Testing the Big Idea

Hypotheses about evolutionary phenomena are tested just like hypotheses about other scientific topics: the trick (as in most science!) is to figure out how to formulate your question so it can be tested. The big idea of evolution, that living things have shared common ancestors, can be tested using the if-then approach—testing by consequences—that all scientists use. The biologist John A. Moore suggested a number of these if-then statements that could be used to test whether evolution occurred:

1. If living things descended with modification from common ancestors, then we would expect that "species that lived in the remote past must be different from the species alive today"

(Moore 1984: 486). When we look at the geological record, this is indeed what we see.

There are a few standout species that seem to have changed very little over hundreds of millions of years, but the rule is that the farther back in time one looks, the more creatures differ from present forms.

2. If evolution occurred, we "would expect to find only the simplest organisms in the very oldest fossiliferous [fossil-containing] strata and the more complex ones to appear in more recent strata" (Moore 1984: 486). Again going to the fossil record, we find that this is true. In the oldest strata, we find single-celled organisms, then simple multicelled organisms, and then simple versions of more complex invertebrate multicelled organisms (during the early Cambrian period). In later strata, we see the invasion of the land by simple plants, and then the evolution of complex seed-bearing plants, and then the development of the land vertebrates.

3. If evolution occurred, then "there should have been connecting forms between the major groups (phyla, classes, orders)" (Moore 1984: 489). To test this requires going again to the fossil record, but matters are complicated by the fact that not all connecting forms have the same probability of being preserved. For example, connecting forms between the very earliest invertebrate groups are less likely to be found because of their soft bodies, which do not preserve as well as hard body parts such as shells and bones, which can be fossilized. These early invertebrates also lived in shallow marine environments, where the probability of a creature's preservation is different depending on whether it lived under or on the surface of the seafloor: surface-living forms have a better record of fossilization due to surface sediments being glued together by bacteria. Fossilized burrowing forms haven't been found—although their burrows have. It might be expected to find connections between vertebrate groups because vertebrates are large animals with large calcium-rich bones and teeth that have a higher probability of fossilization than do the soft body parts of the earliest invertebrates. There are, in fact, good transitions that have been found between fish and amphibians, and there are especially good transitions between reptiles and mammals. More and more fossils are being found that show structural transitions between reptiles (dinosaurs) and birds. Within a vertebrate lineage, there are often fossils showing good transitional structures. We have good evidence of transitional structures showing the evolution of whales from land mammals, and modern, large, single-hoofed horses from small, three-toed ancestors. Other examples can be found in reference books on vertebrate evolution such as those by Carroll (1998) orProthero (2007).

In addition to the if-then statements predicting what one would find if evolution occurred, one can also make predictions about what one would not find. If evolution occurred and living things have branched off the tree of life as lineages split from common ancestors, one would not find a major branch of the tree totally out of place. That is, if evolution occurred, paleontologists would not find mammals in the Devonian age of fishes or seed-bearing plants back in the Cambrian. Geologists are daily examining strata around the world as they search for minerals, or oil, or other resources, and at no time has a major branch of the tree of life been found seriously out of place. Reports of "man tracks" being found with dinosaur footprints have been shown to be carvings, or eroded dinosaur tracks, or natural erosional features. If indeed there had not been an evolutionary, gradual emergence of branches of the tree of life, then there is no scientific reason why all strata would not show remains of living things all jumbled together.

In fact, one of the strongest sources of evidence for evolution is the consistency of the fossil record around the world. Another piece of evidence is the fact that when we look at the relationships among living things we see that it is possible to group organisms in gradually broader classifications. There is a naturally occurring hierarchy of organisms that has been recognized since the seventeenth century: species can be grouped into genera, genera can be grouped into families, and on and on into higher categories. The branching process of evolution generates hierarchy; the fact that animals and plants can be arranged in a tree of life is predicted and explained by the inference of common descent.

we can test not only the big idea of evolution but also more specific claims within that big idea. Such claims concern pattern and process, which require explanations of their own.

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