRAPOD LIMB)

Current issues concerning the nature of ancestry and homology are disc ussed with reference to the evolutionary origin of the tetrapod limb. Homologies are argued to be complex conjectural inferences dependant u pon a pre-existing phylogenetic analysis and a theoretical model of th e evolutionary development of ontogenetic information. Ancestral condi tions are inferred primarily from character (synapomorphy/homology) di stributions within phylogeny, because of the deficiencies of palaeonto logical data. Recent analyses of tetrapod limb ontogeny, and the diver se, earliest morphologies known from the fossil record, are inconsiste nt with typological concepts such as fixed ancestral patterns or baupl ans, emphasising the incompatibility of these with evolutionary contin uity. The evolutionary origin of the tetrapod limb is also examined in the light of its recent discussion in developmental genetics. While t his field promises to reveal more of the fundamental ontogenetic conte nt of homology (identity), at present it is concerned mostly with the abstraction of a new set of types, rather than investigating diversity and change.