The Major Transitions in Evolution Revisited

13. The Small Picture Approach to the Big Picture: Using DNA Sequences to Investigate the Diversification of Animal Body Plans

13The Small Picture Approach to the Big Picture: using DNA Sequences to Investigate the Diversification of Animal Body Plans Lindell Bromham The Adaptive Radiation of the Metazoans The Metazoa (animal kingdom) is divided into approximately three dozen phyla (figure 13.1). The first undisputed fossils of around half of the animal phyla appear in the Cambrian , the geological period that runs from around 543 million years ago (Myr) to 488 Myr. At least a third of animal phyla have no fossil record to speak of (Valentine 2004), but we can infer from phylogenetic relationships that many of these lineages must be at least Cambrian in age. On the basis of this fossil evidence, it has been suggested that all of the major kinds of animals were generated in a period of around 10 to 15 million years (e.g., Carroll 2005; Levinton 2001; Valentine 2004). This inferred explosive radiation of animals in the Cambrian has been considered the signature of a phenomenal rise in diversity and complexity of animal life, and creating more complex ecosystems (e.g., Bambach, Bush, and Erwin 2007). An earlier Precambrian fauna, known as the ediacarans, were relatively simple, softbodied creatures (Xiao and Laflamme 2008). With the possible exception of Kimberella, which has been interpreted as having a muscular foot and scraping radula like a mollusc (Fedonkin and Waggoner 1997), none of the ediacarans show clear evidence of appendages specialized for locomotion, and there are relatively few complex trace fossils (marks made in the sediment) that would bear witness to directed bilaterian movement in the Precambrian period (Jensen, Droser, and Gehling 2005). Due to the general lack of evidence of mouths, claws, teeth, eyes, or other equipment associated with hunting or foraging, the ecology of ediacaran communities has also been regarded as being fairly simple (Bambach et al. 2007; Xiao and Laflamme 2008). By contrast, the Cambrian fauna provides abundant evidence of animals with sense organs, appendages for locomotion and feeding, and defensive structures. Unlike the floor-bound ediacarans, some Cambrian animals moved down into the sediment by active burrowing, and some moved up into the water column by directed swimming. For the first time, there is clear evidence of specialized, mobile animal predators. figure 13.1 Approximate number of described species per metazoan phylum. There is no central database for described animal species, so these figures are open to debate. In most cases, it is a fair assumption that the number of described species will underestimate the actual diversity of the phylum, and in some cases only a relatively small proportion of the species thought to exist have been described. In addition, there is no definitive list of animal phyla, because there is disagreement over which taxa should be given phyletic status. Some taxa listed here are contained within other phyla in some systematic treatments (e.g., Echiura within Annelida). Some single species or genera have been elevated to phylum level on the basis of morphology (Micrognathozoa) or phylogeny (Xenoturbellida). Here, phyla are labeled according to superphyletic groupings (see figure 13.2). Assignment to superphyla is controversial in some cases. For example, Myxozoa (an important group of parasites of fish and other animals) have variously been classified as protists, cnidarians, or as a separate bilaterian phylum. The Small Picture Approach to the Big Picture 273 The metazoan radiation itself is not surprising. The great diversity of animals today must have been ultimately derived from a simple common ancestor, so we know the metazoans made the transition from blobs to bugs at some point. Adaptive radiations are common throughout evolutionary history, where a single ancestral lineage diversifies rapidly to produce a wide range of ecologically specialized descendents. We see evidence of rapid adaptive radiations on many oceanic islands, where a colonizing lineage radiates to fill a variety of ecological niches. For example, in as little as 10 million years, a single ancestral lineage of Hawaiian honeycreepers has radiated into more than fifty separate species, with a wide variety of colors and shapes, which occupy a range of niches including insectivores, seedeaters , frugivores, nectarivores, and snail-eaters (Grant 2001; Lovette, Bermingham, and Ricklefs 2002). What is remarkable about the Cambrian radiation of metazoans is its apparent suddenness and uniqueness: It seems that more fundamental evolutionary change in animal complexity and diversity occurred in this relatively short period than in any equivalent time period before or since the Cambrian. In the time that it took Hawaiian...