The Intelligent Design people argue, among other things, that organisms manifest 'irreducible complexity' (IC hereafter) that cannot be explained by us 'evolutionists'. They say that things like, say, our complex, focusing, camera-like eye, for example, can't have evolved one bit at a time, because the parts make no sense until the whole thing is put together. In fact, this was an example chosen by Darwin to exemplify the potentially devastating explanatory problem that his theory of gradual evolution by natural selection posed. Complex things simply can't arise out of thin air, by any process known to evolutionary biology, after all!
The IC people are essentially arguing against spontaneous generation, one might naively think: that was the idea, mooted about even by Aristotle, that a bit of goo and slop might come suddenly together and--poof!--arise as a ready-made organism, like a fly. Of course famous experiments by Spallanzani in the 1700s (and replicated by yours truly in one of my Evolutionary Anthropology columns only a few years ago) clearly discredited that idea, showing that maggots may appear to arise in dead meat as if by spontaneous generation, but in fact that only happens if a fly happens to have, modestly, laid eggs in the meat.
Darwin's fears were groundless, for two reasons. First, he suggested some possible paths from simple light perception to complex eyes. And second is the minor fact that he has been shown to be right! We now know that eyes of all sorts, simpler and more complex, have arisen many times independently, but usually sharing at least some developmental genetic basis, such as Pax genes in development and opsin genes in light-detection. And, to assuage his worst fears, all sorts of 'intermediate' types of eyes and light-sensitivity have been found.
Still, the IC campaign, a thinly disguised Christian biblical fundamentalism, carries on, apparently oblivious to its own manifest non-sensicallity.
But there is another sense in which IC may be a more profoundly true and legitimate aspect of biology and consequence of evolution (maybe we should not use the abbreviation in this case). A Commentary by Gray et al. in the 12 November 2010 issue of Science ("Irremediable Complexity?", p 920-921, accessible with subscription), asks whether there may be, in fact, an aspect of irreducible complexity in living Nature. (Yes, November 2010 -- I'm working my way through my "Urgent!" reprint stack this week!)
The authors argue that some macromolecular processes in life seem to have "gratuitous complexity". Among the examples they cite is the spliceosome, the structure that takes a messenger RNA molecule copied from a DNA 'gene', and removes the non-coding sequences (introns) that interrupt the coding parts (the exons), and then splices the exons together to form the single protein code. Why introns? Why such a complex structure to do this? How could organisms have evolved to need to spend--waste!--the energy (and hence needed food intake) to splice all their genes in all their cells all the time? It's not necessary to life, as bacteria clearly show (they don't splice). How inefficient!
Strict Darwinian selectionism that is argued to optimize everything all the time must answer such questions, but the suggested answers are often as post hoc and contorted as, well, as a spliceosome! But Gray et al. argue that understandable evolutionary processes have integrated bits of the current structure over time (again contra the allegation of the IntelligentDesigners!). Some steps, or perhaps the initial step may have evolved in the usual way for some reason that may or may not have been related to the current function in which the gene is used. Other steps may have arisen when causing no harm, or such trivial harm that in spite of that they became fixed in the genome by chance. If they happened--'chanced'--to have some effect, say by binding to the strongly selected gene-products, that could have been integrated into the genome (fixed in the very ancient species involved), and then as a result been impossible to remove without harm. Like a ratchet, step by step, complex structures could arise over zillions of years.
In this scenario, previously independent functions could become dependent, as illustrated in the figure from Gray et al. The two components arose on their own, for whatever reason, but mutationally later became hitched at the altar to form a new function or modify an existing one. Once the marriage is made, it cannot be rent asunder by selection, yet neither was installed specifically by selection.
This is the kind of partnership (what we call 'cooperation' in MT) that is so pervasive in life, indeed is so much at the core of how life works. It is in no way incompatible with reproductive success--due to chance or selection--but it was not selected 'for' as is implied by classical Darwinian explanations. It violates no principles of biology or evolution--and certainly doesn't constitute a 'paradigm shift'! It's just plain old evolution.
In this way, constructive evolution can also be selectively neutral evolution. Racheted increasing complexity may be an important or even fundamental aspect of life. We know from many examples that things once established can be removed or made simpler by mutation and selection, but ratched complexity can become so entrenched--so complex--that it can't just be disassembled in any old way to regain the beatific simplicity of its yesteryear.
This of course has implications for selective arguments about evolution. At the molecular level, complexity can be installed without specific strong or directional selection, and this can also be true at the organ or organismal level as well. Selection on higher-level traits, like say, complex behaviors, could occur, even strongly, and yet be distributed over so many different contributing genes that none of them would show statistically detectable evidence of selection.
Even behaviors could have evolved from simple to more complex, bit by bit, in the same ratcheted--neutral--way. Behaviors, like problem solving, often integrate multiple systems including vision, hearing, olfaction and so on, just as spliceosomes and their ilk evolved in augmented, incremental ways. Behaviors also involve perception, synthesis, strategizing, and choice of actions, each with its own only partially interdependent connections.
A point we repeatedly harp on is that the evolution of organisms is the evolution of multiply interacting factors that must cooperate to be successful. It builds piecemeal, and yet is 'reducible', even if we don't know all the details by any means. But even if not 'irreducible', simple reconstructive explanations may be difficult to construct. In this sense, adaptive explanations can also be very far from what actually happened. We can see a net result today, but the evolutionary history of its assembly may be easy to imagine, but difficult to prove or reconstruct. Not food for IntelligentDesigners, but irreducibly complex in retrospect nonetheless. Again, selection works on organisms, no matter how they work.