[Yesterday's post](http://scienceblogs.com/authority/2007/01/how_fast_can_evolution_work.php) on evolutionary speed limits and Haldane's Dilemma has sparked some interesting discussion, and some of the comments have already started to move beyond the very simple scenario that I outlined. Next week, I'll post a couple of more complex examples, and look at the effect of things like a lower frequency of mutants in the starting population, what can happen with two mutations being selected at the same time, and whether mutations need to be fixed to be evolutionarily meaningful. I'll also go over a couple of basic concepts that might help in understanding those scenarios.
Today, I'm just going to respond to part of [one of the comments](http://scienceblogs.com/authority/2007/01/how_fast_can_evolution_work.php#comment-325441) that was left on the last post. This is mostly because it's an interesting question that deserves a thorough response, partly because the question involves some basic concepts that should be explained before I dive into more detail, and partly because it's Friday and I really don't want to spend the time plugging numbers in to work up another example.
Caligula, fairly early on in the comments, raises a point that involves a concept that is very basic to evolutionary biology: fitness: