Instinct

One item that I had originally overlooked when thinking about Evolution is that fact that Natural Selection operates on behaviors, as well as the physical structures of an organism. (Of course, here the discussion is limited to animals - as plants to not have "behaviors" in the same fashion.) I suspect that this is due partially to the fact that I don't own any pets or working animals - and therefore have lacked the experience of watching an animal exhibit a complex behavior without having had the opportunity to learn that behavior from an older animal.

Like I said, I'm not trying to write a book report, so I'll refrain from a page-by-page retelling of the chapter. But there is some interesting stuff here, if you ever get around to reading it yourself. The basic gist is simple - certain inborn behaviors show grant a population of animals a competitive advantage of some sort, and is therefore retained. As further modifications that amplify the effect appear, they are retained, and the behavior is effectively reinforced. Again, domesticated animals are a useful model here - intentional selection has created a number of working dogs that show very specific behaviors (and, at the same time, lack others).

The trick with demonstrating the action of natural selection on behavior isn't much different that with an organism's physicality, although it is more difficult. The fossil record can show us certain physical traits of an animal - but is commonly mostly (if not completely) silent concerning behavior - witness the back-and-forth over whether or not Tyrannosaurs were predators or scavengers. In The Origin of Species, Darwin, with the help of the naturalist community is able to track down several variations on specific species (like cuckoo birds), and from their differing behaviors, put together a simple narrative on how the instincts may have evolved. It's not often a very complete picture, but it does illustrate how Natural Selection works on instinctive behavior, and broadens the scope of the Theory beyond the simply physical.