From U.C. Berkeley’s Understanding Evolution site:

Biological evolution, simply put, is descent with modification. This definition encompasses small-scale evolution (changes in gene frequency in a population from one generation to the next) and large-scale evolution (the descent of different species from a common ancestor over many generations).

I had a drunken hot tub conversation the other night about evolution. It was the kind of discussion that would probably have been a lot more interesting if it had started much earlier, but it was still fun trying to hang on to a train of thought.

People get funny about evolution. Some people will claim that it doesn’t happen, but they will freely accept that, for example, we each get a random mix of traits from our parents. Given this premise, it’s impossible for populations not to evolve. People will be different, and those who thrive better will pass on their traits to more children so that their genetic legacy will have a stronger influence on the future population than those who are less successful. You really can’t have sexual reproduction without evolution.

I suppose one problem is the wide variety of definitions for the word, it often just means “change over time”, so we can talk about the evolution of someone’s personality, or stellar evolution, or the evolution of modern football rules. But in the sense of the biological theory, none of those examples count.

The biological theory of evolution is all about how populations change. Not individuals, so the Star Trek episode where the guy “evolves” into a glowy light show is way off-base. That was more like metamorphosis, not evolution. It doesn’t take any showy special effects, or dramatic changes, all it takes is little differences, tiny variations; not in individuals, but in successive generations. The woman who looks a little different from her mother, and whose daughter looks a little different from her, and her daughter’s a little different, etc. Over enough generations, little differences become big ones, much the way enough inches can add up to miles, seconds tick by into centuries, loose hydrogen molecules can gather into stars so massive they collapse into black holes.

Even without mutation there would be an awful lot of variation going on. A recent study found that the average human has about 60 mutations. For the most part these don’t really do anything, but every now and then one of them will have a pro or a con that may be passed on to future generations.

That’s basically how we get variety, you have DNA from two different people combining in new and interesting ways, plus a sprinkling of mutations to add an extra random factor. Which traits become common in the general population is determined by selection pressure.

Charles Darwin identified three different types of selection, Natural, Artificial, and Sexual. Natural selection is mostly reacting to changes in the environment, so that when an ice age starts, the hairier mammals survive and reproduce better, so populations tend to get hairy. Artificial selection is deliberate breeding by humans, whether we’re breeding for hairless cats, strong horses, or high yield grain. Sexual selection is stuff that doesn’t have anything to do with ability to survive, but ability to mate, so if parrots think that blue feathers are sexy then after a few generations you have noticeably bluer parrots.

Variation and selection together are evolution. It’s really hard to imagine how it could not happen.

This is my totally-not-a-biologist understanding. I’m sure an actual biologist would find problems with this, but I feel pretty confident that the general outline is correct. Tune in next post where I’ll talk about people who don’t like evolution, the things they do about it, and why I think it’s important to keep them away from schools.