Jul 23 2007
In response to my Skeptics Circle entry last week, reader Tim left a comment decrying my reference to intelligent design proponents as “deniers.” In response I challenged Tim to give me one legitimate argument against evolution – the implication being that if there are none then the rejection of evolution as scientifically established is denial. Tim then responded:
“You are try to sucker me, aren’t you? Sorry. Not going to fall for it. I’m not aware of any evidence that disproves evolution.
It’s just that there is insufficent evidence to prove it. So I remain skeptical. For example, the existence of fossil evidence that show there at one time existed animals who in some way resembled horses, doesn’t prove that that horses are descended from those ‘proto-horses’. There is NO scientific evidence of descent. Possible but not scientifically proven.”
There are two significant errors in Tim’s response, one factual the other logical. The logical error is in the confusion of the concepts of “proof” vs “evidence,” which Tim seems to think are interchangeable.
Science, of course, does not deal with absolute proofs. Proofs are for closed systems of logic, like math. Science deals not with proofs but with evidence, deduction, and inference. Science also works with probability, not certainty. This is because we can never have a complete set of data – we are always extrapolating from a limited set of data.
So Tim frames his questions completely wrong. I was not asking for evidence that disproves evolution. What I actually wrote was, “Give me one single valid scientific argument against evolution and I will apologize for calling creationists deniers.” Tim took this to mean “disprove” evolution.
A more appropriate model of how science works, however, is that scientists propose models (theories) of how nature works and then they test those models. Evidence can either support or refute a theory, or perhaps may help distinguish one theory from another. Scientific theories that account for existing data and are confirmed by accurately predicting future data survive and flourish. Those that cannot account for or predict data are either abandoned or modified. In this way science builds an ever more complex and ever more accurate model of reality.
Philosophically there is a persistent discussion of whether or not science’s model of reality has any relationship to actual reality – the ultimate metaphysical reality of things, but I argue that this does not matter (and in fact we cannot know). All that matters from an operational point of view is that scientific theories are internally consistent, logically valid, and accurate in their predictions.
Let’s now look at the question of evolution from a proper scientific perspective. Evolutionary theory is a scientific model to explain the origin of life on earth. The theory states that all life descended from a common ancestor through a branching pattern of relationships. Incidentally, natural selection is a distinct theory proposing a mechanism for change over time and is independent of the theory of common descent.
Tim’s comment refers specifically to the question of common descent, and he concludes that “There is NO scientific evidence of descent.” This is a gross misstatement of fact, and actually he contradicts himself when he refers to the fossil evidence for horses. He refers to evidence, then says there is no evidence – but again, this is probably because he was confusing the concepts of proof and evidence. But now that this misconception is cleared up, let’s take a quick look at was evidence there is for common descent.
The fossil evidence is often what people consider first and foremost, although I think the genetic evidence is far and away more compelling. But the fossil evidence is solid. Evolutionary theory – the common descent part – predicts that fossils will exist in a temporal and morphological pattern that is consistent with a branching pattern of “descent with modification,” as Darwin himself stated it. This means we don’t find horses in the Cambrian era, we don’t find derived species appearing in the fossil record long before their parent species, and we don’t find species or branches of species that have no morphological connection to other life. Anatomical features should not appear out of nowhere. We should not see identical derived structures in different groups (who did not share a common ancestor with that structure).
We can make many such statements about patterns in the fossil record that are compatible with common descent and those that contradict common descent. What we have found in the fossil record is a pattern prediction by evolutionary theory – a pattern perfectly compatible with common descent. Of course, there are gaps in this record, and deniers will always appeal to those gaps, but that is a logical fallacy for another time.
With regard to horses, the notion of common descent predicts that when we investigate the fossil record we will find extinct species going back in time that are morphologically similar to horses but that fit into a morphological and temporal space between extant horses and a pre-horse common ancestor. This is exactly what we found. No, this does not metaphysically prove that any specific sequence of horse-like species actually evolved one into the other, but this is irrelevant. The fossils confirm what evolutionary theory predicts, and in this way it is evidence for common descent.
There are multiple other independent lines of evidence that also support common descent. When embryos develop they do not take a straight anatomical path to their final form, but rather take many twists and turns. This tortuous developmental path reflects the evolutionary history (history of common descent) of the species. The bones that make up the inner ear is mammals start out in embryonic jaw tissue, reflecting the fact that the inner ear bones evolved from reptilian jaw bones. Deniers, again, dismiss such “evidence” as “not proof,” but this is a non sequitur. It is evidence. It must be considered will all the other evidence – which, by the way – all points toward common descent.
There is also copious evidence from biochemistry. Common descent predicts that when we look at the biochemical composition of species we will find a branching pattern of relationships that closely overlaps with the morphological pattern. Species that look more closely related because of the details of their anatomy will also appear more related when we look at he sequence of amino acids in their proteins. What we will not find is, for example, that humans and copperheads have more similar hemoglobin than humans and panda bears. Within this field of evidence there have literally been thousands of opportunities to disconfirm common descent, but so far the evidence is all compatible with evolution (i.e. biochemistry provides copious evidence for common descent).
And, as stated, the genetic evidence is the most robust. The sequence of base pairs in DNA provides the ability to do powerful statistical analysis. DNA has certain features and complexity – like redundancy, and useless bits of dead viruses contaminating our DNA – that give us yet another completely independent layer of comparison among species. Yet again here is an opportunity to either confirm or disconfirm common descent. And yet again we see a pattern that is exquisitely compatible with common descent. We can also say that the odds of this pattern (even for a single gene, let alone the entire genome of every species) appearing in the pattern it does by chance alone is infinitesimally small – small enough to treat as zero. There is no scientific testable theory that can explain the pattern of DNA we find other than common descent. Not one.
Given the mountain of evidence for common descent, that multiple independent lines of evidence support common descent, that there is no competing theory that can explain this evidence, that the theory of evolution has survived countless opportunities for disconfirmation, it is intellectually absurd to conclude anything other than that common descent is an established scientific fact. Referring to such absurdity as “denial”, far from being unfair, is too kind.
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