XXIII.6.1 Under favorable conditions, synapomorphies can be identified on the basis of paleontological data, by comparing the situation in sister taxa or by study of ontogenesis
Similarly as it is frequently difficult in practice to differentiate between homologies and hompolasies, it is also generally equally difficult to differentiate plesiomorphies and apomorphies.The study of fossils can provide a certain guideline, as the plesiomorphic forms of a trait should appear in the paleontological record sooner than apomorphic forms.The paleontological record is, of course, frequently not complete, the fossils of the representatives of an older taxon may be preserved only in younger layers, so that the results obtained in this way are sometimes misleading.
Study of the situation in sister taxa can also provide some information.If the particular trait occurs in the studied taxon in forms A and B and occurs in the sister taxon only in form A, then it is more probable that the common ancestor of the two sister taxa also had the same form A and that this is a plesiomorphic form.Once again, it holds that we might not be capable of correctly recognizing the sister taxon and that its erroneous identification could also mean erroneous recognition of the plesiomorphic form of the trait.
The plesiomorphic form of the trait can also be identified in some cases on the basis of the order of discovery of the individual forms of this trait in ontogenesis.If the foundations for both forms of the trait appear during ontogenesis, the foundations of the plesiomorphic form should generally appear sooner.This empirically derived rule is described by Haeckel’s recapitulation theory.This states that ontogenesis is accelerated phylogenesis, i.e. that the individual structures are generally established in ontogenesis (individual development) in the same order as they appeared during phylogenesis.Haeckel’s theory is basically an evolutionary reinterpretation of the formerly formulated von Bauer’s first law of embryology.According to this law, in the ontogenesis of each species, the individual structures are formed gradually from structures that are common to all the members of the highest taxon to the structures of common members of gradually lower and lower taxa, to which the given species belongs.The historically older von Bauer’s law, which Darwin also mentioned as one of the important documents for the validity of his theory of evolution, is actually substantively more correct that the newer Haeckel’s recapitulation theory.However, there are many exceptions to both the recapitulation theory and von Bauer’s law.In a great many species, the route through which the ontogenesis of a certain structure reaches a certain stage can be modified and some stages can even be omitted in some species (see XII.7.3).It is, however, true that it rarely occurs that the order in which the individual stages appear is reversed and that there would thus be a flagrant breaking of von Bauer’s law and this also the recapitulation theory.