The Animal Tree

There are about 35 living animal phyla. To understand the origin and evolution of any feature found in one or more of these groups, it is necessary to have a picture of the phylogenetic relationships among animals. Ideally, the fossil record would present a complete, ordered, unambiguous picture of the branching pattern of the animal tree. Unfortunately, it does not. As the divergence of most bilaterian phyla appears to have predated the emergence of recognizable members of modern phyla in the fossil record, we must make our inferences from later, more derived forms.

Constructing an accurate picture of metazoan relationships has been challenging, and many alternative schemes of animal phylogeny have been proposed and scrutinized over recent decades and continue to be evaluated. Most approaches have relied on anatomical and embryological comparisons. In general, phylogenies are determined according to shared characters that are presumed to be derived and therefore reflect a close relationship. For example, all animal phyla are thought to be more closely related to each other than to any other nonanimal phylum, because of similarities in animal multicellularity, cell structure and morphology, and cell signaling. Members of the most closely related protist group, the choanoflagellates, share a similar cell architecture with sponges but are not multicellular. What is most difficult to determine is whether apparent similarities between animals (for example, segmentation in arthropods and annelids) are due to common ancestry, are superficial, or evolved independently. Also, different tree topologies can emerge when different characters are used or when the same characters are weighted differently.

One way to circumvent the reliance on morphological comparisons is to use molecular genetic characters to construct animal phylogenies. As taxa diverge, the sequences of DNA, RNA, and protein molecules diverge as well; the relative degree of divergence can therefore be used to infer phylogenetic relationships. In addition, the presence or absence of particular genes, or the linkage of a group of genes on chromosomes, can be used to construct phylogenetic trees. New methods based on molecular sequences have been combined with morphology-based approaches to both prune and strengthen the animal tree.

We now recognize shared morphological, developmental, and genetic traits that suggest that the Bilateria can be organized into three great clades (a set of species descended from a common ancestor) (Fig. 1.4):

• The deuterostomes, including chordates, echinoderms, ascidians, and hemichordates. The deuterostomes are named for a shared feature of early embryonic development in which the mouth forms from a site separate from the blastopore, an opening in the early embryo.

• Two groups of protostomes, in which the mouth develops from the blastopore. The protostomes are divided into the lophotrochozoans, including annelids, molluscs, and brachiopods, many of which share a trochophore larval stage in their life cycle, and a clade consisting of the arthropods, onychophora, and priapulids.

Figure 1.4

Metazoan phylogeny

Figure 1.4

Metazoan phylogeny

The current picture of metazoan phylogeny showing representatives of three major bilaterians clades—the deuterostomes, the Lophotrochozoa, and the arthropod + onychophora + priapulid clade.

Within these great clades, the branching order has been less well resolved, such that it is unclear which phyla are more closely related. It is worth noting that the recent assignment of arthropods and annelids to two different protostome clades and the assignment of pseudo-coelomate phyla among different clades are major changes from previous portraits of the animal tree. The phylogenetic placement of the nematodes, including the model organism Caenorhabditis elegans, remains controversial, because their rapid molecular clock complicates analysis. Some phylogenies place the nematodes close to the arthropod + onychophora + priapulid clade and others more basally near the common ancestor of all bilaterian phyla.

The anatomical and developmental features of the Bilateria are very distinct from those of the basal metazoans (cnidara, ctenophores, and porifera). The evolutionary links between basal metazoans and the bilaterians are difficult to perceive. Indeed, as we will see in Chapter 4, major differences exist between the genetic toolkit of these two groups, and the differences are much more substantial than those between most bilaterians. Because of the long divergence time since the radiation of these groups, the phylogenetic relationships between cnidarians, sponges, and ctenophores and the last common ancestor of the Bilateria are uncertain. Many extinct animal lineages, as yet unknown from the fossil record, may have branched off of the metazoan tree between the last common ancestor of all animals and of the Bilateria (Fig. 1.4).

The gaps in the fossil record; the great differences in anatomy, development, and genome content between radially symmetrical animals and bilaterians; and the cryptic early history of bilaterians, make inferences about the morphological transformations involved in the origin of animal body plans very speculative. Paleontologists have introduced the concept of disparity to refer to differences among body plans and use the term diversity to refer to the number of species within a group. The genetic and developmental bases of the morphological diversification of a particular body plan within a phylum are far more accessible than is the origin of different body plans. Therefore, we will focus primarily on evolutionary trends within a few select phyla, such as the arthropods and chordates, making the implicit assumption that the same sort of genetic mechanisms involved in the evolution of large-scale morphological diversity within phyla also gave rise to fundamental differences in body plans.

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