Bat Bird Cladograms

Figure B3.1.2 Because each pair of ciadograms in Figure B3.1.1 is commutatively equivalent, there are really only three ciadograms under consideration.

include computers, since a digital watch has the shared, derived characters of computers.The cladogram suggests that the term "watch" does not describe an evolutionarily meaningful (monophyletic) group, in the sense that a cladogram that includes digital watches, wind-up watches, and quartz watches must also include computers, as well as a variety of more conventional mechanical timing devices (such as stop watches). Rather; the term "watch" may be thought of as some other kind of category: it describes a particular function (timekeeping) in conjunction with size (relatively small).

In this example, we are fortunate in that, should we so choose, we can test the cladogram-

based conclusions by studying the historical record and find out about the evolution of wrist watches, digital watches, and quartz watches. Obviously this is not possible to do with the record of the biota because there is no written or historical record with which to compare our results.The characters of each new fossil find, however; can be added to existing ciadograms and the hypothesis of relationship that shows the least complexity will be favored according to the principle of parsimony. In our discussions of the biota, we attempt to establish categories that are evolutionarily significant (monophyletic groups), and avoid groups that have less in common with each other than with anything else.

The cladogram need not depict every organism within a monophyletic group. If we are talking about humans and carnivores, we can put them on a cladogram and show the derived characters that diagnose them, but we might (or might not) include other mammals (e.g., a gorilla). As with the motor vehicles example, if the hierarchical relationships that we have established are valid, the addition of other organisms into the cladogram should not alter the basic hierarchical arrangements established by the cladogram. Figure 3.9 shows the addition of one other group into the cladogram from Figure 3.8. The basic relationships established in Figure 3.8 still obtain even with the new organisms added.

Parsimony It may be apparent by now that, in an evolutionary context, a cladogram is actually a "hypothesis of relationship," that is, an hypothesis about how closely (or distantly) organisms are related. With a given set of characters, it may be possible to construct several possible ciadograms (as we saw that there were in the example of the pick-up truck, the car, and the motorcycle). We can distinguish among different hypotheses of relationship using the principle of "parsimony." Parsimony, a sophisticated philosophical concept first defined by the

fur- or hair-bearing

Figure 3.8. A cladogram showing humans within the larger group Mammalia. Mammalia is diagnosed by warm-bloodedness and possession of fur (or hair); many other characters unite the group as well. Carnivora, a group of mammals that includes bears and dogs (among others), is shown to complete the cladogram. Carnivores all uniquely share a special tooth, the carnassial, and humans all uniquely share various gracile features of the skeleton. Note that all mammals (including humans and carnivores) are warm-blooded and have fur (or hair), but only humans have the gracile skeletal features, and only members of Carnivora have the carnassial tooth.

fur- or hair-bearing

Figure 3.8. A cladogram showing humans within the larger group Mammalia. Mammalia is diagnosed by warm-bloodedness and possession of fur (or hair); many other characters unite the group as well. Carnivora, a group of mammals that includes bears and dogs (among others), is shown to complete the cladogram. Carnivores all uniquely share a special tooth, the carnassial, and humans all uniquely share various gracile features of the skeleton. Note that all mammals (including humans and carnivores) are warm-blooded and have fur (or hair), but only humans have the gracile skeletal features, and only members of Carnivora have the carnassial tooth.

Carnassial

warm-bloodedness fur- or hair-bearing

Figure 3.9. Addition of the genus Gorilla.The addition of gorillas to the cladogram does not alter the basic relationships outlined on the cladogram shown in Figure 3.8.

warm-bloodedness fur- or hair-bearing

Figure 3.9. Addition of the genus Gorilla.The addition of gorillas to the cladogram does not alter the basic relationships outlined on the cladogram shown in Figure 3.8.

Bird Cladogram

Figure 3.10. Two possible arrangements for the relationships of birds, bats, and humans. Part (a) requires wings to have evolved two times; part (b) requires birds to have lost fur and mammary glands.These as well as many other characters make (a) the more parsimonious of the two cladograms.

Cladogram With Multiple Characters

Figure 3.10. Two possible arrangements for the relationships of birds, bats, and humans. Part (a) requires wings to have evolved two times; part (b) requires birds to have lost fur and mammary glands.These as well as many other characters make (a) the more parsimonious of the two cladograms.

fourteenth century English theologian William of Ockham, states that the simplest explanation - that is, one with fewer steps than another -is probably the best. Why resort to complexity when simplicity is equally informative? In other words, why take more steps when fewer can provide the same information?

