Metazoa: Systematics

Phylogenetic Hypothesis for Animals

"Why do evolutionary biologists care who's related to whom, and how do scientists find out how different animals are related?"

Scientists care because phylogeny is the fundamental product of evolution. Therefore, a phylogenetic hypothesis is essential if you want to understand biological phenomena, most of which have an evolutionary explanation.

Since many scientists would like to know how animal diversity and animal body plans came to be, presently there is a great deal of work on resolving the evolutionary relationships among the major groups of animals. Much of this research has relied upon morphological characters, especially those expressed in early development (e.g. embryological characters). More recently, a significant advance in our understanding of animal phylogeny has been brought about by the study of molecules (in particular genes and their protein products) contained within animal cells. The phylogeny presented here is a relatively conservative guess based upon various published studies of 18S ribosomal RNA sequence data. As you can see, there are quite a few unresolved branches, and therefore a great deal of work to be done in this area.

Note that the phylum Porifera (the sponges) is paraphyletic. A few lines of independent evidence suggest that one group of sponges is actually more closely related to non-sponge animals than it is to the other sponges. This is an important finding for it implies that the lineage leading to all other animals (including ourseleves!) was directly descended from an animal with a sponge body and a sponge life style.

Visit the Tree of Life for more information concerning the systematics of animals.

For some classic images of invertebrate animals and protists, created in the 19th century by the great zoologist Rudolph Leuckart, click here to visit the Marine Biological Laboratory at Woods Hole, Massachusetts.

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