How birds, bark and soot changed the colour of a moth population

How to use this simulation

1. Look at the tree bark on the left. Each small shape is one moth: pale ones and dark ones. The strip under the bark shows the soot level, and the bark colour follows it.
2. Press play. Every generation lasts a couple of seconds. A bird swoops across the bark, the moths it caught fade out, and the survivors breed to make the next generation. The graph on the right records the share of dark moths.
3. Drag the soot slider up. The bark darkens, the pale moths now stand out, and the graph climbs more steeply for the dark form. Drag it down and the opposite happens.
4. Drag the selection-strength slider. A hungry bird that takes a bigger share of the visible moths makes the change much faster; at zero nothing changes at all.
5. Change the share of dark moths at the start to see how the starting point shifts the whole curve left or right.
6. Read the survival bars under the graph and the readings below: the two survival chances, the selection coefficient and how many generations the favoured form needs to reach 90 %. Press reset to return to the defaults and check your numbers against the solved problems.

The moth that changed colour

Picture a pale tree trunk covered in pale lichen, and a small moth resting on it by day. A pale moth is almost invisible there. A dark one stands out like a dot on a white shirt. A hungry bird, scanning the trunk, does not see the pale moth and eats the dark one.

Now cover the trunk in soot from nearby factories. The lichen dies, the bark turns black, and the picture flips. The dark moth melts into the bark, and the pale moth is the dot on the shirt. Same bird, same moths, opposite result.

This is the story of the peppered moth, one of the best-known examples in biology. Around industrial towns in Britain, pale moths were the common form when the trees were clean. As soot built up, dark moths became common. When pollution was cleaned up, pale moths made a comeback. Careful field experiments later showed that bird predation was a main driver of the change.

Notice what did not happen. No moth turned dark because it needed to. The colour a moth has is fixed by the genes it inherits. What changed was which moths lived long enough to pass those genes on.

Four ingredients, all required

Darwin and Wallace saw that selection needs exactly four things. If any one is missing, there is no evolution by natural selection. The simulation has all four, and you can switch some of them off by moving a slider.

Words you need first

Evolution has a vocabulary that is easy to mix up. Learn these before the numbers.

How the simulation works

There are 60 moths on the bark in each generation. A moth is either dark or pale, and it passes its colour on unchanged. This is a simplification of real genetics, where one gene can have a dominant and a recessive allele, but it keeps the arithmetic honest and shows the pattern clearly.

Two numbers decide which moth is easy to see. Soot is the darkness of the bark, from 0 % (clean and pale) to 100 % (black). A dark moth is visible in proportion to how pale the bark is, 100 % minus the soot. A pale moth is visible in proportion to the soot itself. At 80 % soot, a dark moth has only 20 % visibility and a pale one has 80 %.

The second number is the selection strength, the share of a fully visible moth that a bird takes in one generation. At 40 %, a perfectly visible moth would be eaten 40 times in 100. A moth's survival chance is 1 minus strength times visibility. So at the defaults, a dark moth survives with 92 % chance and a pale moth with 68 %.

After the birds have eaten, the survivors breed. The new generation has the same proportions as the survivors. That is all the model does: eat, breed, repeat.

Reading the frequency graph

The graph has the generation number along the bottom and the share of the population up the side. There are two lines that always add up to 100 %: the dark form and the pale form.

• Where does the line start? At the share you chose, 10 % dark here, so 90 % pale.
• What is the shape? An S. Slow at first, because there are few of the favoured form to multiply. Fastest in the middle, when both forms are common. Slow again at the end, because the less fit form is so rare that the birds hardly meet it.
• Does the line reach 100 %? In theory it only approaches 100 %. The model never removes the last moth of the less fit form, which is why real populations keep a little variation.
• Where is the steepest part? Always where the two forms are equally common, at 50 %, because that is where selection has the most variation to work on.

The soot slider: the same moth, a different verdict

Selection is not a property of an animal. It is a property of an animal in a place. Move the soot slider and the same dark moth switches from a liability to an advantage.

The strength slider: how fast, not which way

The soot decides the direction. The selection strength decides the speed. A weak predator slows the process to a crawl, and a ruthless one finishes it in a few generations.

