Senator Brownback has an op-ed in the New York Times explaining his views on evolution. Here are highlights:
The question of evolution goes to the heart of this issue. If belief in evolution means simply assenting to microevolution, small changes over time within a species, I am happy to say, as I have in the past, that I believe it to be true. If, on the other hand, it means assenting to an exclusively materialistic, deterministic vision of the world that holds no place for a guiding intelligence, then I reject it.
There is no one single theory of evolution, as proponents of punctuated equilibrium and classical Darwinism continue to feud today. Many questions raised by evolutionary theory — like whether man has a unique place in the world or is merely the chance product of random mutations — go beyond empirical science and are better addressed in the realm of philosophy or theology.
The most passionate advocates of evolutionary theory offer a vision of man as a kind of historical accident. That being the case, many believers — myself included — reject arguments for evolution that dismiss the possibility of divine causality.
Ultimately, on the question of the origins of the universe, I am happy to let the facts speak for themselves. There are aspects of evolutionary biology that reveal a great deal about the nature of the world, like the small changes that take place within a species. Yet I believe, as do many biologists and people of faith, that the process of creation — and indeed life today — is sustained by the hand of God in a manner known fully only to him. It does not strike me as anti-science or anti-reason to question the philosophical presuppositions behind theories offered by scientists who, in excluding the possibility of design or purpose, venture far beyond their realm of empirical science.
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This statement on evolution demands a response. First, the key distinction that Brownback tries to draw--accepting microevolution (evolution within a species) but implicitly rejecting macroevolution (evolution from one species into another)--is one that no serious biologist would accept. The process that would lead to microevolution is exactly the same as that would lead to macroevolution. The only real difference is time--macroevolution takes much more time.
Creationists accept microevolution because the evidence for it is really irrefutable and because it is consistent with a young earth. But if you accept that the Earth is 4 billion years old (and not 6000 years old), the same process that leads to microevolution should result in massive macroevolution as well.
Second, while there is some debate within the scientific community about evolutionary details, the overwhelming scientific consensus is that evolution lead to the development of species (including humans) and that natureal selection was the vehicle for doing so. Thus, Brownback's handwaving about punctuated equilibrium and classical Darwinian theory displays a profound misunderstanding of evolutionary science.
Finally, the dichotomy that Brownback offers between accepting microevolution versus "an exclusively materialistic, deterministic vision of the world" is a false one. Dr. Francis Collins is a good example of a devout Christian who rejects an exclusively materialistic vision of the world, but who also accepts macroevolution.
For a more detailed take on the Brownback op-ed (albeit from a very athiestic point of view), check out P.Z. Myers' blog.