How natural selection works
The short answer
Natural selection in four steps: variation is inherited, more are born than survive, the fittest pass the filter, and deep time draws the tree of life.
Chapters and transcript
- 0:00
A mechanism, not a mystery
In 1859 Darwin proposed a mechanism simple enough to watch in action. Four steps, turning on a loop — this film walks through each one.
- 0:10
Step 1 — no two are alike
Every population carries variation: size, speed, beak depth. A species is a cloud of differences, never a shelf of identical copies.
- 0:20
And much of it is inherited
Offspring resemble their parents. Beak depth runs in families — and variation that is not inherited can never be selected.
- 0:30
Step 2 — more are born than can live
Darwin reckoned one elephant pair would leave about 19 million descendants in 750 years — if every single one survived.
- 0:40
The world pushes back
Food is finite. Predators hunt. Disease spreads. Winters bite. In every species, most individuals born die before breeding.
- 0:50
The environment sets the filter
Which variations help depends on the year. After the 1977 drought on Daphne Major, only large, hard seeds were left on the ground.
- 1:00
Measured, not imagined
The Grants banded the finches of Daphne Major. After the drought, average beak depth rose about 5% in one generation — evolution, with calipers.
- 1:10
Step 4 — survivors pass it on
No finch changed its own beak. The small-beaked birds starved; the large-beaked ones raised large-beaked chicks. The average moved.
- 1:20
Repeat for a few million years
Stack that shift across thousands of generations, and populations drift apart until they can no longer interbreed — new species.
- 1:30
One ancestor, over a dozen species
Darwin's finches: one colonist species, thirteen-odd descendants, each island keeping its own beak. The tree is written across the map.
- 1:40
Three things it is not
Individuals do not transform during life. Evolution is a bush, not a ladder with us on the top rung. And nothing is steering toward a goal.
- 1:50
The whole mechanism, once more
Variation, inheritance, struggle, selection — four steps on a loop that has been turning, without pause, for nearly four billion years.