Me, I love to learn. Back before I used RSS feeds, I checked my daily funnies* by hand; first clicking individual bookmarks, then wheel-clicking folders to open them ALL AT ONCE. Glory! Anyway, when I was gifted with wheel-clicking by Firefox, I added five random wikipedia pages to my funnies list. It was pretty neat. There are a lot of small European villages with their own wikipedia articles**. Nowadays I have the RSS feed, so I get tons of articles from atheist blogs, skeptical blogs, feminist blogs, science blogs, and my daily funnies. So I'm reading less wikipedia, getting more informed about some niche advocacy groups, stretching my brain a bit to accommodate unfamiliar science (there are more biologist bloggers than physicist bloggers, I find), and getting my daily chuckles.
So why not post a brief intro on evolution? Even intelligent, well-educated people are ignorant about a lot of things. Why, I have friends who pay for homeopathic "medicine"! And they went to college!
But that's another post.
Essentially, evolution boils down to two principles. Descent with modification and natural selection.
Descent with Modification
You are like your parents, but not exactly like your parents, and a similar observation is true of all living things***. This isn't just that your DNA is a mix-and-match of your parents', but also that each offspring contains multiple mutations. When the DNA gets read and copied, the copy contains errors. For example, each human receives somewhere between 60-120 mutations. Given that there are billions of base pairs in the human genome, that's remarkably high fidelity.
Not all mutations are negative (a common fundamentalist myth). To be sure, many are; any living organism is a highly evolved machine, and a mutation can really throw a monkey in the wrench. However, mutations can also be beneficial. For example, roughly 10% of people of European descent have a copy of a gene that renders them resistant to AIDS. Having two copies makes you resistant to infection by HIV in the first place, while having only one slows its development into full-blown AIDS.
That said, the vast majority of mutations are neutral. As I said, we're all walking around with dozens of mutations, but we're mostly not obviously supermen and also not casting as extras in post-apocalypse films. Nevertheless, mutation is vital, because without it, there would be no change, no differences from one generation to the next, nor between individuals, and without differences, there would be no...
This isn't really "survival of the fittest", popular though that phrase is, but that's a part of it. In the end, evolution is about reproduction. Really, it's about having grandchildren. Who have grandchildren. Who have grandchildren. Fitness has a mathematical definition: the proportion of individuals displaying a trait after selection compared to the proportion before. In this case, "selection" means the process whereby the group decides who has offspring and who doesn't. Mating season for bears, say, or the prom for drunk teenagers.
Notice that the item in question isn't an individual organism, but a specific trait of that organism. What precisely is being selected has been a source of contention in biology for some time now. Richard Dawkins was long a proponent of the idea that the gene is the natural unit of selection; see his book The Selfish Gene. Others have argued, with merit, that the unit of selection is the genotype, the phenotype, the organism, populations of organisms, or even entire species. Not only do all these arguments have merit, I suspect that all are true, to a degree.
An individual gene can be particularly beneficial or particularly harmful in itself, and be selected for or against. No gene acts in isolation, though; rather they act as cartels, cooperating to build the organism and its parts, and those cartels can be selected for or against. And those cartels can only be considered in the context of the genotype of which they are a constituent, and its ability to build and maintain an organism. And that genotype isn't immune to how it interacts with the environment to produce a phenotype; a genotype that thrives in one environment may produce a different, and indeed inferior, phenotype eleswhere. Once the organism has been constructed, it is then tested and selected in its environment, first as an individual, then as a member of its particular group, which is then considered as a component of an ecosystem, and then the species as a whole is tested against all the systems they've managed to penetrate. I can't imagine that any of these can be neglected (though the timescale under consideration surely favors some over others), and I expect they all inform one another to remarkable degrees.
There is such a thing as selection against a trait. You don't have children? You're a failure.**** You have children? Good for you! But if they don't have children, you're a failure. You have grandchildren? Good for you! But... All individuals die, all species go extinct, but their DNA can go on as they mutate and evolve into something new, something fit, something beautiful, however temporary. The dinosaurs, for example are all gone, but not entirely.
There is also a way to select for, rather than against. A truly exceptional individual can see to it that his genes propagate quite well into the next generation, and the generation beyond. Like I said, there's a measure of fitness, and a fitness score of greater than one means you're really spreading your seed. However, whatever trait(s) makes an individual fit is always temporary and provisional, because it's fitness as proved against a certain environment. Also, once the trait is spread through the entire population, the best you can hope for is a fitness score of 1. Still, there is always that burst of glory.
So there it is. My understanding of evolution. Next time I'll talk about some of the common myths (a few were mentioned), and why they're wrong.
* You know, funnies, comics. The things that, in newspapers, are three panels of minimalist drawings coupled with mediocre humor and that, online, are freed from all constraint and delve into varying styles of artwork that ranges from acceptable to gorgeous and humor that covers the gamut from chuckles to guffaws and stories anywhere from slice-of-life to wild speculative fiction. In other words, awesome symphonic taste sensations as opposed to the bland, de-flavored, luke-warm sugar free tapioca you get in the paper.
** Essentially, "BLANK is a village in the BLANK province of BLANK. It has a population of BLANK. They fucking LOVE football."
*** Evolution is more complex for bacteria, which can exchange genetic information, forming less a tree of life and more a web of life, but let's not confuse the issue.
**** No you're not. Among other things, because it's not just about you, it's also about your genes, your family, and your species. So long as any of those survive and thrive, thanks to your actions, then you're not a failure; you're a success. Also, thanks to writing and whatnot, it's not merely our genes that can survive our deaths, but the knowledge and works we've created. Nature invented immortality when it discovered life; we've invented another kind.