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I think Darwin's main theory included:
1. Heritability
2. Variability
3. Adaptation
4. Natural Selection/selection pressure.
and 5. Reproductive Isolation.
Suppose you had several members of a species of bird who landed on an empty island (similar to the Galapagos islands), and there was very little selection pressure, so that after a while the island was filled with birds. For the most part, the birds look like the original members of their ancestors, although there is variability (via 2). Now, imagine that selection pressure occurs (such as the weather creating an abundance of large nuts but fewer small ones). Those birds who are adapted (3) to crack those nuts (such as those with a large beak) are more likely to survive, and thus reproduce, than the rest. So, the next generation of birds will contain more large-beaked members (via 1), and fewer of the rest. Suppose this happens for two or three years (via 4, selection pressure), so that a third of the birds have distinctly larger beaks than the first generation, and perhaps are physically larger than the rest. Now, suppose things change, so that the weather supports only small nuts. The large birds are now at a disadvantage, but so are the medium-sized birds, relatively speaking. Only the few small-beaked, small-sized birds thrive. Suppose this goes on for a couple of years, so that you have, say, 10% large birds, 60% medium, 30% small.
Suppose the weather continues to alternate like this, creating each time favoring either the small or large birds, but never the medium. Eventually, suppose there are no individuals left who resemble the original ancestors with their medium size. Now, there are only large birds and small birds. The size difference, and perhaps, with enough time, genetic incompatibility, now makes reproduction impossible between the two. You have reached reproductive isolation (5). The two species now cannot reproduce.
This same process need not happen in the one place, but could also occur because a few members migrate to a different region with different selection pressures.
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