Definition
Genetic drift is the change in how common a gene is within a population, caused purely by chance rather than by how well anyone fits their environment. A gene doesn't spread or vanish because it is especially helpful. It can sweep through the whole population or disappear completely just because of simple probability.
A chance event can reshape the next generation
Picture a forest path where green beetles and brown beetles live side by side. Someone walks by without looking down and steps on the path. By bad luck, they squash only green beetles, and the brown ones survive.
The brown beetles didn't survive because they were smarter or tougher. They just weren't standing in the wrong spot. It was pure chance and luck. The surviving brown beetles have babies, and the next generation has far more brown beetles.
When a population's genetic makeup shifts because of random events or the luck of reproduction, and not because some individuals fit the environment better, we call it genetic drift. The name comes from the way gene frequencies wander randomly, like a raft drifting on the sea.
The smaller the population, the stronger the pull of chance
Flip a coin 1,000 times, and heads and tails will come out very close to half and half. But what if you flip it just 4 times? Getting heads four times in a row happens about 1 time in 16. That's not as rare as you'd think.
The same idea works in living populations. In a huge group of tens of thousands, a few animals dying by accident barely changes the overall gene mix. The more individuals there are, the closer things stay to the average.
But if a group has only about ten members, everything changes. If just one or two fail to have offspring by chance, a gene can disappear forever in an instant. The smaller the population, the more powerful and dramatic genetic drift becomes.
To be more precise: the key difference from natural selection
When people think of evolution, they usually think of Charles Darwin's natural selection first. In natural selection, living things with helpful traits, like better feather color or stronger wings, survive and reproduce more. There is a clear direction: toward whatever helps survival and reproduction.
Genetic drift, to be more precise, is random change with no direction at all. It has nothing to do with how fit or healthy an individual is. Even a trait that doesn't help much, or a slightly harmful gene, can spread through a whole population if luck is on its side.
Two classic examples are the 'bottleneck effect' and the 'founder effect.' In a bottleneck, a huge disaster like a wildfire or flood kills most of a population and only a tiny number survive. In the founder effect, a few individuals cross over to a new island and settle there. In both cases, the genes of a small group that happened to make it decide the population's future.
Common misconceptions
Evolution always happens because the best-adapted, strongest individuals survive.
Evolution comes from natural selection, but also from genetic drift. Genes that don't help survival, or even hurt it, can spread widely to the next generation through pure chance.
Genetic drift is stronger in huge populations with lots of individuals.
It's the opposite. The smaller the population, the bigger the effect of chance. In a large population, random events tend to cancel each other out, so the change in a single generation is much smaller. That said, drift doesn't disappear in large populations.
Where you meet it
After a wildfire kills most of a forest's squirrels, the few that happened to hide in rock crevices survive, and their fur color comes to dominate the next generation.
A small group of lizards accidentally drifts to a new uninhabited island, and as they multiply, their genes end up very different from the mainland population.
Someone accidentally tramples a patch of flowers, and only plants with one petal color get broken and fail to leave offspring.
In one sentence
Genetic drift is when the mix of genes changes from one generation to the next purely by chance, regardless of adaptation or fitness.