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Peppered moth

Biston betularia

The famous experiment had real flaws, the critics were largely right about them, and a seven-year re-run got the same answer. That sequence is the good part.

A dark form of this moth rose from rarity to dominance in industrial Britain and fell again as air cleaned up. The classic experiments had real methodological problems, the criticisms were largely fair, and a seven-year re-run that fixed them reached the same conclusion.

This is the most famous example of natural selection in action, the most simplified, and — because of how the argument played out — one of the most instructive. The bare facts are not in dispute. The peppered moth occurs in a pale speckled form and a black one. The black form was a rarity in the early nineteenth century, first recorded near Manchester in 1848, and became overwhelmingly the commoner form in industrial regions within decades. After clean-air legislation it declined again. The proposed explanation was equally clear: soot darkened the surfaces moths rest on and killed the pale lichens, birds took whichever form contrasted more, and the proportions shifted. Kettlewell tested it in the 1950s by releasing marked moths of both forms in polluted and unpolluted woods, and the results matched the prediction. Decades later the methods came under serious and largely justified criticism. Moths were released in daylight at unnaturally high densities, and often placed on exposed trunks — and cage experiments then showed that peppered moths overwhelmingly prefer to rest on the undersides of branches, so the classic setup was not reproducing the situation in which selection actually operates. In some popular accounts this became “the peppered moth story was faked”, which it was not; the correct reading was that the experiment had not adequately tested its own hypothesis. What happened next is the part worth telling. Michael Majerus spent seven years running it properly — moths settling in their own resting positions, at natural densities — and recorded predation on nearly five thousand moths. Melanic moths were taken at significantly higher rates in that now-unpolluted wood, at a rate sufficient to explain the decline. It was published after his death by colleagues.

Early coverage · 35% complete · reviewed 2026-09-03

What this page covers

One species, across Britain, Europe, Asia and North America. The famous work is British; comparable melanic rises and falls were recorded in industrial regions elsewhere.

Often confused with: A story about moths turning black, when what changed was the proportion of two forms in a population; A discredited experiment, which is what the criticism is often reported as having established

Quick facts

What changed
The proportion of two existing forms, not the colour of individual moths
The criticism that mattered
Moths rest under branches, not on exposed trunks
The answer to it
Seven years, natural resting positions, same conclusion
The mutation
A transposable element, dated to about 1819 — first melanic recorded 1848

A result, a serious objection, and an answer

Told as three stages, because the middle one is usually either omitted or overstated.

The classic experiments had real methodological problems — including that the moths were put on tree trunks, where they do not normally rest. A seven-year experiment that fixed them reached the same conclusion.

Established

Specialists would state this without hedging. Multiple independent lines of evidence agree.

Early selection experiments on Biston betularia used artificial release densities and placement on exposed trunks, contrary to the species’ natural resting preference for the undersides of branches. A subsequent seven-year field experiment allowing natural resting positions and densities recorded significantly higher predation on melanic morphs in unpolluted woodland, at a magnitude sufficient to account for the observed decline in melanic frequency.

Who this applies to
The peppered moth in British woodland.
Studied in
Biston betularia
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

The specific methodological objections were identified, taken seriously, and addressed by an experiment designed around them, which reached the same conclusion with far better methods. The causal mutation has since been identified and dated consistently with the historical record.

Caveats

  • The re-run covers one wood during a period when melanic frequency was already declining, so it measures selection against melanics rather than the original rise.
  • Bird predation is the principal mechanism rather than the only one; migration and other selective factors contribute and were debated throughout.

Still unanswered

  • How much of the geographic pattern of melanic frequency is explained by predation alone versus by movement between populations.

Last reviewed 2026-09-03

The evidence (4 studies)
How the question was settled
StageWhat was doneWhat it established
1950sMarked moths released and recaptured; birds filmed taking moths from trunksDifferential predation matching the prediction
1984Resting-site choice tested in a cageMoths prefer branch undersides — the setup was wrong
1990s–2000sPublic argument over whether the result stoodMostly heat; the objection was methodological, not fraud
2001–2007Seven years of releases at natural densities and positionsHigher predation on melanics, sufficient to explain the decline
2016The causal mutation identified and datedA transposable element, arising around 1819

The distinction the middle rows depend on is between "this experiment did not adequately test the hypothesis" and "this hypothesis is wrong". The first was true and was worth saying loudly. The second did not follow, and some popular retellings made the jump anyway — which is why the moth acquired a reputation as a debunked example among people who had only encountered the debunking.

