Stephen Jay Gould's paraphrase of Darwin in Ever Since Darwin:
- Organisms vary, and the variations are inherited by their offspring.
- Organisms produce more offspring than can possibly survive.
- On average, offspring that vary most strongly in directions favored by the environment will survive and propagate. Variation will therefore accumulate in populations by natural selection.
With this in mind, notice that survival of the fittest has nothing to do with the better triumphing over the worse: it is a tautology. 'fittest' is defined as that which survives. In any environment of replicators featuring variation, selection and heredity, evolution just naturally occurs: traits for longevity, fidelity of reproduction, and fecundity will be passed on in greater proportion than the alternatives with each generation (what these traits are depend on the environment, which is subject to change -- one of the things that keeps things so interesting.) This has nothing to do with 'progress' in the sense of things getting 'better' (though there is a tendency toward greater complexity.)
Something I have to make clear: none of this operates for our benefit. It has nothing to do with making our lives better, or making us happier. It's just something that happens; something that has happened for all of the history of life -- a process we're stuck in the middle of.
If tomorrow a mutant superpeacock were born, that was stronger, faster, smarter than every other peacock in the world because he had no tail and thus had so much biological wherewithal to expend elsewhere, all those traits would die with him, because he couldn't get laid. Peahens have undergone selection to want to mate only with peacocks with impressive tails. Sexual selection is a powerful force, and once a snowball like "big tails good" starts rolling, it can pick up a lot of speed.
But mortality selection is powerful, too. Imagine a change in the environment that began to select against big-tailed peacocks -- say there was some new predator that could only find, or could only catch, or just plain preferred the taste of big-tailed peacocks. Suddenly the superpeacock would start to look awfully good in a last-man-on-earth kind of way. And it's not only the case that tail-less genes would be winning out over big-tail genes by default because those with the latter were getting killed, but the genes of the peahens most willing to mate with tailless peacocks would be getting passed on more often than those of peahens who held out for big tails: there would be selection for peahens preferring taillessness.
Some generations down the line, all peacocks would be tailless, and if some throwback had a magnificent tail, then he couldn't get laid.
Kurt Vonnegut understood this in his Galapagos in which he changes humanity's environment such that we don't have the physical means to create technology, and diving is our only source of food. So a pointy streamlined head offers a greater survival advantage than intelligence. Thus, over the course of a million years, evolution results in a human race of pointy-headed idiots who are good at diving.
Now, I'm not immune to falling prey to the fallacy that human intelligence is some evolutionary pinnacle -- it took some effort for me to wrap my mind around that one. But it checks out. (Now if only someone could explain to me why this is supposed not to be science fiction. Oh yeah. Because Vonnegut is a savvy and cynical marketer and most people would sooner let a label on a book spine determine their opinions than think for themselves.)
A lot of people make the mistake of assuming that because some trait exists there must be some specific reason for it. For instance, in a discussion on one blog I read, a young woman wondered: "what could possibly be the evolutionary benefit to menstrual cramps?"
There isn't one. There doesn't have to be. Mutation happens and the absence of selection against something can be all it takes for it to persist. In the environment in which humans evolved, women were pregnant or lactating for pretty much all of their short adulthoods. Menstrual cramps would have had so little opportunity to occur that they'd have been inconsequential as a force in evolution. Again, evolution doesn't exist for our benefit; we exist as an effect of evolution.
And at any given moment, it's the case that the traits that exist have not been selected for their suitability to the current environment. They are, rather, those traits which have been selected by all past environments. Like the prospecti say, past performance is no guarantee of future results. If a trait is prevalent, we can say it has been successful, but no matter how prevalent it is, how successful it has been, it's meaningless in the face of a changing environment. Mother Nature doesn't extend credit -- she's always asking "What have you done for me lately?" If the current environment selects against your traits, i.e. if you can't survive or reproduce, you're out of the gene pool with no possibility of appeal.
Susan Blackmore in The Meme Machine writes:
Darwin's theory of evolution by natural selection is, to my mind, the most beautiful in all of science. It is beautiful because it is so simple and yet its results are so complex. It is counter-intuitive and hard to grasp but once you have seen it the world is transformed before your eyes. There is no longer any need for a grand designer to explain all the complexity of the living world. There is just a stark and mindless procedure by which we have all come about -- beautiful but scary.