EVOLUTION AND SYSTEMATICS (cont.)

RECONSTRUCTING PHYLOGENY AND CLASSIFICATION: WHY BOTHER?
Having presented the basics of phylogenetic methodology, how can we use the results that we obtain? Why should we bother to go through the process of constructing phylogenetic relationships? More than simply summarizing the available evidence that was used to construct a pattern of relationships, a phylogenetic hypothesis allows us to predict, in an evolutionary framework, the distribution of characters that were not included in the original analysis, or that may be difficult or impossible to fossilize. Consider physiology. We can't measure body and ambient temperature in terrestrial dinosaurs. What does the hypothesis of relationship we just constructed (Figures 4, 7) suggest about terrestrial dinosaur physiology? In this example, unfortunately, the cladogram does not suggest a simple, definitive answer. Either dinosaurs share endothermy with birds, as a derived character, or they share ectothermy with crocodiles, as an ancestral character. Other evidence — the absence of turbinate bones in dinosaur nasal cavities (Ruben et al., 1998), the presence of downy feathers on small theropod dinosaurs (Chen et al., 1998) — must be examined in order to distinguish between these two possibilities. If crocodiles happened to be endothermic, we could predict with greater confidence that dinosaurs are likely to have been endothermic also. If birds happened to be ectothermic, we could predict with greater confidence that dinosaurs are likely to have been ectothermic. Instead, we are left with an intriguingly complex ambiguity.
Predictions about the phylogenetic position of additional taxa can also be generated and tested from cladograms. Where do snakes fit in Figure 4? We might predict that they are the sister group to the tetrapods, since they are vertebrates but do not have limbs. Testing this prediction by examining more characters, we would have to recognize that snakes share many derived features with the clade that includes crocodiles and birds (Archosauria); thus, our initial prediction was incorrect. Archosaurs and snakes, not tetrapods and snakes, are sister groups in this example.
Taxa considered to be "missing links" can be interpreted more easily with respect to a phylogenetic diagram. Archaeopteryx has long been considered to be the ancestral bird, primarily because of the presence of feathers in the few specimens that have been collected. Feathers are actually one of the very few characters that Archaeopteryx shares with living birds. It lacks a breastbone and does not possess a bird-like shoulder joint. It possesses many features that are also present in certain theropod dinosaurs, such as sharp teeth in the jaws, a long bony tail, fingers with claws (and now even feathers; Chen et al., 1998). The many similarities between Archaeopteryx and theropods were somewhat confusing and interpreted by some biologists as homoplastic similarities, not shared by common ancestry. If we consider the mosaic of characters that Archaeopteryx possesses in a phylogenetic context, the similarities are no longer as confusing. Archaeopteryx shares derived characters, including feathers, with birds (and other derived theropod dinosaurs, further strengthening the link between birds and dinosaurs; Chen et al., 1998; Dal Sasso and Signore, 1998; Ji et al., 1998). It also shares many primitive features, including teeth, tail, and claws, with theropod dinosaurs lacking feathers.
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The process of creating a cladogram forces us to follow a careful procedure of observation and comparison. It also requires us to be honest and straightforward about exactly what pieces of evidence were used to make decisions about phylogenetic relationships. As is true in all fields of science, the ability to repeat another investigator's methods and obtain the same results is a critically important element in the process of testing hypotheses. Phylogenetic methods require all conclusions about relationships to be stated clearly and defended with evidence.
These examples all underscore one main point — that having an understanding of the ancestry of organisms, even in the form of a hypothesis of patterns of relationship, gives us a fundamentally important insight into the process of evolution, a process that is difficult to observe directly. It makes an enormously significant difference in our understanding of the evolution of locomotion or physiology or reproductive behavior to know if flight, or endothermy, or parental care has evolved more than once among the vertebrates and, if so, from what primitive states. Knowledge about the ancestry of organisms provides an essential foundation for understanding the environment in which evolutionary change has occurred.
It can be difficult and confusing to make scientific sense out of complex patterns of similarity and difference in morphology or behavior without reference to a phylogenetic hypothesis. Patterns of common ancestry cannot be clearly distinguished from patterns resulting from similar function or similar selective pressures without a phylogenetic point of reference provided by systematics. As evolutionary biologists and paleontologists, reconstructing phylogenetic patterns is surely one of our most significant goals.

REFERENCES CITED
Benton, M.J. 1995. Testing the time axis of phylogenies. Philosophical Transactions of the Royal Society of London, Series B 349:5-10.

Carlson, S.J. 1995. Systematics, p. 391-402. In W. A. Nierenberg (ed.), Encyclopedia of Environmental Biology, Volume 3. Academic Press, New York.

Chen, P.J., Z.M. Dong, and S.N. Zhen. 1998. An exceptionally well-preserved theropod dinosaur from the Yixian Formation of China. Nature 391:147-152.

Dal Sasso, C. and M. Signore. 1998. Exceptional soft-tissue preservation in a theropod dinosaur from Italy. Nature 392:383-387.

De Queiroz, K. 1988. Systematics and the Darwinian Revolution. Philosophy of Science 55:238-259.

De Queiroz, K. and J. Gauthier. 1990. Phylogeny as a central principle in taxonomy: phylogenetic definitions of taxon names. Systematic Zoology 39:307-322.

De Queiroz, K. and J. Gauthier. 1992. Phylogenetic taxonomy. Annual Review of Ecology and Systematics 23:449-480.

Fisher, D.C. 1991. Phylogenetic analysis and its application in evolutionary paleobiology. p. 103-122. In N.L. Gilinsky and P.W. Signor (eds.), Analytical Paleobiology, Short Course in Palaeontology, no. 7. University of Tennessee, Knoxville.

Foote, M. 1996. On the probability of ancestors in the fossil record. Paleobiology 22:141-151.
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Hennig, W. 1966. Phylogenetic Systematics. University of Illinois Press, Urbana.

Huelsenbeck, J.P. 1994. Comparing the stratigraphic record to estimates of phylogeny. Paleobiology 204:470-483.

Ji, Q., P.J. Currie, M.A. Norell, and S.A. Ji. 1998. Two feathered dinosaurs from northeastern China. Nature 393:753-761.

Mayr, E. 1942. Systematics and the Origin of Species. Columbia University Press, New York.

Wagner, P.J. 1995. Stratigraphic tests of cladistic hypotheses. Paleobiology 212:153-178.

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