What does "Survival of the Fittest" mean? Imagine that some predatory animal happens to be born with slightly better vision than average. This improved vision will give it an advantage while hunting, so it'll be more likely to survive and pass on the genes for that good eyesight to future generations. Eventually, the individuals with good eyesight genes will dominate over those without them, and the species as a whole will change, becoming more "fit".
This simple example is a pretty good description of how evolution works. Unfortunately, nature is rarely so simple. The same genes can often control more than one physical trait at a time. For instance, what would happen if the "good eyesight" genes in our example also resulted in purple toes? Our hunters would have to evolve this extra trait too, despite the fact that it doesn't necessarily make them any fitter than before.
While purple-toed, keen-eyed hunters are just a made up example, a real animal that demonstrates this is the three-spined stickleback fish of western Canada. These fish moved into glacier-carved lakes thirteen thousand years ago. Over time, they evolved into two separate species. The original is short and thin, and has a small mouth, while the other is long, fat, and wide-mouthed. Why did they evolve this way?
Biologist Dolph Schluter has recently discovered that all three traits-length, thickness, and mouth size-are linked together on the same genes. Although the fittest possible stickleback might be both slender and wide-mouthed, these linked traits make such an evolutionary jump unlikely.
So evolution doesn't always take the straight road to fitness. It often detours along a crooked path.