Evolution by natural selection: three conditions and a consequence
Nothing is trying to improve. The ones that survive are simply the ones that survived.
Darwin's idea is often described as complicated. It is not. It requires three conditions, all of which are plainly observable, and the conclusion follows whether you like it or not.
The three conditions
- Variation. Individuals in a population differ - in size, speed, colour, resistance to disease.
- Inheritance. Some of that variation passes to offspring.
- Differential survival. More are born than can survive, and which ones make it is not random with respect to those differences.
If all three hold, then traits that help survival and reproduction become more common over generations. Not because anything wants them to - simply because the individuals carrying them leave more descendants. That is the whole mechanism.
What it is not
Not individuals changing
A giraffe does not stretch its neck and pass on a longer one. Within any generation, some necks are longer; in an environment where reaching higher food matters, those individuals do slightly better, and the average shifts. Populations evolve; individuals do not.
Not survival of the fittest, as usually meant
'Fitness' in biology means reproductive success, not strength or athleticism. An organism that lives a short life and leaves many surviving offspring is fitter than one that lives long and leaves none. The phrase is Spencer's, not Darwin's, and it has caused a century of misunderstanding.
Not progress toward anything
There is no ladder and no goal. Selection responds to the current environment only. Change the environment and traits that were advantageous become costly - which is why so many extinct species were highly successful right up until conditions shifted.
No new species; just the frequency of an existing variation tracking the environment, in both directions, within human lifetimes. The reversal is the strongest part - it shows the mechanism responding to conditions rather than heading anywhere in particular.
Where the variation comes from
Darwin had no idea. He knew variation existed and was inherited; the mechanism was unknown until genetics, and the two were not properly joined until the 1940s.
The source is mutation - copying errors in DNA - plus, in sexual reproduction, the reshuffling of existing genes. Most mutations do nothing. Some are harmful. A small fraction are useful in a given environment, and selection acts on that fraction.
It is worth noticing that mutation is random and selection is not. That combination is what people find hard to accept: a random source of options with a ruthlessly non-random filter produces results that look designed, because the filter is doing the work.
Why antibiotic resistance is the same thing, happening now. A population of bacteria varies. Antibiotics kill the susceptible ones. The few resistant ones survive and reproduce, and within days the population is resistant. Finishing a course matters because stopping early leaves the hardest-to-kill bacteria alive with no competition - which is selection working exactly as described, against us.
Deep time is the hard part
The mechanism is easy to accept over decades, as with the moths. The difficulty is believing it produces eyes and wings.
That is mostly a failure to feel the timescales. Life has been evolving for about 3.8 billion years. A billion is not a big million - a million seconds is eleven days, a billion seconds is thirty-two years. Small changes compounding across that span produce results that no amount of intuition about a human lifetime prepares you for.