Natural selection1
Natural selection, stripped down to its essence, predicts that if a self-replicating organism varies between generations, and if that variance can be inherited through generations, and if that variation affects the ability of the organism to survive and reproduce itself, then the variations will effectively be ‘selected’ by the environment, shaping the evolution of the organism in a fashion that adapts it to the environment.2
This (so elegantly simple as to be almost trivial) idea, then, is the point of the Black Smoker applet. The applet uses your system’s random number generator to rough up the genomes of the organisms that are reproducing, during reproduction events, on an occasional basis (the occasions themselves selected randomly), in varying ways (for more details, read the metabolism page). But the rules that determine whether an organism, based on its genetic complement, ultimately survives to reproduce or dies, are purely mechanistic, and take into account precisely-defined features of the environment around the organism (this includes, naturally enough, features affected by its own, or by other organisms’ activities).
Further reading
On the modern synthesis of the theory of evolution—the canonical work is still John Maynard Smith’s excellent The theory of evolution. My copy is Cambridge, 1997.
Notes
1 It’s likely to seem almost absurd to many readers of this page it’s even necessary to provide this definition. But, as there is occasional misunderstanding and even misrepresentation of the theory in popular literature, I suspect a precis will prove useful to some.
2 Implicitly, then, natural selection is not a directional process. It does not necessarily lead to more and more complex organisms, in the sense of the Victorian belief in a great chain of being culminating in human kind. The theory just says random variation will go where the environment will let it. If being complicated increases an organism’s ability to survive and reproduce in an available niche, that organism may become pretty complicated (as did free-living vertebrates, for example) but it may also lead to simpler and simpler body plans (as in many parasitic organisms).