Chance and Complexity
It helps to understand what they really mean. They do not mean that it is irreducibly random in the way that physicists do when talking about quantum mechanics. Darwinists are piously devout determinists. They believe that the variations arise from natural processes that obey the laws of physics and chemistry.
When the Darwinists say that the process is random, they mean that those processes are indifferent to evolution. They in no way direct evolution. And why are they so certain? Essentially, they believe nothing else influences evolution because they just cannot imagine anything else. Unfortunately, the limits of the universe are not necessarily coextensive with the present limits of our imaginations.
There are circumstances in which it would be legitimate to declare that events occur “by chance” (even though scientists and philosophers cannot agree on what “chance” really is). If we had what we could consider a complete understanding of all the living processes that are involved in bringing about variation and could confidently say that none of them have a role in directing evolution, it would be legitimate to say that variations arise by “chance.” Biologists are not even remotely close to being able to make such an assertion. But although Darwinists cannot conclusively prove randomness, they might be able to at least say that the process could be random: that a completely random process is quite sufficient and no other explanation is necessary.
This is where real controversy arises. A great many intelligent, well-informed, and thoughtful people have argued extensively that natural selection is not sufficient to explain evolution and that something more is necessary. They are invariably denounced as crackpot creationists. Such characterizations are false. Virtually none of these particular critics of Darwinism are creationists and only some of them are proponents of the theory of intelligent design (which, despite what you may have read, is not just another version of creationism). Most of them are highly respected scientists and mathematicians.
This is not the place to restate their arguments in full (and my purpose in any case is not to join their side of the dispute but to point out important things that are ignored by both sides of the debate). Still, it would be worthwhile to provide an overall sense of their reasoning so that you can see that it is not trivial or foolish. There are several different lines of thought, but most have to do with the way chance operates in highly complex systems. (If you are interested in more detail, look at Michael J. Behe's Darwin's Black Box, William A. Dembski's Uncommon Dissent, or Michael J. Denton's Nature's Destiny).
The fundamental problem is that living things are extremely complicated things. No human invention — even the product of many inventors — is even one one-hundredth as complex as any one of its inventors. Organisms are complex at every level. Each microscopic cell in your body is like a busy factory with many different molecular machines humming away bringing in raw material, producing a stream of products, and dispensing with wastes. The cells are complexly organized into organs which all have intensely complicated and dynamic relations with each other. The organism sustains itself and interacts with an ever-changing and unpredictable environment. Nothing in the non-biological world is anything like it.
Complexity by itself would not rule out random variation and natural selection as the sole cause of evolution. If it could be established that there is unbroken continuity in the design of all living things — a line of progression in design in which each variation was only slightly different from the proceeding one — then natural selection could easily explain how they might have all evolved from a single ancestor.
Try to imagine how random variation and natural selection might influence the design of a human product — aircraft, for example. It is certainly true that the design of airplanes has evolved in the past one hundred years, and there is at least a degree of continuity in the design. That is, the airplanes built in one year are only somewhat different from those built in the previous year and those only slightly vary from the year before. But the continuity is broken in many places, and it is hard to envision its not being broken. It is easy to imagine a gradual, continuous evolution in the shape of the wing — from a straight wing to a delta wing, for example. But how could one evolve from a piston engine to a jet engine? It is difficult if not impossible to imagine a series of intermediate steps that would be “fit” (in Darwinian terms). They would not work, and thus would not survive the selection process. The critics of Darwinism maintain that biological systems are filled with such discontinuities.
The Darwinists respond by showing that the critics have not proved that continuity is impossible and have suggested some potential ways in which it could possibly have occurred. They point out that variants that are first merely neutral (they do not make the organism either less or more fit to survive and reproduce) could latter become useful and finally necessary. They cite the fact that a number of apparent discontinuities have been plausibly, if conjecturally, filled in. They argue, quite correctly, that the fact that one cannot imagine how a particular gap could have been filled does not mean the gap is empty: it could simply be a failure of imagination.
Then, instead of offering positive arguments for Darwinism, they usually turn their attention to attacking the alternate theories of intelligent design and creationism (often insisting that they are really the same thing) as unscientific — which they assuredly are.
So, the Darwinists cannot specify how these
necessary transitions occurred; they merely insist that they could have occurred and that they are
certain they did occur. Their belief is firm even though they are talking about systems — biological
processes — that continue to be extremely mysterious. As the mathematician David Berlinski asks,
could a system we do not completely understand be constructed by means of a process we cannot
completely specify?
[emphasis in original] His answer, and the only sensible answer: we cannot possibly
know. We cannot
answer that question at this time. We are ignorant.