Introduction to Natural Selection

An organism is stuck with the genes it was born with. It cannot decide to grow a thicker coat or a longer beak because conditions demand it. What can change is the population: the proportion of individuals carrying each version of a trait. That proportion shifts when individuals with different heritable traits leave different numbers of offspring. Over generations, traits linked to more surviving offspring become more common, and traits linked to fewer become rarer. Nothing inside any one organism is transformed. The arithmetic of who reproduces more is what moves.

The mechanism has four pieces that always show up together. First, variation: individuals in a population already differ in heritable traits, like beak depth or coat thickness, before any environmental pressure acts. Second, competition: resources like food, space, or mates are limited, so not every individual gets enough to survive and reproduce. Third, differential survival and reproduction: individuals whose traits suit the current environment better tend to survive longer and produce more offspring. Fourth, inheritance: offspring tend to resemble their parents, so the advantageous traits get passed on. Run this cycle generation after generation and a small edge compounds — the offspring of the better-suited individuals make up a larger share of the next generation, and their offspring do too.

This is the foundation for everything else in the unit. Evidence like the fossil record, biogeography, and molecular similarities all document that allele and trait frequencies really have shifted this way over time. And when populations become isolated and accumulate enough of these shifts independently, they can no longer interbreed — that is speciation, the next idea built directly on this mechanism.