Animal phylogeny

In the Origin of Species, Darwin used the metaphor of a tree to describe the relationship of life; indeed a tree is famously the only illustration in the book. The origin of species didn’t include any explicit phylogeny of animal life, yet Darwin’s book is said to have prompted the German biologist Ernst Haeckel to abandon his medical practice and turn to the study of evolution. In 1866, Haeckel produced a tree depicting the evolutionary relationships of living taxa including the animals (Figure 1). Looking at Haeckel’s tree we can observe two contradictory things. First, we can see that much of the detail is very familiar in today’s phylogenies — we recognise groups such as echinoderms, arthropods, molluscs and vertebrates; these groups represent the animal phyla. Second, many of the relationships in Haeckel’s tree between these phyla (molluscs with the seasquirt (Himatega), echinoderms with annelids and arthropods) do not accord at all well with a more modern concept of animal phylogeny (Figure 2). This dichotomy seems to derive in large part from the definition of a phylum. Members of a phylum typically show a well-defined body plan that is not obviously related to that of any other group. What this means practically is that it is easy to recognise an echinoderm, arthropod, mollusc or annelid but much harder to see how they might be related to one another. This apparent uniqueness of phyla probably stems in part from their definition as distinct groups and in part from the manner of phylogenesis in the Precambrian during which they originated. Whatever the explanation, it is this dichotomy that has made understanding phylum level animal relationships so difficult and so controversial. Primer Molecular systematics Inevitably, the use of molecular genetic data has had an enormous impact on unraveling animal phylogeny. Far and away the most important contribution has come from the study of the small and large subunit ribosomal RNA genes (SSU and LSU). Our current view of animal phylogeny, which is often referred to as the ‘new animal phylogeny’, is in large part derived from studies of SSU but various conclusions regarding this tree have been strengthened by a diversity of additional sources of molecular evidence — in particular from the ‘phylogenomic approach’, which entails the use of whole genome sequences and large EST datasets to assemble datasets of many tens or hundreds of concatenated genes for phylogenetic analysis. Further support for individual groups has come from the discovery of ‘rare genomic changes’, which are complex genetic novelties such as a change in mitochondrial genetic code or gene order; their complexity and rarity should, in principle, exclude the possibility of convergent evolution in unrelated groups.