Why do moths near cities turn dark?
After you watchWhy do moths near cities turn dark?
The short answer
No single moth changes its own color. Soot darkened the tree bark, so dark moths were better hidden from hungry birds while pale moths stood out and got eaten. The dark survivors had more babies, so over many generations the whole moth population shifted dark. That is natural selection, not individual moths repainting themselves.
Try this next
- What if you start with way more pale moths than dark ones on the sooty tree? Stack the first generation heavily pale, then predict before you let the bird hunt: will dark still take over, and will it take more generations to get there?
- What if the bird is a slow, clumsy hunter that misses a lot? Imagine letting the bird eat fewer moths each round. Predict first: does the population still shift dark, just slower, or does it stop shifting at all?
- What if the soot never lands and the bark stays pale? Run it on a clean pale tree and predict which color disappears this time — then check that it's the opposite of the sooty-tree result.
Now you — bend it
- What if On the free-play tree above, drag the soot slider all the way to CLEAN, then watch the two 'easy to spot' meters under it. Now nudge it slowly up toward SOOTY and find the exact spot where the dark meter drops below the pale meter — the moment dark stops sticking out and starts being the hidden one.Use the soot slider: the pale meter rises and the dark meter falls as the bark darkens. Predict where they cross — at a little smoke, halfway, or only at thick black soot? That crossing point is when the bird would start eating pale moths instead of dark ones.
- What if Thought experiment (no slider for this): the generations run always starts at a 50/50 mix of pale and dark. Imagine instead that dark moths are vanishingly rare at the start — only 1 in 100, like the real melanic form was before the soot arrived.You can't set the starting mix, but you CAN watch how fast 50/50 tips dark by tapping 'Let the bird hunt' round by round and reading the dark % each generation. Each round multiplies the dark share, so predict: starting from just 1% instead of 50%, would it need a few extra rounds or many to reach the same dark majority you just watched?
- What if Thought experiment (no slider for this): the generations bird is a sharp hunter that eats most pale moths it can see. Imagine instead a slow, clumsy bird that misses far more often.There's no dial for the bird's skill, but use the soot slider above as a stand-in for 'how big the spot-gap is': on thick soot the pale and dark meters are far apart (easy hunting), on grey bark they're close (clumsy hunting). Predict whether a clumsier bird still tips the group dark — just over more generations — or barely shifts it at all.
Can you prove it?No individual moth ever changes colour — the population darkens only because the survivors pass their fixed colour to their young. — Make your guess at the prediction gate, then in the generations panel pick one specific moth on the sooty bark and watch it across the whole run, tapping 'Let the bird hunt' round after round — confirm its shade never shifts. Meanwhile track only the numbers: read the dark % under the population bar each generation. If you see it climb (say roughly 50% -> 65% -> 78%...) while every single moth stays one colour, the change must live in WHO survived to breed, not in any moth's body. Tap 'Start over' and run it again to confirm the count moves the same way every time. Colour is inherited and fixed; the count is what selection moves.
Design your own test:Before you drag it, predict the relationship: as you slide soot from clean to sooty, will the pale moth's 'easy to spot' meter climb in a smooth straight line, or will it jump fastest somewhere in the middle? And predict the soot level where the pale moth first becomes the easier one for a bird to catch.
Explain it to a 6-year-old: No moth painted itself dark — the birds kept eating the moths they could see on the smoky tree, so only the hidden dark ones were left to have babies.
The whole story
How it works
A moth's color is inherited and fixed for life, but a population always has a mix of pale and dark moths. When city soot blackened the tree trunks, the dark moths blended in and the pale moths popped out against the dark bark. Birds spotted and ate more of the easy-to-see pale moths, so more dark moths survived to breed. Each new generation inherited its parents' colors, so the proportion of dark moths in the population climbed generation after generation. The world quietly kept the moths that matched and removed the ones that did not.
What people get wrong
The biggest mistake is thinking each moth darkened its own body to match the smoky trees, the way you might get a suntan. That never happens — an individual moth is one color its entire life and cannot change it. What actually changed was how many of each kind survived: dark moths got eaten less, so the population as a whole became darker over generations, even though no single moth ever changed color.
The catch
Dark is not simply better. Dark moths only won because they matched the sooty bark. When cities cleaned up their smoke and the bark turned pale again, the pale moths were suddenly the hidden ones and they bounced back. Neither color is the good one; whoever matches the environment of the moment is the one that survives, so the advantage can flip when the world changes.
Questions kids ask
Do moths actually change their own color to match the trees?
No. A moth is born one color and stays that color its whole life. The population looked like it changed because birds ate more of the moths that did not match the bark, so the survivors who did match had more babies and became the majority over generations.
If dark moths win, why are pale moths still around?
Dark only wins while the bark is dark. When cities cut their smoke, tree trunks turned pale again, and now the pale moths were the hidden ones. Birds ate more dark moths, so the pale form came roaring back. The winner is always whoever matches the environment right now.
How fast does this happen?
Surprisingly fast in nature, over a few dozen generations. Because moths breed every year, a clear shift in how many are dark versus pale can show up within a human lifetime, which is why people in smoky cities really did notice the moths getting darker.
Is this the same as how the giraffe got its long neck?
Yes, it is the same idea: natural selection. Animals that happen to be born with a trait that helps them survive in their world live to have more babies, so that trait becomes more common over generations. No animal reshapes its own body during its lifetime to fit its surroundings.
Talk about it
- Guess first: did the moths change color to hide, or did something else change? What's your best theory before we look?
- If being dark was so good for the moths, why didn't every moth in the world turn dark?
- Can you think of an animal you've seen that's hard to spot where it lives — and what would happen to it if its home suddenly looked different?
For grown-ups
This is the peppered moth (Biston betularia), the classic textbook example of natural selection. Colour is inherited and fixed for life; no individual changes colour. During the Industrial Revolution, soot blackened tree trunks and killed the pale lichen, so the rare dark (melanic) form became far better camouflaged from bird predators, and its frequency rose toward roughly 95 percent in heavily polluted regions. After Clean Air legislation cut pollution, the bark lightened and the pale form rebounded. Selection acts on the population's allele frequencies across generations by changing who survives to reproduce, not on any single moth's body.
Keep going
What else makes you wonder?
- If matching the bark is what keeps a moth safe, how does a baby moth end up the right color when it can't see where it will land?
- Birds find moths by sight — what would change if the hunter used smell or sound instead?
- Cities cleaned up their smoke and the bark turned pale again. What else have humans changed that might be quietly flipping which animals survive right now?