Nick's mostly right, I think. The lower in the animal kingdom you go, the more that raw strength and fearlessness is responsible for the "dominance" that defines the alpha; it's the one all the rest back down from.
Thing is, humans build such complex social interrelationships that fearlessness and raw strength alone usually aren't enough to achieve dominance. Domineering personalities (ie "alphas") probably do possess those traits to a greater degree than other people, but they also bring other qualities to the table, especially as you move up the pyramid of social leadership.
How much of this can be developed by those who don't normally possess such traits? Probably the level of general fearlessness characteristic of an alpha is something you're born with and can't really be altered too much. But fear in certain contexts is very open to change. Once you've learnt to deal effectively in one context, there's just no reason to feel fear in that context any more. It's like once you've realised that approaching a strange chick is no big deal, there's no reason to feel afraid of it anymore; typically, guys no longer do. But in new situations, the level of general fear/fearlessness plays a greater role; the more fearful person will have to "train" himself to a greater degree to cope with repeat runs of the new situtation than would the less fearful "alpha" type.
There are a huge number of "contexts" in modern life, but nobody lives a life that throws him into all of them. So most of us, with some application, can probably learn to be more effective in the situations we regularly face, and adapt what we learn to situations similar to those. My point, in case it's unclear, is that, for humans, a significant amount of "alpha" type stuff can be developed.
Just a quick example. Nick went on about fighting. This one's obvious. Just because an alpha might not fear fighting doesn't necessarily mean he's good at it. I think it's pretty clear that most people can greatly improve their strength and fighting ability, and even if they are total pyssies and shyt themeslves at the thougth of getting into a fight, after a couple fights they can realize that with their "new powers" they can kick some serious azz. Clearly that's an example of a learned alpha trait. And even if a natural alpha also began training, it's not necessarily teh case that he would resume his advantage over a non-natural alpha, because if they are both equally trained, and both equally fearless (in the context of fighting, anyway), then other natural traits will be decisive (eg height, weight, speed, reflexes, etc), and I don't think there's any particular reason to suppose that the natural alpha will be necessarily endowed with an advantage in any of those. (Of course, fighting ability, though often important, is hardly decisive, or even the most important element, when it comes to human dominance relationships; I just used it as an example.)
In my own case, how much have I developed? I wouldn't say I'm a "constant alpha". I just can't be bothered with trying to "dominate" every group situation I find myself in. I don't particularly mind it if some other guy takes charge in a classic alpha way, but it totally depends on how he goes about it. I have very little tolerance for domineering wankers, so if it's like that, I make it pretty clear that *I'm* not gonna be putting up with any carp; how he deals with the rest of the group isn't much of a concern to me, unless he's being too unfair or obnoxious, in which case, yeah i will step in. I guess "leading from the background" is one way to look at it. I really don't care that this isn't "real" or "classic" alpha; it's not, and I couldn't give a fkk.