Re: What predictions has the evolutionary model made?

The theory of evolution makes several predictions, and I'll tell you about two of the most important, in my mind.

From the idea that everything has evolved from some primitive common ancestor, we predict that the fossil record should show a progression of complexity in organisms. Simpler organisms should appear earliest in the record (i.e., their lineages should be much older), and more complex organisms should appear later. For example, the 'simplest' organisms are prokaryotes, bacteria and other extremely organisms that have no nuclei or organelles. We predict that these should be the oldest fossils. Along the animal lineage, we would predict invertebrates to appear first, followed by fish, amphibians, reptiles, and mammals. This is exactly the order they appeared in the fossil record. The oldest known fossils (around 3.7 billion years old) are prokaryotes. Invertebrates appeared 700 million to 1 billion years ago. Fish appeared after that, followed by amphibians, reptiles, and mammals, in that order. The same type of progression from primitive to advanced organisms is found in the plant fossil record. For references to these successions, see Strickberger (1990) or Ridley (1996).

A second prediction that evolutionary theory makes is that as organisms evolve to different habitats and their body parts take on different functions, some organisms should have useless remains of once important structures. These are called vestigial structures, and they are found in many organisms. Good examples of these (also from Ridley, 1996) are found in the hind limbs of whales and pythons. Whales have no external hind limbs, yet they do have some bones we have in our hind limbs: a pelvis, femur, and ischium. These are vestigial structures. Pythons and other related snakes do have an external hind limb, but it is very small, and difficult to see. Snake workers refer to them as spurs. Skeletally, they consist of a femur and an ilium.

People often speak of vestigial structures as disappearing over time. Whether or not this occurs often depends on how much it costs the organism to keep the vestigial structure (Does it take a lot of energy to make the structure? Does it make the animal more vulnerable to predators or less able to function?). If it costs a lot, there could be selective pressure to lose the vestigial structure, depending on how much variability is present in that structure. With whales, they obviously pay a price to make their hind limbs, and perhaps they could swim or function better without them (I cannot say); however, if all whales make a pelvis, natural selection cannot operate to reduce the limb size.

You also have parts that are considered vestigial. Your appendix is one. If there were selective pressure against people having an appendix and some people who did not have an appendix (naturally, not through surgery), it might disappear through several generations. Right now, though, everyone has an appendix or the genetics that gave him or her one. Furthermore, with modern medicine, the selective pressure against having an appendix is not very strong; if you get appendicitis, it is removed and you usually recover. On the other hand, if everyone with appendicitis died and some people had no appendix, the story would be different. Thus, vestigial structures are likely to hang around in many cases. They are useful in giving us ideas of evolutionary relationships.

References:
Ridley, M. 1996. Evolution, 2e. Blackwell Science, Inc. 719pp.

Strickberger, M.W. Evolution, 2e. Jones and Bartlett Publishers International, London. 670pp.

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