Whew! That middle took us quite a while, but it was finally the combination of [SUN]YATSEN (36D: Revered Chinese figure) (Thank goodness Frannie has been reading Pearl S. Buck!) and [THU]CYDIDES (37D: Athenian general who wrote "History of the Peloponnesian War") (Thank goodness we're both such nerds!) that finally did it for us. We thought of the latter first, but when we had everything but the first letter for the first one, and it just had to be "Sun," well, then the rest of the puzzle flew by like a short work-week. The last, REMAIN[SAT] (15D: Doesn't leave) is, by far, the weakest of the bunch, but the rest are so far above it that no sour taste is left.
The rest of the fill is mostly solid, with some very good and some less-than-good. Loved 31D: Some keep waiting for them (TIPS), and 40D: Nonspeaking role on "CSI" (DEADBODY) was very nice. I yelled out "They got me AGAIN!" after putting in TAU (27D: Leader of ancient Troy?). When, oh when, will I be prepared for those?!
Frannie took issue with VARIATION (25A: Basic process of genetics), calling it a "result" and not a "process," but I think that it could be seen the other way, too. I mean, the imperfections inherent in the DNA system lead to variation, and it is through variation and adaption that gene mutations are carried on. Still, the wording is not perfect. Neither is it perfect, I don't think, in the ARE[WED]ONE (26D: "Anything else that you require?") clue. It seems that the tone of the two statements is too disparate. The clue seems polite, reverential even, and the answer is crude and rude.
Still, overall, it was a good Thursday challenge.