Evolution - or survival of the fittest (or luckiest) - is a readily provable fact, and one not limited to species: Evolution also applies to ideas (memes) with areas as diverse as religion, music, fairy tales, and urban legends.
The concept of evolution is simple: That which successfully reproduces, survives. If pressures (due to competition or predation) limit the growth of something which has a natural variation (a choice of religions, or music genres, or tales, or an ecosystem, or apes), those variants which most successfully reproduce will succeed versus those less suitable, or less lucky.
Note how few successful religions abound which forbid sex. There have been short term experiments in this direction. More subtly, religions that don't have a strong philosophy of proselytism tend to be dominated by those that do. Remember, survival of the fittest is really just survival of those that successfully reproduce. See The Purpose of Life.
Evolution of species continues today. Mankind is forcing change, which drives evolution. Some of this is merely speeding long term trends (such as reduced success of amphibians in general, or the loss of many marginal species like the spotted owl). Other changes are more worrisome (such as the evolution of anti-biotic resistant bacteria).
Especially with animals (including humans), there are two forces that dominate evolutionary pressures. In addition to reproductive success due to superior survival characteristics, there is reproductive success due to sexual selection (ie, how members of each sex choose mates). More colorful animals are easier to find, and the most showy male is likely to get the most females and successfully reproduce even though he is also the most visible to predators and ends up living a shorter life. Sexual selection may also explain extremely large sauropod dinosaurs; perhaps the males/females liked (or could see) tall females/males better - and sought them as mates - leading to runaway selection for this feature.
Even mankind continues to evolve in several ways, and indeed demonstrates evidence of very recent evolution.
For example, there is strong evidence that people have been evolving for external sexual characteristics. Human females have proportionally larger breasts, narrower waists, and broader hips than other primates. (As a human male, I do love that shape.) Human males have shapes that illustrate upper body strength, and have a penis that is larger in proportion to body size than any other ape. Apparently males have been selecting for large breasts and hips (especially in contrast to waist size). And women have been selecting men with broad shoulders, large muscles, and a big penis. And bad boys, at that.
Watching many reality shows (and especially MTV) suggests that human females are still actively selecting for strength, size, and sexual prowess; intelligence is clearly not a requirement. Likewise for human males, actively selecting voluptuous, athletic females with exotic eyes, long hair, and aggressive sexual attitudes.
Our technology is also having a significant effect on the human specie: we are becoming less diverse, as our ability to travel globally is reducing regional and racial disparities at measurable rates. In a few thousand years, there may be no remaining significant racial differences at all as we continue to interbreed and blend. Personally, I think this is a good thing.
We are also enabling the survival and allowing the reproduction of humans who would never live to adulthood without technology and/or large social organizations to care for them. I think this is a bad thing (when genetically caused), as I prefer that our children be smarter, stronger, and healthier. I know many people find my attitude offensive, but really, people, it is not in humanity's best long term interest to support or encourage the reproduction of serious genetic defects or low intelligence. Again, see my post The Purpose of Life.
A few other rambles:
Humanity is the ocean's most effective predator, and our fishing techniques are rapidly changing (evolving) fish to have less desirable characteristics. Fish are maturing younger and at smaller sizes as we select only the largest (previously most successful) fish. Fish that humans like to eat are being selected out - made extinct - versus undesirable, bony, badly tasting, or hard-to-capture fish. A tight school of fish may have worked to confuse dolphins or sharks, but it is an bright sonar target easily capture by our mile-wide nets today. And small and mid-size fish that avoid schooling behaviors make poor (unprofitable) targets.
Note that the world's most successful plants and animals are those whose evolution has made them desirable food for humans (cows, chickens, pigs, wheat, corn, rice, etc.). Then we help them thrive and reproduce, in numbers far exceeding natural populations.
Some people have argued against the use of windmills as a source of renewable electric power, based upon the fact that the turning windmills kill many birds. Tear down the windmills, drill for oil, save the birds. The reality is that more birds are killed by cars and trucks on the highway. (The activists would probably like to outlaw cars and trucks, too.)
I believe in the value of evolution: the birds that learn to avoid the rotating windmill blades will survive and reproduce. We can already see this effect along our highways: fifty years ago it was much more common to hit a bird on the highway, even though speeds were significantly lower then. Think of it as evolution in action (thank you, Larry Niven).
Last, the implications of evolution to a field near and dear to my heart: science fiction.
Contrary to nearly every movie alien, any intelligent life we meet in outer space will not be highly effective carnivorous killing machines. Au contraire, they will be (on their home planet) relatively weak and defenseless, needing superior intelligence to survive and reproduce. A dominant carnivore, or a herbivore that does not need to fear predation due to successful defenses (armor, size, quills, poisons) will cease to evolve. Every intelligent alien ever depicted with huge fangs, great strength, speed, armored skin, etc., is absurd, as they would never have evolved intelligence.
No, the most intelligent species will be those that are slow, weak, need protection from the elements, need to build and use tools to thrive, and need a civilization to defend against the superior strength, speed, and teeth of their planet's versions of lions, and tigers, and bears.
Of course, there is some evidence that intelligence is not a long term survival characteristic. We haven't yet met a single intelligent alien.