a new comic every Tuesday
(Carney is an equal opportunity executioner.)
“By irreducibly complex I mean a single system composed of several well-matched, interacting parts that contribute to the basic function, wherein the removal of any one of the parts causes the system to effectively cease functioning. An irreducibly complex system cannot be produced directly (that is, by continuously improving the initial function, which continues to work by the same mechanism) by slight, successive modifications of a precursor system, because any precursor to an irreducibly complex system that is missing a part is by definition nonfunctional.” (Behe, 1998:39; underline added)This is basically correct! But notice the word “directly“. Biological evolution is not a direct process, like the creation of a comic strip, where the cartoonist goes directly (more or less) from concept to execution. Biological evolution takes twists and turns. Tetrapods go through all the trouble of developing limbs only for snakes and glass lizards to lose them. Basal dinosauromorphs become bipedal only for sauropodomorphs, hadrosauriform ornithopods, coronosaurian ceratopsians, and eurypodan thyreophorans to revert to quadrupedality. Dinosaurian lineages tend to progressively become larger—but birds (and possibly titanosaurs) go against the grain, producing smaller forms as time goes on (Hone et al., 2005). Reversals are rampant in the history of life. What works best in one context (e.g. large body size in the Late Jurassic) may not work as well in another (e.g. large body size at the Cretaceous/Paleogene boundary).
And reversals can explain the emergence of “irreducibly complex” systems. Suppose we see an organism that has feature A and feature B, and neither feature functions without the presence of the other. That is, A alone is impossible, and B alone is impossible. Then we cannot come up with a nice, simple, direct transition like A → AB, because the initial state is impossible. Must we then give up our search for a naturalistic explanation? No, because the combination AB may have come about because of a reversal. Suppose the existence of a feature C, which, like A, enables B. But unlike A, C does not depend on B. Then this incremental pathway is perfectly possible: C → BC → ABC → AB.
But this isn’t the main fallacy “Intelligent Design” proponents commit. More often the features they assume to be codependent are not. IDists have suggested the vertebrate eye as an example of something “irreducibly complex”, but in nature we see examples of eyes with lensless irises (Nautilus, marine snails, some bivalves, abalones, ragworms, etc.), retinas without irises (flatworms, some bivalves, polychaete worms, limpets, etc.), and simple photocells which are not part of any retina (some jellyfish, starfish, leeches, etc.) (Dawkins, 1996:ch. 5). The chain isn’t easy to see (pardon the pun) if you look only at the human eye, but we are fortunate in having a great variety of organisms we can compare.
Anyway, after the past four strips you’re probably all sick of hearing about anti-evolutionary pseudoscience (in the unlikely event you weren’t already), so, starting next week, it’s back to some good old-fashioned holiday fun!
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