Strategy

In this chapter, I will proceed according to the three-stage strategy described by Eldredge (1979). That is, I will first consider possible patterns of relatedness of the major metazoan groups using cladograms; I will then attempt to put approximate times to the important lineage divergence points, thus producing phylogenetic trees; finally, I will bring in some relevant palaeoecological data to produce what Eldredge calls adaptive scenarios – though the treatment of these will be brief, given the criticisms of such scenarios considered in Section 1.3. At each shift (cladogram-to-tree and tree-to-scenario) the proposal being made has a higher information content but also a high probability of being wrong: there are many possible trees for each cladogram, many possible adaptive scenarios for each tree.

The ‘internal’ side of things – that is, the genetic and developmental architecture of the evolutionary changes involved (as discussed in Section 2.4) – will for the moment disappear from view. It will resurface later (in Chapter 5 and beyond) when I deal with it in more detail. Ultimately, it is the connection between that architecture and the phylogenetic patterns on which we will now focus that is most important. I will, consequently, at one stage in this chapter, focus on the relationships between taxa containing species which are used as ‘model systems’ for the study of developmental genetics.