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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  • Cavalier-Smith, T., Allsopp, M. T. E. P., Chao, E. E., Boury-Esnault, N. & Vacelet, J. 1996. Sponge phylogeny, animal monophyly, and the origin of the nervous system: 18S rRNA evidence. Canadian Journal of Zoology 74, 2031-2045.
  • Collins, A. G. 1998. Evaluating multiple alternative hypotheses for the origin of Bilateria: An analysis of 18S molecular evidence. Proceedings of the National Academy of Sciences, USA 95, 15458-15463.
  • Eernisse, Douglas J., James S. Albert, & Frank E. Anderson, 1992. Annelida and Arthropoda are not sister taxa: A phylogenetic analysis of spiralean metazoan morphology. Systematic Biology 41(3):305-330.
  • Garey, James R. & Andreas Schmidt-Rhaesa, 1998. The essential role of "minor" phyla in molecular studies of animal evolution. American Zoology 38(6): 907-917.
  • Garey, J. R., Schmidt-Rhaesa, A., Near, T. J., Nadler, S. A. 1998. The evolutionary relationships of rotifers and acanthocephalans. Hydrobiologia 387-388: 83-91.
  • Halanych, K. M., J. D. Bacheller, A. M. A. Aguinaldo, S. M. Liva, D. M. Hillis, & J. A. Lake. 1995. Evidence from 18S ribosomal DNA that the lophophorates are protostome animals. Science 267: 1641-1643.
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  • Ruiz-Trillo, Iñaki, Marta Riutort, D. Timothy J. Littlewood, Elisabeth A. Herniou, & Jaume Baguñà, 1999. Acoel flatworms: Earliest extant bilaterian metazoans, not members of Platyhelminthes. Science 283: 1919-1923.
  • Valentine, James W., David Jablonski, & Douglas H. Erwin, 1999. Fossils, molecules and embryos: New perspectives on the Cambrian explosion. Development 126(5): 851-859.
  • Wallace, Robert Lee, Claudia Ricci, & Giulio Melone, 1996. A cladistic analysis of pseudocoelomate (aschelminth) morphology. Invertebrate Biology 115(2): 104-112.
  • Winnepenninckx, Birgitta, Thierry Backeljau, & Rupert de Wachter. 1995. Phylogeny of protostome worms derived from 18S rRNA sequences. Molecular Biology & Evolution 12(6): 641-649.
  • Winnepenninckx, B., T. Backeljau, L. Y. Mackey, J. M. Brooks, R. de Wachter, S. Kumar, & J. R. Garey. 1995. 18S rRNA data indicate that Aschelminthes are polyphyletic in origin and consist of at least three distinct clades. Molecular Biology & Evolution 12(6): 1132-1137.
  • Wirz, A., Pucciarelli, S., Miceli, C., Tongiorgi, P., Balsamo, M. 1999. Novelty in phylogeny of Gastrotricha: Evidence from 18S rRNA gene. Molecular Phylogenetics and Evolution 13(2): 314-318.
  • Zrzavy, Jan, Stanislav Mihulka, Pavel Kepka, Ales Bezdek, & David Tietz, 1998. Phylogeny of the Metazoa based on morphological and 18S ribosomal DNA evidence. Cladistics 14(3): 249-285.

* Since the development of this webpage in 1994, biologists have continued to research evolutionary relationships and have uncovered new information about how these organisms are related. Our most current understanding of these relationships is reflected in the scientific literature.