Pattern and Process

Pattern. Consider that if evolution is fundamentally an aspect of history, then certain things happened and other things didn't. It is the job of evolutionary biologists and geologists to reconstruct the past as best they can and to try to ascertain what actually happened as the tree of life developed and branched. This is the pattern of evolution, and indeed, along with the general agreement about the gradual appearance of modern forms over the past 3.8 billion years, the scientific literature is replete with disputes among scientists about specific details of the tree of life, about which structures represent transitions between groups and how different groups are related. Morphologically, most Neanderthal physical traits can be placed within the range of variation of living humans, but there are tests on fossil mitochondrial DNA that suggest that modern humans and Neanderthals shared a common ancestor very, very long ago—no more recently than 300,000 years ago (Ovchinnikov et al. 2000). So are Neanderthals ancestral to modern humans or not? There is plenty of room for argument about exactly what happened in evolution. But how do you test such statements?

Tests of hypotheses of relationships commonly use the fossil record. Unfortunately, sometimes one has to wait a long time before hypotheses can be tested. The fossil evidence has to exist (i.e., be capable of being preserved and actually be preserved), be discovered, and be painstakingly (and expensively) extracted. only then can the analysis begin. Fortunately, we can test hypotheses about the pattern of evolution— and the idea of descent with modification itself—by using types of data other than the fossil record: anatomical, embryological, or biochemical evidence from living groups. one reason why evolution—the inference of common descent—is such a robust scientific idea is that so many different sources of information lead to the same conclusions.

We can use different sources of information to test a hypothesis about the evolution of the first primitive amphibians that colonized land. There are two main types of bony fish: the very large group of familiar ray-finned fish (e.g., trout, salmon, sunfish) and the lobe-finned fish, represented today by only three species of lungfish and one species of coelacanth. In the Devonian, though, there were nineteen families of lungfish and three families of coelacanths. Because of their many anatomical specializations, we know that ray-finned fish are not part of tetrapod (four-legged land vertebrate) ancestry; we and all other land vertebrates are descended from the lobe-fin line. Early tetrapods and lobe-fins both had teeth with wrinkly enamel and shared characteristics of the shoulder girdle and jaws, plus a sac off the gut used for breathing (Prothero 1998:

Figure 1.2

Are tetrapods more closely related to lungfish or to coela-canths? Courtesy of Alan Gishlick.

Figure 1.2

Are tetrapods more closely related to lungfish or to coela-canths? Courtesy of Alan Gishlick.

358). But are we tetrapods more closely related to lungfish or to coelacanths? Is the relationship among these three groups more like Figure 1.2A or Figure 1.2B? We can treat the two diagrams as hypotheses and examine data from comparative anatomy, the fossil record, biochemistry, and embryology to confirm or disconfirm A or B.

Anatomical and fossil data support hypothesis B (Thomson 1994). Studies on the embryological development of tetrapod and fish limbs also support hypothesis B. Now, when contemplating Figure 1.2, remember that these two diagrams omit the many known fossil forms and show only living groups. It isn't that tetrapods evolved from lungfish, of course, but that lungfish and tetrapods shared a common ancestor, and they shared that common ancestor with each other more recently than they shared a common ancestor with coelacanths. There is a large series of fossils filling the morphological gaps between ancestors of lungfish and tetrapods (Carroll 1998) and more are being discovered (Shubin, Daeschler, and Jenkins 2006).

Another interesting puzzle about the pattern of evolution is ascertaining the relationships among the phyla, which are very large groupings of kinds of animals. All the many kinds of fish, amphibians, reptiles, birds, and mammals are lumped together in one phylum (Chordata) with some invertebrate animals such as sea squirts and the wormlike lancelet (amphioxus). Another phylum (Arthropoda) consists of a very diverse group of invertebrates that includes insects, crustaceans, spiders, millipedes, horseshoe crabs, and the extinct trilobites. So you can see that phyla contain a lot of diversity. Figuring out how such large groups might be related to one another is a challenging undertaking.

Phyla are diagnosed on the basis of basic anatomical body plans—the presence of such features as segmentation, possession of shells, possession of jointed appendages, and so forth. Fossil evidence for most of these transitions is not presently available, so scientists have looked for other ways to ascertain relationships among these large groups. The recent explosions of knowledge in molecular biology and of developmental biology are opening up new avenues to test hypotheses of relationships—including those generated from anatomical and fossil data. Chordates for a long time have been thought to be related to echinoderms on the basis of anatomical comparisons (larvae of some echinoderms are very similar to primitive chordates) and this relationship is being confirmed through biochemical comparisons (e.g., ribosomal RNA) (Runnegar 1992). Ideas about the pattern of evolution can be and are being tested.

Process. Scientists studying evolution want to know not only the pattern of evolution but also the processes behind it: the mechanisms that cause cumulative biological change through time. The most important is natural selection (discussed in chapter 2), but there are other mechanisms (mostly operating in small populations, like genetic drift) that also are thought to bring about change. One interesting current debate, for example, is over the role of genetic factors operating early in embryological development. How important are they in determining differences among—and the evolution of—the basic body plans of living things? Are the similarities of early-acting developmental genes in annelid worms and in primitive chordates like amphioxus indicative of common ancestry? Another debate has to do with the rate and pace of evolution: do changes in most lineages proceed slowly and gradually, or do most lineages remain much the same for long periods that once in a while are punctuated with periods of rapid evolution? We know that individuals in a population compete with one another, and that populations of a species may outbreed one another, but can there be natural selection between lineages of species through time? Are there rules that govern the branching of a lineage through time? Members of many vertebrate lineages have tended to increase in size through time; is there a general rule governing size or other trends? All of these issues and many more constitute the processes or mechanisms of evolution. Researchers are attempting to understand these processes by testing hypotheses against the fossil and geological records as well as other sources of information from molecular biology and developmental biology (embryology).

Natural selection and other genetically based mechanisms are regularly tested and are regularly shown to work. By now there are copious examples of natural selection operating in our modern world, and it is not unreasonable to extend its operation into the past. Farmers and agricultural experts are very aware of natural selection as insects, fungi, and other crop pests become resistant to chemical controls. Physicians similarly are very aware of natural selection as they try to counter antibiotic-resistant microbes. The operation of natural selection is not disputed in the creationism/evolution controversy: both supporters and detractors of evolution accept that natural selection works. Creationists, however, claim that natural selection cannot bring about differences from one "kind" to another.

Pattern and process are both of interest in evolutionary biology, and each can be evaluated independently. Disputes about the pattern of evolutionary change are largely independent of disputes about the process. That is, arguments among specialists about how fast evolution can operate, or whether it is gradual or punctuated, are irrelevant to arguments over whether Neanderthals are ancestral to modern Europeans and vice versa. Similarly, arguments about either process or pattern are irrelevant to whether evolution took place (i.e., the big idea of descent with modification). This is relevant to the creationism/evolution controversy because some of the arguments about pattern or process are erroneously used to support the claim that descent with modification did not occur. Such arguments confuse different levels of understanding.

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