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?
If you guessed PZ Myers, think again. It was John Derbyshire at the National Review.
What's the world coming to when John Derbyshire is the right wing's voice of reason?