Who cares about lack of evidence

One of the common strategies of creationists and proponents of 'Intelligent Design' to discredit the theory of (Darwinian) evolution is to point out the supposed lack of evidence the theory has. Aside from the obvious reason that the overwhelming majority of falsification of evidence having been brought forth by these people in the past ranged from misunderstandings to utter nonsense, this claim always bothered me on a more principal level.
I think I now understand why. Even if they were right and all the little pieces of evidence that have been collected over the years were just plain wrong - it would not really matter that much. The assumption that it would shows a deep misunderstanding of the way science works. Let me explain why.

First we have to make clear what we are talking about. The "theory of evolution" creationists et al. make so much fuss about is essentially the assumption of modern biology that evolutionary processes (in the wider sense, i.e. including drift, gene flow and extinction) are solely responsible for the emergence of the diversity of life on earth from a single ancestral life form. (Many creationists confuse that with theories on the origin of life, but that is a different matter.)

Digression I: kinds of evolution

Creationists like to distinguish between micro-evolution (changes in a population over time) and macro-evolution (appearance and disappearance of species) claiming that the two are qualitatively different phenomena. Although this disctinction is not part of mainstream biology we will keep it here just for the sake of the argument.
Anti-evolutionists usually (nowadays) have no problems with biologie's understanding of "micro-evolution". The thing they hate and that the whole debate is about is how "macro-evolution" is explained by evolutionary theory.

I assume the goal of anti-evolutionists is to discredit evolutionary theory and replace it with something more to their liking such as the "theory" of Intelligent Design which involves some divine intervention.
In science theories that explain something reasonably well are kept around until either they are proven invalid or a theory that gives a better explanation comes around. If anti-evolutionists want us to abandon the theory of evolution they therefore either have to prove it invalid or provide something better.

Digression II: kinds of theories

I think what most people have in mind when they talk about a "theory" is something like Newtonian mechanics or relativity. A clearcut, simple (as opposed to complex) mathematical model that describes some part of the "inner workings" of our world.

The theory of evolution is very different. It consists of three statements which in very simplified form look like this:

1. If there is heritable variation between individuals in a population which matters for their chances of survival or reproduction then evolution will take place which ultimately also can lead to the split of populations into distinct species.
2. The physiology and ecology of actual biological individuals provides reproduction/survival-relevant heritable variation so that actual populations can evolve.
3. The process of evolution has occured in the past and is (solely) responsible for the diversity of life on earth.

Epistemologically these are very different kinds of statements.

The first part is a essentially a theory about a specific type of emergence in complex systems. It describes how given certain conditions concerning the elements of a system certain mechanisms lead to a specific behaviour of the system. In this sense the first part is entirely a logical statement and has nothing to do with reality.
The second part is the assumption that the mentioned conditions can occur in our world and that the mentioned mechanisms are compatible with the laws of physics/chemistry/etc.
The third part is the statement that these processes have actually occured in the past and are responsible for the appearance of a certain aspect of the world as we see it.

In short proving the theory of evolution invalid could be done by showing that the theory is logically inconsistent (i.e. that evolution a priori can not happen) or that it contradicts more fundamental laws of e.g. physics or chemistry (i.e. that evolution can not happen in our universe or on our planet).

Digression III: kinds of being wrong

I usually tend towards a rather constructivist point of view but just for the sake of the argument let us for a moment assume there is some objective describable reality with respect to which our theories can actually be wrong.

The theory of evolution (and similar theories about the history of complex systems) can be wrong in three ways:

1. It can be logically inconsistent, i.e. the assumed conditions do not lead to the described processes happening or the assumed mechanisms have a different outcome.
2. It can be inconsistent with the laws of physics, i.e. although evolution might happen the way we describe it, it can not do so in our world since the preconditions can never be met or the mechanisms can not take place.
3. It can be historically inaccurate. This means that although evolution could happen in our world it did not do so in the past or at least not on a sufficient scale to actually produce the diversity of life as we know it.

'Lack of evidence' however (even if it would apply) does not disprove the theory of evolution, at best it weakens its explanatory success.
That only leaves the second option - coming up with a better theory. As with every other topic many clever people have spent their lives thinking about what constitutes a good theory. The current mainstream version goes something like this:
A good theory has to be logically consistent and able to explain the phenomenon in question. It has to be falsifiable - a theory that can not possibly be proven wrong belongs to faith and not to science. Given two good theories, the more parsimonious, i.e. the one needing fewer assumptions is considered better.
The assumption of the existence of a supreme being with limitless power does not strike me as particularly parsimonious not to mention the fact that its alleged unpredictability makes every theory based on its behaviour by definition unfalsifiable.

Therefore, even *if* creationists et al. were right concerning the lack of evidence for evolution, I have to say, given the rather lousy alternatives, I will stick with it. Feel free to prove me wrong.