bd,
Let me offer the disclaimer that I’m no expert but do spend a fair amount of time trout fishing here in the East.
As far as I’m concerned (and I think the trout feel the same way) match the color, size and profile of the natural and you’ve got it. Without getting into a biology discussion and latin lesson, I’d say they’re is not a big difference.
Here’s a much more detailed discussion if your interested. I find it fascinating:
http://www.flyanglersonline.com/features/nqento/part6.html