The seven-year answer

Moths choosing their own resting places, at densities that occur in nature.

How we know

Running the famous experiment properly

The classic peppered moth experiments put moths on tree trunks, where the species does not normally rest. Does the result survive if the moths choose their own positions?

Nearly five thousand peppered moths were released in a single wood over seven years. Rather than being placed on trunks, they were allowed to settle in natural resting positions, at natural densities, and disappearance attributable to bird predation was recorded for pale and melanic forms.

What happened

Melanic moths suffered significantly higher predation than pale moths in this now-unpolluted wood, at a rate sufficient to account for the observed decline in melanic frequency.

What it shows

That differential bird predation is the principal mechanism, with the central methodological objection to the earlier work addressed rather than argued around. It is an unusually clean case of a field taking a criticism seriously, doing the work, and finding the original conclusion stood.

What it does not show

It measures selection against melanics in clean air — the mirror image of the historical rise rather than a re-enactment of it. One wood, during a period when melanic frequency was already falling, and the analysis was completed by colleagues after the designer’s death.

The controls — what makes this evidence rather than a story
  • Natural resting positions, which is the specific methodological objection the experiment exists to answer.
  • Natural release densities, addressing the criticism that earlier work concentrated moths unnaturally.
  • Seven years of data from one site, so the result does not rest on a single season’s conditions.
  • Predation events observed directly where possible, rather than inferred solely from disappearance.

From Selective bird predation on the peppered moth: the last experiment of Michael Majerus

One detail is worth stating because it changes how the result should be read. This experiment ran in a wood that was no longer polluted, so it measured selection against the melanic form rather than the original selection in its favour. That is the direction the population was moving at the time, and it is the prediction the hypothesis makes for those conditions — but it is the mirror image of the historical episode rather than a re-enactment of it.

The mutation, and a date

Most textbook examples of selection have no identified genetic change. This one does.

In 2016 the specific change responsible for the black form was identified: a transposable element — a piece of DNA that copies itself into new positions — inserted into a single gene. Dating the insertion from the surrounding sequence put its origin at around 1819. The first melanic peppered moth on record was caught near Manchester in 1848. A mutation arising a few decades before the first sighting, and then spreading, is what the account predicts, and the fit between a genomic estimate and a Victorian collecting record is better than anyone had a right to expect.

The moths did not change colour. Both forms already existed; what changed was how many of each survived to breed. That distinction is the whole difference between selection and the Lamarckian story it is often mistaken for.

Related

  • Camouflage

    Background matching, and what happens when the background moves

  • Adaptation

    Populations change; individuals do not

  • Predation

    The selective agent here was birds finding moths

The research behind this page

4 studies, newest first. Each one has a page explaining what it found and what it could not show.

2016Nature

The industrial melanism mutation in British peppered moths is a transposable element

The melanic form is caused by a transposable element inserted into a single gene, and the insertion is dated to around 1819 — consistent with the first melanic moth being recorded in Manchester in 1848.

2012Biology Letters

Selective bird predation on the peppered moth: the last experiment of Michael Majerus

Melanic moths suffered significantly higher predation than pale moths in this now-unpolluted wood, at a rate sufficient to account for the observed decline in melanic frequency.

1984Biological Journal of the Linnean Society

On the selective forces acting in the industrial melanism of Biston and Oligia moths

Moths overwhelmingly chose to rest on the undersides of branches rather than on exposed trunks, indicating that the resting position used in the classic experiments was not the natural one.

1955Heredity

Selection experiments on industrial melanism in the Lepidoptera

Melanic moths were recaptured at higher rates in the polluted wood and pale moths at higher rates in the unpolluted wood, and birds were directly observed taking the form that contrasted more with the trunk.

This page is a stop on a longer route

A guided journey reads several subjects in a deliberate order, with an argument for why one follows another. You can join in the middle.

Where to go from here

Each of these follows from something on this page — a relationship in the evidence, a claim people ask about, or the next mechanism along.

How complete this page is, and what it is still missing

NatureHQ publishes its own gaps. This page is at 35% completeness against what we would call a finished subject, and was last reviewed on 2026-09-03. It carries 1 claims and answers 6 mapped search questions.

  • no research from the last few years is attached — check for newer work
  • more experiments could be explained in plain English
  • no popular claim about this subject has been checked yet
  • The re-run measured selection against melanics in clean air rather than the original rise, which is the mirror of the historical episode.
  • Migration between populations contributes to the geographic pattern and is not covered here.
  • The moth’s general biology beyond this episode is outside the page’s scope.