At the default 40 %, the dark form takes 15 generations to reach 90 %. At 80 % it takes 6, and at 20 % it takes 33. Notice that the destination is the same in all three: only the time differs.

This matters for real evolution too. A strong pressure such as a new disease can transform a population quickly. A weak pressure may need so many generations that other forces, such as chance, get in the way.

What the model leaves out

Every model is a simplification. These are the big ones here, so you do not mistake the picture for the whole truth.

• Real colour genes have dominant and recessive alleles, which hide the pale allele in dark moths. This slows the loss of a rare allele.
• Mutation can create new variation. Here the variation is only what you start with.
• Chance matters. In a small population, a form can vanish or spread by luck alone, a process called genetic drift. This model is deterministic, so it ignores drift.
• Moths move between areas, so a polluted town and a clean village next door exchange moths. That is gene flow.
• Predators learn, and prey that is rare is sometimes overlooked. Selection strength can change as the mix changes.

Natural selection and the wrong ideas around it

Jean-Baptiste Lamarck proposed that an animal could change during its life and pass that change on, as in a giraffe stretching its neck. If that were true for moths, a moth that sat on sooty bark would darken and have dark children. It does not work that way. A change in a body during a lifetime is not written back into the genes that go to offspring.

Darwin’s idea is different. Variation already exists, and the environment filters it. The giraffe story under Darwin is that giraffes born with longer necks reached more leaves and left more offspring.

Another common slip is thinking that selection has a goal. The bark did not want dark moths. It just made pale ones easier to eat. There is no plan, only whoever survives.

Try it in the simulation

Spend ten minutes on these experiments. Each is a question an examiner could ask.

• Leave everything at the default. How many generations does the dark form need to reach 90 %? Compare your reading with 15.
• Drag the soot slider to 0 % and press play. Which form wins now? Does the graph still look like an S?
• Set the soot to exactly 50 %. What happens, and why?
• Set selection strength to its minimum and then its maximum. What changes: the direction or the speed?
• Start with 90 % dark moths and the soot at 20 %. How long before the pale moths take over?
• Watch the number of moths eaten per generation as the dark form spreads. Does it go up or down, and why?
• Set the soot high and the selection strength low. How far does the graph get in 30 generations?

Solved problems

Each solution states the formula first, then substitutes the numbers. Every figure is computed by the same code that drives the simulation.

Common mistakes

These are the slips that cost marks every year.

• Saying individual moths changed colour to match the bark. They did not. The mix of the population changed.
• Writing that organisms evolve because they need to. Need does not create variation; selection only sorts what already exists.
• Reading “survival of the fittest” as the strongest wins. Fitness means passing on more offspring, and a small, camouflaged moth can be the fittest.
• Thinking acquired characters are inherited. Changes made during a lifetime do not go to offspring.
• Calling a trait good or bad without naming the environment. Dark is an advantage only on dark bark.
• Forgetting the time. One generation is a tiny step; the S-curve comes from repeating it.
• Mixing up natural selection (the filter) with mutation (a source of new variation).

Where you see it in real life

Bacteria that survive an antibiotic pass on the genes that let them survive. Over many rounds of treatment, the survivors are the resistant ones. This is the same engine as the moths, with the antibiotic playing the bird.

Insects exposed to the same pesticide year after year face the same filter. Farmers see it as a spray that works worse each season. The pest has not learned anything; the susceptible ones have simply been removed.

Farm crops and animals show the human version, called artificial selection. Farmers keep seeds from the best plants and breed from the best animals, so people play the role of the bird.

The lesson from the moths is practical: if a population is under strong selection and has variation, expect it to shift.

Exam corner

Typical questions, and what a full-mark answer contains.

Revision summary

Natural selection needs variation, inheritance, unequal survival and time. In the moth model, soot sets how visible each colour is, and the survival chance of a form is 1 − strength × visibility. At 80 % soot and 40 % selection, dark moths survive at 92 % against 68 % for pale ones, so the dark share rises from 10 % to 13.1 % in one generation and to 90 % in 15. The soot decides the direction, the strength decides the speed, and at 50 % soot nothing changes. Individuals do not change; the mix of the population does.

Frequently asked questions