Conclusions and evolutionary perspectives

Abstract

As formulated by Hennig (1966), Brundin (1968) and Lovtrup (1977), phylogenetic systematics starts from the assumption that every group in a phylogenetic system has a corresponding twin or sister group and that both are derived from a common ancestor (Fig. 14.la). In this dichotomy, one of the sister groups retains to a greater extent the characters of the ancestral group (m1) and is referred to as plesiomorphic (or in Lovtrup’s terminology, plesiotypic) while the other, diverging more from the ancestral type, is described as apomorphic or apotypic. In making these distinctions it is clear that we must attempt to understand both the conservative character of the organism and its evolutionary innovations and be able to distinguish between the ‘old’ (primitive) and new (specialized) characters (Hansen, 1977). If, following Lovtrup, we apply this system to the relationship of the two cyclostome groups to the gnathostomes, the most likely interpretations are those represented by Figs. 14.1b, c and d. In Fig. 14.Ib, the myxinoids and lampreys are portrayed as sister groups, here referred to as secondary twins, diverging from a common ancestor with the gnathostomes at t. The gnathostomes together with the myxinoids and lampreys are also sister groups constituting the primary twins. In the second and third alternatives, Fig. 14 c, d, each of the two cyclostome groups in turn is considered to be the secondary twin or sister group to the gnathostomes, sharing with the latter a common ancestor at t 2, while either the myxinoids or the lampreys and gnathostomes constitute the primary twins originating in the common vertebrate ancestor at t. A decision on the choice of these various alternatives rests on the degree of relationship that we can establish between the two cyclostome groups and the gnathostomes. Are the myxinoids and lampreys more closely related to one another as in Fig. 14.1b where they share a common ancestor than is either group to the gnathostomes? According to the principles of phylogenetic analysis the answer to this question rests on our ability to discriminate between plesiotypic and apotypic characters. Thus, those two groups that share the greatest number of apotypic characters are regarded as the secondary twins and for this purpose, the common possession of plesiotypic characters is irrelevant. Since apotypic characters are inherited from the immediate common ancestor, these alone are an indication of close relationship. Open image in new window Fig. 14.1 Phylogenetic principles and the relations of cyclostomes to the gnathostomes. Based on Brundin, 1968 and Lovtrup, 1977. (a) Variation from an ancestral form A of plesiotypic (b) and apotypic (c) sister species. t. time; m. extent of variation from ancestral form. (b), (c) and (d) represent alternative interpretations of the position of the lampreys and myxinoids in relation to the gnathostomes (Gnath).