It has come to my attention recently that I might have readers who are unfamiliar with evolution, so I’ve decided to start a series of posts on the topic. And at the risk of being too basic, I’m going to start with the basics. (This is a slightly modified re-posting of something I said on a message board some time ago. The other posts in the series will be original.)
“Evolution”, in this sense, refers to the fact that species change over time. The “theory of evolution” refers to the accepted explanation of this fact, which has itself evolved over time starting from Charles Darwin’s original version. The theory is that some traits make individuals more likely to survive than others (this is called natural selection) or more likely to reproduce (sexual selection), and that these traits become more common while traits having the opposite effect become less common.
Actually, if you think about it, natural selection is the common sense part of the theory, and it’s kind of surprising that it took until the 19th century before anyone figured it out. (Even after it was realized that life must evolve in some way, it still took decades before Darwin came along and pointed out what is really the most obvious part of the entire process.) I mean really, the chain of reasoning is essentially:
1. some traits make individuals more likely to survive or more likely to reproduce (obvious, and not disputed by anyone to my knowledge)
2. offspring generally inherit the traits of their parents (also obvious and not disputed; no one knew how heredity worked in Darwin’s time, but everyone knew it existed)
3. therefore, traits that encourage survival and/or reproduction are more likely to be passed on to the next generation
Of course, the fact that no one knew how heredity worked was a notable gap in the theory’s original form. So after Darwin, the next important development in evolution came with the discovery of genetics by Gregor Mendel, which unfortunately remained obscure for several decades.
The more serious gap was that Darwin didn’t know where new traits came from, but that was also filled in by genetics. Sometimes genes don’t copy exactly; this is a mutation.
That’s how evolution works. As to the extent of what it does, the evidence suggests that all life on Earth (with the possible exception of viruses if they qualify as alive) is ultimately descended from a common ancestor.
Evolution is the single most important theory in biology, and much of biology can’t be completely understood without evolution.