Living things change over time; this process is called evolution.
Any single living organism, over the course of its life, will experience changes—for humans, unfortunately timed voice cracking is probably the most embarrassing among them—but this individual development is not to be confused with changes in populations of living things over geologic (read: long) periods. Because of the genetic variation inherent in all populations, over time, traits will change, and future generations of a population may look, behave, and function differently from their ancestors. Who knows? Maybe in a thousand years, everyone will have six fingers on their right hands. Some species will go extinct. New species will appear. This process is called evolution, and the most important concept in evolution is natural selection.
Charles Darwin, who, incidentally, had a beard that could give Dumbledore a run for his money, and his colleague, Alfred Russel Wallace, first proposed natural selection in 1859. Darwin, or Charlie, as we like to call him, usually gets more credit because, well, he thought of it first. You’ll read a lot more about natural selection and other mechanisms of evolution, but in a nutshell, natural selection is based on four main points:
- In most species, more offspring are produced than will ultimately survive and reproduce. Sad, but true.
- Variation exists within a species. Human variation is obvious to us, but we tend not to realize that populations of other organisms vary, too. Variation exists in every living species, from skunks to amoebas to lima beans.
- Taking points 1 and 2 a step further, variation contributes to who survives and who doesn’t. In other words, those who succeed in surviving and reproducing themselves are successful in part because of the specific traits they possess.
- Some of the traits that vary and contribute to success are heritable, meaning they can be passed on from generation to generation. How many times does Harry Potter hear that he has his mother’s eyes?
Putting this all together, here’s the bottom line: not everyone is going to make it, and of those who do, not everyone will be equally successful, or "reproductively prolific." Since there is variation in the population, some individuals will have traits that make them better able to survive and reproduce. Maybe they’re faster at escaping from predators, keener at sniffing out food, or more tolerant of harsh conditions. These individuals will be the ones that pass their traits along to their offspring.
Over time, whoever can produce the most offspring that live to reproduce will be the one whose traits prevail in a population. Natural selection is the idea that traits are selected based on the benefit they provide to their possessor; that benefit, in turn, depends on environmental conditions, like predators, disease, resource availability, climate, and so on. The possibilities are endless.