The problem I have with biology is that there is so much of it. Each level of structure seems to be a whole world in itself: cells interact in so different a manner than organs; an organism's metabolism operates so differently than an ecosystem. And all these levels interact with each other. What does it mean to have a grasp of such a large, complicated field? Or, less ambitiously, what is the make-up of a basic understanding of biology? I initially wanted to write one post to address this question, with each paragraph discussing one area of biology I thought important. However, the paragraph on evolution seemed thrown together, touching on many loosely related topics. I took this as an indicator that I should do two posts on biology, with the first dedicated solely to modern evolutionary theory. After all, it is because of the self-modification of biological systems, as described in evolutionary theory, that there is this interaction among the various levels of biology.
First, even though it is the most obvious reference, I have debated with myself as to whether I should recommend Charles Darwin's On the Origin of Species; after all, a good college-level biology textbook should present the basic ideas of the work, in a framework involving information that Darwin didn't have, such as the genetic code -- or even genetics. On top of this, I have never read the book myself, and I would feel uneasy recommending it on the word of others. However, I have seen many online arguments with people arguing against evolution by citing passages from the book, with other people rebutting that the passages in question have been misunderstood. For those who wish to follow such arguments, it is clearly important to read the original source, and understand its arguments. On the Origin of Species is available on Google Books, but there is also an Internet archive of Charles Darwin's writings. Be warned, however: the book is not an easy read.
According to dictionary.com, the word "evolution" is defined as "any process of formation or growth; development." Biological evolution, as defined in the Audesirks' Biology, is "the process by which characteristics of individuals in a population change over time." To clarify, it is the distribution of traits in a population that changes over the generations; this is not to be confused with change in a single individual over its lifetime. Being a math geek with an interest in dynamical systems, I conceive of evolutionary theory as follows. There is a population of long letter-chains (DNA), which reproduce; that is, they add new letter-chains to the population, which can differ from the parent chains (by mutations and genetic drift). Through complicated selection rules (natural selection), members of one generation have varying levels of success at reproducing. (Note that time is considered discrete, being measured in generations.) That is, there are at least two important aspects to evolution: variation happens, and selection is made from among that variation; the selection part seems to be often overlooked. Other things to keep in mind: there is no known ultimate goal to evolution, and no scale to declare any individual as better or worse, more or less evolved (no matter what you've heard about Social Darwinism). Of course, there are complications to this simple description; one is the subject of the neutral theory of molecular evolution, which states that some genes have exactly the same effects, and therefore cannot be distinguished by selection. Also, there is a big debate in biology concerning the relative importance of natural selection versus genetic drift (Lecture 12) in biology, a debate I would imagine to be on part with nature versus nurture in the field of psychology.
It can be astonishing just how powerful such a seemingly simple process can be, and a sense of the diversity of forms so generated can be conveyed through the idea of common descent. Wikipedia puts it well: "[a] group of organisms is said to have common descent if they have a common ancestor." Most evidence for common descent comes from living organisms and fossils having similar biological structure, which some detractors suggest might instead reflect common design by a God figure (I will leave it to those better at biology than me to explain why, divinity or no, common descent is the most scientific explanation for the evidence). In fact, modern biology holds that all life on earth has a common ancestor, a concept known as universal common descent. To be sure, evolution would still occur without universal common descent; just consider a scenario like Star Wars -- or especially The Mist: selective pressures would still be present. However, the common biological structure that extends down to the most basic levels through all known life is considered strong evidence for universal common descent.
Thus far, this post has addressed life coming from life, ignoring the question of how life began in the first place. Technically speaking, life origins (also called abiogenesis) is a different field from evolutionary theory, just as chemistry is a different field from physics, even though the laws of chemistry are based on the laws of physics. However, I feel that an overview of evolutionary theory without a mention of life origins would be unsatisfying, as both are important to answering the Big Question, "How did we get here?" Using the principle that the present is the key to the past (which for the purposes of this paragraph means that the laws of physics and chemistry have not changed near earth since the planet's formation), scientists have developed scenarios in which life could have arisen. For nontechnical discussions of life origins, try Exploring Life's Origins or The Bio Review. Clearly, chemistry is very important in this field.
Before I close, I would like to offer my opinion concerning the claim that high school teachers should "teach the controversy", that evolution has huge gaps in it -- above and beyond the gaps to be found in any still-active science. As I see it, even if Intelligent Design were indeed the next big thing in biology, it still should not be taught before college. The strengths and weaknesses of Newtonian physics are not part of any high school physics curriculum, even though physicists have repeatedly shown that its predictions fail at near-light speeds; instead, those students who are interested can go on to learn special relativity, once they have a firm grasp of the basics. On top of that, the fact that those who do have a firm grasp of the basics in biology are not convinced by such arguments suggests that Intelligent Design might not even be the next big thing in biology. However, I will concede a point to opponents of evolution: self-organization is counterintuitive -- which is precisely what I find so utterly fascinating about it! I feel that, with so much feedback on so many levels, the mathematics of evolution is highly nonlinear, rendering useless the intuition of a person who has not worked for years in a biology lab. I think this is also the reason so many engineers seem opposed to evolution -- at least, it seems as if the Intelligent Design movement is stuffed full of engineers.