How natural selection works
The short answer
Natural selection needs inherited variation: beak depths differ, and chicks resemble their parents. More are born than can live, and when a drought leaves only hard seeds, the deep-beaked birds survive and breed, so the average beak moves deeper over generations. The film's generations come from a model; the real observation it rests on is the larger beaks of Daphne Major finches after the 1977 drought.
Chapters and transcript
- 0:00
A population is a cloud of differences, never a shelf of copies.
No two finches in a population are identical. Beak depths vary, and that variation is the raw material for natural selection.
- 0:11
Beak depths vary; a child inherits a parent's beak, plus a small change.
A child resembles its parents, so a deep-beaked parent tends to have deep-beaked chicks, with a small random change. Only inherited variation can be passed on.
- 0:27
More are born than can live: seeds are hard, and only deep beaks crack them.
The food is finite. Some seeds are too hard for a shallow beak to crack, so only birds with deep beaks can feed on them.
- 0:33
Here, 200 birds are born; only the ones above the threshold survive.
In the simulation, 200 birds are born each generation. The chance of surviving rises with beak depth past the threshold, so most survivors are deep-beaked.
- 0:45
A drought leaves only the hardest seeds. The threshold moves up.
In a drought, the soft seeds run out and the hardest seeds are left. Now only birds with deeper beaks can crack them.
- 0:52
The birds that survive are the deep-beaked ones, and they breed.
The survivors breed, so the next generation's average beak depth moves toward the deep-beaked birds that lived.
- 1:05
Repeat for generations: the average beak depth moves toward the drought.
Repeated over generations, the average beak depth rises. Each step is one generation of birds passing on its beaks.
- 1:20
In the real Daphne Major finches, birds after the 1977 drought had larger beaks.
On Daphne Major in the Galápagos, the 1977 rainy season brought only 24 mm of rain. Peter and Rosemary Grant found that the birds of the years after the drought had measurably larger beaks.
- 1:31
This is a model; the one real fact is the 1977 change in the Galápagos finches.
The generations on screen come from a computer model with a fixed seed, not from measured birds. The 1977 change is the field observation the model is built on.
Sources
Every figure in the film comes from these. The animation is illustrative.