What is parsimony, anyway?

Jeff Poling

INTRODUCTION

When reading through the debates on the Dinosaur Mailing List, or recent textbooks dealing with dinosaurs, one will often see the words parsimony and parsimonious. A given debater will insist that his cladogram or his view on certain dinosaur physical characteristics is the most parsimonious.

Well, what is this parsimony thing, anyway?

The principle of parsimony is defined as "a scientific rule that states that if there exists two answers to a problem or a question, and if, for one answer to be true, well-established laws of logic and science must be re-written, ignored, or suspended in order to allow it to be true, and for the other answer to be true no such accommodation need be made, then the simpler of the two answers is much more likely to be correct."¹ Put a simpler way, parsimony is "a principle that states that the simplest explanation that explains the greatest number of observations is preferred to more complex explanations".²

The application of parsimony is similar when one is talking about phylogeny and cladograms, or about whether an extinct animal might have had certain physical characteristics. However, the application of the principle can look very different. By treating the two topics separately it should be clearer not only how parsimony is applied, but what it is all about.

PARSIMONY AND CLADISTICS

Cladistics is a phylogenetic taxonomy system where organisms are grouped based on their hypothesized phylogeny, or ancestral and familial relationships. The result of a cladistic analysis, a clade, can be considered a hypothetical family tree.

Clades have two components: a definition and a diagnosis. The definition tells what the clade is, such as clade Dinosauria defined as "the most recent common ancestor of birds and Triceratops, and all its descendants." The diagnosis of clade is a listing of what derived, or advanced and possibly unique, physical characteristics will indicate that a given animal is a member of a given clade. The diagnosis of clade Dinosauria might be three sacral vertebrae and an open hip socket.

At the heart of any given cladogram are evolutionary steps, such as the loss of one character, or the modification or gain of another. Each cladogram will have a certain number of evolutionary steps. When two possible cladograms for the same set of organisms are compared, the one with the least number of evolutionary steps is chosen. Because the cladogram with the least number of steps is the simplest, it is considered the most parsimonious and is the preferred choice.

For example, consider two cladograms of birds, bats and humans:³

In cladogram A, the following evolutionary steps occur: At a wing structures feathers At b hair mammary glands At c wing structures
In cladogram B, the following evolutionary steps occur: At a hair mammary glands At b wing structures At c loss of hair loss of mammary glands feathers

Cladogram A requires five evolutionary steps while cladogram B requires six. Because cladogram A requires fewer evolutionary steps it is the simplest, and parsimony dictates that this is the preferred cladogram. Further, one could legitimately argue that the combination of the loss of hair and mammary glands and the evolution of feathers would require that the "well-established laws of logic and science must be re-written, ignored, or suspended in order to allow" cladogram B to be "true." No such accommodation need be made for cladogram A, thus, again, making it the most parsimonious and preferred cladogram.

It should be noted that there is no law that states that natural systems must behave simply, or parsimoniously. Systems in nature are often wonderfully complex.⁴ However, the simplest explanation is often the best place to start. It should also be noted that choosing different characters to consider can change which cladogram under consideration is the most parsimonious, or lead to multiple equally parsimonious cladograms. This is often at the heart of disagreement between phylogenetic taxonomists.

PARSIMONY AND ANATOMY

The science of cladistics bases its hypotheses upon the physical characteristics of the organisms in question. In our diagnosis of Dinosauria, we named three sacral vertebrae and an open hip socket as advanced characteristics of members of that clade. However, in our cladistics example above, we used such characteristics as fur and feathers. Skeletal features such as vertebrae and hips are readily available in the physical fossil evidence of an extinct organism. Soft features such as organs and integument are rarely available.

Because certain characteristics are not obvious in the fossil record, scientists formulate hypotheses on whether certain organisms possessed certain characteristics. As with cladograms, parsimony is applied to several hypotheses to help determine which is the preferred hypothesis.

When formulating hypotheses about anatomical characteristics, an important part of the methodology is Larry Witmer's "Extant Phylogenetic Bracket Method."⁵ This methodology uses an extant ingroup, or a group of organisms within the same clade as those organisms being considered, and the closest living outgroup, or group of organisms closest to those organisms being considered but not in the same clade. By comparing an extinct animal to these two extant groups, hypotheses of varying degrees of confidence can be formulated as to whether or not a particular character was present within the animal. Parsimony helps choose the preferred hypothesis from the varying hypotheses.

