Evolution: The change in the frequency of alleles in a given population.
The theory of evolution is remarkable in its conceptual simplicity, matched only perhaps by Einstein's theories of relativity. Instead of, as discussed on the previous page, a monstrous conglomeration of ideas and hypotheses in unrelated fields, the evolutionary hypothesis is a beautiful little idea that happens to have far-reaching consequences.
A discussion of Darwin's whole years-long thought process in arriving at his hypothesis is beyond the scope of this document; regardless, you probably have some idea of Darwin's journey on the HMS Beagle, his observations in the Galapagos, and his careful gathering of data for the next two decades. His reasoning -- which is still remarkably applicable today, after countless revisions of the details of his theory -- is as follows:
That's it. That's the gist of the evolutionary hypothesis. The rest of the page will be spent explaining it, but if you can wrap your mind around this list and its implications, you'll have a fairly good grasp of the basics of evolution. Something to keep in mind: evolution has three criteria for it to occur. They are: Heredity, Variation, Competition. If they are present in a population (and they invariably are), then evolution will occur; if they are not, then evolution will not occur.
Evolution is actually an elegant solution to several glaring problems in naturalism. So, several observations that prompted the hypothesis:
The evolutionary hypothesis very neatly explains all of these patterns. It can be summed up as "descent through modification by natural selection" -- Darwin actually eschewed the term "survival of the fittest" in favor of this more descriptive (and less deceptive) summary. The geographic distribution, classification, and fossil patterns can all be explained by descent away a common ancestor, changed from that ancestor and from others around by modification and the selection of successful varieties by the particular environment. Of course, the adaptation can be described by natural selection working on the the species and its environment.
This evolutionary hypothesis also generates many predictions, and can be tested quite extensively. For example, here's a short list:
These predictions just scratch the surface of the tests that the evolutionary hypothesis can be put through. In fact, Darwin spent nearly two decades after his return to England meticulously gathering and recording data, testing his idea. The chief reason his hypothesis quickly gained support is because he packed On the Origin of Species chock-full of data, analysis, and predictions that scientists after him could use to put his idea through more tests.
The evolutionary hypothesis has stood conceptually for over a century, although its details have been revised and retested; for example, Darwin's proposed mechanism for inheritance has been discarded in favor of Mendel's theory of genetics. The details are not relevant, though, to this page, which focuses on the underlying ideas.
Evolution has two simple components: mutation and selection. Since with any organism -- even viruses -- heredity is a given (i.e., genetic material is passed on from generation to generation), let us focus on the other two criteria for evolution, variation and competition. It is simple fact that in any population, given a characteristic, the population is not uniform but various. Similarly, because resources are scarce, the population will compete for them.
So, you can think of evolution in terms of these components: mutation generates the variety from which competition selects. Mutations occur much more frequently than is commonly supposed; the average human, for example, actually has between 100 and 200 mutations; less complex organisms mutate many times more frequently. The HIV virus, for example, has a mutation rate a thousand times that of humans. The vast majority (90-98%) of all mutations are neutral, while only a few percent effect significant change in the organism as it develops.
Even so, evolution acts on time scales and population sizes that are so vast, those few percent do add up eventually. Take the flu virus, for example; the population of flu viruses in any given flu season must be at least in the trillions, so it is no wonder that we continuously must update our flu vaccines. Those few beneficial mutations add up quickly, while deleterious mutations take themselves out of the population.
Now, let's conduct a short thought experiment for ourselves here.
Suppose there's one initial species of cats (maybe the size of ocelots?) living on one side of the mountain range. Now, we'll say a group of them hunts across to the other side and can't find their way back. So, they continue living there. Now, you'll agree that natural selection works on them, right? So, this environment over here is, we'll say, colder. The prey animals that live here are larger and stronger than the prey animals on the other side. So, what happens? They adapt, right?
Natural selection favors the cats in the population who have longer fur and are slightly larger. Moreover, it selects animals who work well together to hunt in packs. Well, there's no "information" being gained, right? It's only weeding out the animals who have genes that give them shorter fur or smaller stature. Okay, let's continue extrapolating this thought experiment. Fur becomes gradually longer and longer until the cats are very well adapted to their environment. Also, natural selection favors the stronger and larger ones. Over, say, three hundred thousand years, the cats double in size and fur length. They can no longer interbreed with the species that exists on the other side of the mountain range (of course, that species has been evolving as well, but it still appears to be similar to its ancient form). They also hunt in packs and bring down their prey as a group, like wolves.
Are you still with me? Let's keep the thought experiment rolling. What's happened so far is speciation, which is, of course, okay under the creation model of "variation within kinds". Okay, so here's what we'll put our cats through next. A small group of the big, hairy cats goes off on a hunt, and they arrive at an oceanside. They're lost and can't find their way back to the original population, so they stay put. It's a little warmer here, so natural selection favors the ones who have slightly shorter fur. The cats also change their diet: they can catch fish in the ocean's lagoon, while the big herd prey animals are much less prevalent.
So, since the environment's changed, our cats decrease in size and increase in swimming ability. Natural selection favors genes that contribute to swimming ability and general adaptation for the water to maximize the ability to catch fish. So, we see that our cats in this new environment, as time goes on, will develop webbed feet, perhaps, greater lung capacity, smaller and sleeker size.
Let's extrapolate this five hundred thousand years into the future. We've got the relatively stable (but genetically drifting) original cats and big cats, and we've got our new oceanside cats, who have paddle-like feet, sleek bodies, are developing blubber, and can spend five to ten minutes underwater.
Okay, so let's fast-forward another million years. This is an almost unimaginably long timescale. What could have happened in that time? Well, let's suppose that the original population (though now very subtly genetically different because of drift) and the big-cat population (also different from drift) have remained relatively stable in terms of their physical appearance. However, the oceanside cats have continued to be under selection pressure from their new lifestyle. Selection has favored cats who are more efficient swimmers, have limbs that are wider and resemble paddles, have tails that are wider, stronger, and more paddle-like, and have more fat instead of hair, since fat insulates better underwater. Their skulls have changed shape and now are longer, sleeker, and their lungs have doubled in size. Their bodies are more efficient at using oxygen, and they can stay underwater for periods of time ranging from a half-hour to forty-five minutes. They are efficient fish-catchers. In short, they look a whole lot like seals, but with vestigial claws and hind limbs.
I could keep doing this all day, but I'm sure you get my drift. The key term here is gradual. It is the gradual change in the average of the population that signifies evolution, and it is that gradual change, given mutations to work with and shaped by natural selection, that is what makes evolution such a beautiful and powerful theory.
Evolution is such a beautifully complex and detailed theory that I can't make any pretensions of understanding all of it or being able to explain more than the basics. Even so, there are several things you should take from this introductory essay. First, you should understand that evolution is simply the change in the genetic makeup of a population. Second, you should realize that evolution occurs exactly when heredity, variety, and competition are present in a population. Third, you should get how mutation and selection interplay in the evolutionary process. If you walk away from reading this with a deepened understanding of the basic ideas behind evolutionary theory, I'll have done my job.
You don't have to read this, but it's my website and I feel like adding it, so there. Fix R^n as a "gene space" and identify each axis with a separate gene. Then each organism will lie on a point x in R^n because each organism can be described in terms of his genes. Given a population, consider a function f:R^n -- (0,1) that describes the distribution of its population about a mean x_m. Evolution predicts, among other things, that f will be continuously parameterizable by time. You can visualize this as a bell curve moving along an axis as time goes on, natural selection favors one side of the curve while pruning down on the other side, and mutation permits the bell curve to not run up against a wall.