A bird, a human, and a bat will serve as a simplified example. We will start with the following characters: wings, fur, feathers, and mammary glands. Figure 3.10 shows two cladograms that are possible from these animals and their characters. In the one in which the bird is most closely linked with the bat, the bird has to lose ancestral mammary glands and it has to replace fur with feathers. In the cladogram in which the bat and the human share a most recent common ancestor, wings must be invented by evolution twice. The cladogram in which the human and bat are most closely linked is the simpler of the two because it requires fewer evolutionary events or steps. The cladogram linking the human and bat together remains uncomplicated by the addition of more characters; by contrast, the addition of virtually any other characters that pertain to the creatures in question (e.g., the arrangement, shape, and number of bones, particularly those in the skull and wings, the structure of the teeth, the biochemistry of each organism, etc.) only further complicates the cladogram that most closely links birds and bats. Based upon parsimony, therefore, the cladogram is preferred that shows bats and humans to have more in common with each other than either does with a bird. And indeed, as a hypothesis about the evolution of these vertebrates, it is extremely likely that bats and humans share a more recent common ancestor with each other than either does with a bird (which, obviously, is why they are classified together as mammals).

In this case, the use of shared, derived characters has led us to the most parsimonious conclusion with regard to the evolution of these three creatures.

Science and Science is an approach to gaining insight into certain kinds of issue testing hypotheses that is rooted in a particular type of logic. In this sense it is nothing more (or less) than a tool for solving a restricted series of problems. Indeed, there is a variety of potentially significant problems that are not particularly amenable to a scientific solution. Examples of such questions are "Is there a God?", "Does she love me?", "Why don't I like hairy men?", and "Is this great music?" Such questions might be answerable, but it will never be by means of science that the answers are discovered.

Other questions, however, are more amenable to scientific inquiry. For example, a scientific hypothesis (although a very simplistic one) is: "The sun will rise tomorrow." This statement can be thought of as a hypothesis with specific predictions. The hypothesis (that the sun will rise tomorrow) is testable; that is, it makes a prediction that can be assessed. The test is relatively straightforward when the right kinds of observation are made: we can wait until tomorrow morning and either the sun will rise or it will not. If the sun does not rise, the statement has been falsified, and the hypothesis can be rejected. If the sun does rise, the statement has not been falsified, and the hypothesis cannot be rejected. For a variety of relatively sophisticated philosophical reasons, scientists do not usually claim that they have proven the statement to be true; rather, the statement has simply been tested and not falsified. One of the basic tenets of science is that it consists of hypotheses that have predictions which can be tested. We will see many examples of hypotheses in the coming chapters; all must involve testable predictions. Our ability to test these will determine the importance of these hypotheses as scientific contributions.

In an evolutionary context, cladograms are hypotheses of phyloge-netic relationship. They make predictions about the distributions of characters in organisms, both living and extinct. New fossils can test the phylogenetic hypotheses inherent in cladograms because these contain character suites that need to be concordant with the pre-existing cladograms. Living organisms can also test cladograms by the distribution of their characters, including the content of their genetic material (DNA sequences). Cladograms are most "robust" (strongest), if they survive falsification attempts. Parsimony indicates that these are the ones that most closely approximate the course of evolution.

It is now clear how a cladogram is tested. The addition of characters can cause the rejection of a cladogram by demonstrating that it is not the most parsimonious character distribution. In contrast, a tree of life presents more difficulties. Aside from requiring the miniscule probability that ancestors and their direct descendants will be preserved, a tree of life is untestable. How does one identify the actual ancestor and its direct descendant? Given the absurdity of a claim that these have been found, a tree of life is really more of a story or "scenario," than a testable scientific hypothesis. For this reason, here we content ourselves with ciadograms, and do not confuse them with trees of life. As will become evident, much can be learned from ciadograms that will contribute to our desire to know what occurred in ages long past.

Important readings Cracraft, J. and Eldredge, N. (eds.) 1981. Phylogenetic Analysis and

Paleontology. Columbia University Press, New York, 233pp. Eldredge, N. and Cracraft, J. 1980. Phylogenetic Patterns and the Evolutionary Process, Method and Theory in Comparative Biology. Columbia University Press, New York, 349pp. Hennig, W. 1966. Phylogenetic Systematics (translation by D. D. Davis and R. Zangerl). University of Illinois Press, Urbana, IL, 263pp. (Reprinted 1979).

Jepsen, G. L., Simpson, G. G. and Mayr, E. 1949. Genetics, Paleontology, and

Evolution. Princeton University Press, Princeton, NJ, 445pp. Nelson, G. and Platniclc, N. 1981. Systematics and Biogeography, Cladistics and

Vicariance. Columbia University Press, New York, 567pp. Ridley, M. 1992. Evolution. Blackwell Scientific Publications, Inc.,

Cambridge, MA, 670pp. Stanley, S. M. 1979. Macroevolution. W. H. Freeman and Company, San Francisco, 332pp.

Wiley, E. 0., Siegel-Causey, D„ Brooks, D. and Funk, V. A. 1991. The Compleat Cladist, A Primer of Phylogenetic Procedures. University of Kansas Museum of Natural History Special Publication no. 19,158pp.

+1 -1

Post a comment