In one of the comments to my recent posting, “Why Not Teach the Controversy?”, someone called “Navy Davy” repeatedly called for evolutionary biologists to provide the “best evidence … that supports the theory of evolution.” Merely by asking the question in that way, Mr. Davy displays a woeful misunderstanding of science and how science works.
Simply put, there is never a single best piece of evidence; there is no crucial experiment or observation that will validate any theory whatsoever. Theories are accepted according to whether or not they explain data better than some competing theories - a mountain of evidence, if you will.
Let me give a simple, not entirely hypothetical example. Suppose that you have developed a theory that the relative atomic masses (atomic weights) of atoms ought to be integers because the nuclei are made of neutrons and protons and you think that neutrons and protons have the same mass. The relative atomic mass of any atom is proportional to the mass of the atom divided by that of carbon, whose relative atomic mass is defined as 12.
You start by weighing the elements in order and calculating their relative atomic masses. Hydrogen: 1.008. Pretty close. Helium: 4.003. Lithium: 6.941. Beryllium: 9.012. Boron: 10.81. Hmmm; a little off. Nitrogen: 14.007. Oxygen: 16.00. Neon: 20.18. Pretty good agreement, you think; my theory is on the right track. No single measurement, however, is critical; rather, it is the consistency of all the measurements that convinces you that you may have a valid theory.
You go along happy as a clam at high tide until you come to chlorine. True, the masses of the other elements are not exactly integers, but they are close, so you infer that you are on the right track but that something more may be going on. Maybe protons and neutrons have slightly different masses, for example.
Chlorine, however, has a relative atomic mass of 35.453 - almost exactly between 2 integers. What’s going on? You have several choices, among them: put away chlorine for a later date and continue measuring, throw away your theory and become a lawyer, or try to find out what’s anomalous about chlorine.
At first glance, the case of chlorine seems to falsify your theory. Indeed, a single disconfirming measurement is potentially more serious than any number of apparently confirming measurements. It takes only one blond to falsify the theory that all Europeans have black or brown hair. So you decide to examine chlorine more closely. You formulate the ad hoc hypothesis that chlorine has two or more isotopes and naturally occurring chlorine is a combination of two or more isotopes. Further, each isotope (you theorize) will have a relative atomic mass that is an integer. You devise a means for separating the isotopes of chlorine and, sure enough, it has several isotopes whose average mass is 35.453. You go on and measure the rest of the elements and find others that have more than one isotope.
You have evidence now that atoms are made up of protons and neutrons, and you can even figure out how many of each reside in each atomic nucleus. Other evidence includes the (absolute) masses of protons and neutrons, which add up correctly to give the masses of all of your atoms. Radioactive decay of atoms further supports your theory. Finally, you build an atom smasher and physically knock helium nuclei from some heavy atoms.
You have, let us say, three bits of evidence to support your theory: the relative masses of atoms, the actual masses (in kilograms), and the atom-smashing experiments. Which one is crucial? None. What convinces you is the body of evidence.
Still, there is an anomaly in your theory: The mass of an atomic nucleus is not exactly equal to the masses of the constituent protons and neutrons. Does the fact rule out your theory? Not at all; Newtonian physics could not explain why the orbit of Mercury didn’t quite close, but it was a tremendously successful theory all the same. The mass difference is, similarly, a smallish anomaly that you will leave for future researchers. Your theory has been too fruitful to be discarded just because of a small discrepancy, and you have been rewarded by being removed from science and made the department chair.
Along comes a layperson who has a rudimentary knowledge of physics. He does not believe in protons and neutrons, and asks you for your “best” evidence. You reply, well, there is no “best” evidence. Here is the evidence: And you outline your reasoning in detail.
The layman is not satisfied and keeps bugging you. He seems to think that nuclei are little, indivisible billiard balls created by God with their present masses. He has, however, no evidence to support his contention that the nuclei are indivisible and cannot refute your atom-smashing experiments. So he reverts to gaps in your record, such as the discrepancy between the masses. He claims that the nucleus could not stick together because of the electromagnetic repulsion between the protons. When you explain about the strong nuclear force, he becomes impatient and announces that you have just made that up to support your failed theory.
Finally, he asks you to debate. There is, however, nothing to debate. No reputable scientists support his position. His arguments have no scientific merit, and he has proposed no new experiments, made no new observations. You explain these (to you) obvious objections, but he insists on a debate and eventually calls you supercilious and a “dogmatic protonist.” He may even mount a public-relations campaign to have his pet theory of nuclear indestructability taught alongside standard physics and chemistry.
The state of evolutionary biology is analogous to the atomic mass theory. It is supported by several separate but linked streams of evidence: paleontological, morphological, genetic, and developmental, all of which are tied together by a wealth of mathematical, statistical, and computational theory. There is no single “best” bit of evidence, any more than a table has one crucial leg; the theory is a unified whole and is as well supported as any theory in science. It is questioned almost exclusively by laypeople who would not have the audacity to challenge any other settled scientific theory but unaccountably think that evolution is fair game.