A few months back during one of the presidential primary debates someone asked the nominees "Do you believe in evolution?" Some answered 'yes' and some answered 'no', but frankly, both answers are misguided. The problem is that the term 'evolution' is multiply ambiguous, a fact which has resulted in massive amounts of confusion in discussions of Darwinism, creationism, and intelligent design.
Here are a few of the more common usages of the term 'evolution':
- Change in gene frequencies over time within a breeding population.
- The efficacy of 'natural selection' - the differential influence of environmental stresses on reproductive rates - to effect evolution in the sense of (1).
- Common descent - the idea that all organisms that have ever lived on Earth are part of a common 'family tree'.
- The sufficiency of random genetic variation (mutation, recombination, etc.) plus natural selection (construed broadly to include things like 'sexual selection') to account for the diversification of life on Earth.
- (3) + (4).
- Naturalistic 'abiogenesis' (origin of life) + (3).
- (4) + (6).
This list is not exhaustive. I have arranged them in order from less controversial to more controversial.
(2), I take it, includes (1). (3), I take it, includes both (1) and (2). (4) includes (1) and (2) but not necessarily (3) - it allows for the possibility of there being multiple, independent 'family trees'. (5) includes (1-4). (6) includes (1-3), but not necessarily (4). (7) includes all of (1-6).
Now, of these possible ways of construing 'evolution', only (1) and (2) are completely uncontroversial
. No one, not even the much-pilloried young-earth creationists, dispute them. And there's a very good reason for this consensus: We can and do observe these things happening in real-time
. Finch-beak variations and bacterial drug resistance are examples.
(3) is supported by some impressive evidence - the fossil record, DNA comparisons, homology, and such. Put it all together and the case for common descent may
be strong enough to constitute a preponderance of evidence. Whether that is so or not, I don't think that it has been established beyond a reasonable doubt. One of my reasons is this: Common descent is a very strong claim
- it says that all
organisms that have ever
lived on Earth (let's set the issue of ET's aside) are related. If it said "most" or "many" instead of "all" that would be another thing. Another reason for doubt has to do with the seemingly abrupt appearance of some of the major taxonomic categories (phyla, kingdoms, classes, etc.) in the fossil record.
Compared with (3), which has considerable support, (4) and (6) (and by extension (5) and (7)) have very little
(4) runs into a problem that has been acknowledged by Richard Dawkins and developed by Michael Behe. The problem is that many biological systems, especially at the molecular level, give strong appearance of being irreducibly complex
. Of course, this fact doesn't prove
that (4) is false. Critics of Behe have appealed to devices like 'exaptation' to argue that systems like the bacterial flagellum that appear to be irreducibly complex might
have arisen in step-wise fashion by the co-option of simpler components that were previous performing some function other than what they do in the flagellum. But this response is rather lame. It is quite a large step from showing that something might possibly
have happened, to showing that it did
happen, or that it is at all plausible
to believe that it did happen.
Similarly, (6) remains highly speculative. There are a lot of very interesting proposals on the table (see, e.g., here
), but all of the naturalistic proposals on the table face significant, seemingly insurmountable problems. And none, so far as I am aware, addresses the origin of biological information
. Maybe future research will solve the problems. But, then again, maybe it won't. We have no advance guarantee from the cosmos that the origin of life does in fact have a naturalistic explanation. It seems to me that any confidence that (6) is correct must stem not from actual data, but from an a priori
commitment to the idea that the actual explanation must
be a naturalistic one.
So, do I believe in evolution? Well, that depends on what one means by 'evolution'. I'm very confident in (1) and (2). I'm open to (3), but am not sold on it. As for (4) and (6), I'm pretty sure they are false.
I think my judgments on these points are fairly objective and informed, though I admit that I'm just a layman in this area. But regardless of whether I'm right about (4) and (6), I think any fair-minded person should
agree that they are far more dubious than (1) and (2) and that it is possible for intelligent and informed persons to entertain reasonable doubt about them.