I would though like to draw attention to the following bit of weaseling.
"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."
What's going on here is that Brownback has got a whiff of the notion that living species indisputably do change over time. This is so well established that the old creationist position—that species do not change over time—has had to be abandoned. Creationists have retreated to this new position: "Yes, OK, a given species does change over time, but never into a new species." You could summarize this as "micro-evolution yes, macro-evolution no." It's a common creationist line of argument.
The problem with this position is, that you need to observe—or at least, darn it, hypothesize—some mechanism that stops the micro before it goes macro. (Not to mention that you have to posit some mechanism, other than macro-evolution, for the origin of species... But leave that aside for the moment.)
Take, for example, allopatric ("different homeland") speciation. You have a population of living, sexually-reproducing organisms, all belonging to the same species (i.e. able to mate with each other). You observe variations within the population. You further observe, watching across several generations, that some variations (red hair, schizophrenia) are heritable in whole or part, some (appendectomy scars) are not heritable at all.
Now you divide your population in two: Population A and Population B. You separate them geographically. (Hence "diferent homelands.") You observe that A and B have different "menus" of heritable variation (A has more redheads, B more schizophrenics). You further observe that A's and B's environments are different—A's is hot and dry, B's cool and wet.
You sit back and observe for a few thousand generations. Yep, microevolution goes on. A changes, B changes. Because they started out with different menus of heritable variations, and because environmental pressures in the two places are different, they change differently. They diverge. A thousand generations on, the two populations look and behave differently from each other. Ten thousand generations on, they look and behave way differently. Orthodox biology ("Darwinism") says that eventually they will be so different, they can no longer interbreed. Speciation will have occurred. A and B are now two species.
Under Brownbackian evolution—micro yes, macro no—this can't happen. They can't go on diverging. They can only get so different, no more. The divergence must slow down and stop. But... what stops it? What's the mechanism?
I'm not expecting Sam, or any other creationist, to give me an answer. I do wish Sam had at least confronted the question honestly, though. Like this, perhaps:
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 macroevolution, those small changes accumulating all the way to speciation, then I reject it.
And Sam at least has the interests of scientists at heart.
While no stone should be left unturned in seeking to discover the nature of man's origins, we can say with conviction that we know with certainty at least part of the outcome.