Aaah, after a long week of graduate studies, teaching, drinking insane amounts of high-priced Belgian triple-ferments, and insane amounts of Eiswein... I'm taking it easy, drinking Stash Green Tea Chai and hoping to actually get some sleep tonight... What do you think of competition? Vermeij's "Escalation Hypothesis" frames the idea that inter-specific competition is THE greatest single factor in Natural selection. Specifically, he basically claims that mollosc shells observed in the fossil record should become thicker (or at least more dense) the closer to present samples are taken; but also that drillers and shell-crushers should become a tiny bit more successful through geological time as well. Predators become more efficient at chowing down, prey become more efficient, or at least keep pace. Interesting idea. Anyhow, I'm drunk and caffiene buzzed... Good night! Time for the naked hotties...
Certainly, you have to evade your predators long enough to breed; but you also have to out-compete your relatives, who by definition are the individuals with needs like yours. I'm sure this is the reason for sexed size difference in sparrowhawks (European ones, Accipiter nisus ) for instance; it allows males and females to hunt the same territory without excessive competition for prey species.
Basically, speciation is a way of avoiding competition with your relatives.
I also think this is a strong driver in (for instance) bird migration; makes it possible to spread your competition around. And that moving into new habitats at the edge of an existing range is a strong driver for the same reason - less competition from your relatives; which in turn will produce speciation as the organism adapts to its slightly-changed environment.
Not that I'm a biologist, or anything.