The following examples will use, naturally, non-avian dinosaurs as the extinct animal.⁶ They will be compared to their closest living ingroup, the neornithine dinosaurs (modern birds), and closest living outgroup, the crocodilians (definitions of ingroups and outgroups depend upon what specific clade one is looking at, e.g. one other clade that is discussed, Amniota, has neornithines, crocodilians, turtles, lizards and snakes, and mammals as part of the ingroup, with amphibians as an outgroup). Further, these examples use a cladistics viewpoint to simplify the explanation why one hypothesis is more parsimonious than another by comparing the evolutionary steps each hypothesis would require. Whether a particular hypothesis does or does not violate the "well-established rules of logic and science" is left to the reader to ponder. In real life, the issues are likely far more complex than presented here.

Hypothesis 1: Non-avian dinosaurs had hearts.

Hypothesis 2: Non-avian dinosaurs built nests out of vegetation, and sang to their young still in their eggs.

Hypothesis 3: Predentatans had cheeks.

Hypothesis 4: All dinosaurs were feathered.

1. Functional arguments for the flight mechanism in extinct volant birds suggest that feathers must be present to allow flight. This takes feathers down through clade Ornithothoraces to the base of clade Aves.
2. The presence of feathers in Archaeopteryx at the base of Aves suggests that feathers are at least primitive for Aves (one step, feathers evolving once, versus two steps, feathers evolving independently, or three steps, feathers evolving, being lost, then re-evolving for flight). Witmer calls this aspect the Lagerstatten modifier to the Extant Phylogenetic Bracket Method: well-preserved fossils (Lagerstatten) can make some fossil material effectively "extant" in terms of the evaluation of the soft tissue structure.
3. The probable feathers of Compsognathus primus (=Sinosauropteryx prima) pulls feathers down to the base of Coelurosauria, assuming that C. primus was a basal coelurosaur, C. primus's integument really was feathers, and that the most parsimonious explanation for feathers is a single origin (one step, feathers evolving once in the Dinosauria, versus two steps, feathers evolving twice within the Dinosauria).
4. Carnotaurus, as far as can be seen, lacked feathers (at least as an adult). This is the only non-coelurosaurian theropod for which much of the integument is known. It suggests by parsimony that, unless its scales were derived feathers, feathers (or at least feathered adults) developed somewhere between the base of clade including Ceratosaurus up to and including the base of clade Coelurosauria (one step, evolution of feathers in a later Dinosaur, versus two steps, evolution then loss of feathers). The presence of feathers in such dinosaurs as Torvosaurus and Allosaurus, for example, is therefore problematic in cladograms where allosaurids are more primitive members of clade Theropoda than Carnotaurus.
5. The absence of feathers in preserved integument in any sauropodomorph or predentatan is consistent with the hypothesis that feathers evolved within the Theropoda and is not primitive for the Dinosauria as a whole. However, the confirmation of feathers in a small hyspilophodont, for example, in a lithographic limestone would question this part of the argument, as well as that used above for non-coelurosaurs. Such a find would suggest that the most parsimonious hypothesis is that feathers are common to all dinosaurs (one step, feathers evolving once, versus the multiple evolution of feathers in different branches of the Dinosauria).

Adding the known fossil record to the Bracket Method results in feathered coelurosaurs being the most parsimonious hypothesis (one step, feathers evolving once in the Coelurosauria, versus two steps, feathers evolving independently in at least two separate branches of the Coelurosauria, Aves and whatever branch contains C. primus).

Hypothesis 5: All dinosaurs were warm-blooded.

As in cladistics, multiple workers can disagree on what constitutes the most parsimonious hypothesis.

CONCLUSION

Parsimony is a principle in science where the simplest answer is always the preferred. In cladistics, the preferred hypothesized phylogeny, or cladogram, is the one that requires the fewest evolutionary steps and does not violate the tenets of logic and science. Similarly, parsimony can be used to test hypotheses, formed using the "Extant Phylogenetic Bracket Method" with consideration of other evidence, on what anatomical features an extinct organism might have had. Although there is no rule that requires nature to follow the simplest path, and results can vary based which pieces of evidence are used, it is nonetheless a strong basis for the scientific work of paleontologists.

1. Jeff Poling, Dictionary, http://www.dinosauria.com/dml/diction.htm, 10 March 1997, The DOL Dinosaur Omnipedia, http://www.dinosauria.com/dml/dml.htm.
2. David E. Fastovsky and David B. Weishampel, The Evolution and Extinction of the Dinosaurs (Melbourne: Cambridge University Press, 1996), 437.
3. Ibid., 61.
4. Ibid., 61
5. Dr. Thomas R. Holtz, Jr., Re: Parsimony, private e-mail to the author, 21 November 1996.
6. Ibid.
7. Jeff Poling, Fossil of crocodile with erect stance found, http://www.dinosauria.com/jdp/misc/croc.htm, 23 December 1996, Jeff's Journal of Dinosaur Paleontology, http://www.dinosauria.com/jdp/jdp.htm.