Transient Polymorphism: Directional Selection and Allele Replacement

Genes don’t usually change overnight, but populations do. Give a species a strong enough push from its environment, and the balance between two versions of the same gene can flip within a few dozen generations. This slow but decisive takeover, where one allele gradually replaces another until the older form all but disappears, is what geneticists call transient polymorphism. It’s one of the clearest windows we have into natural selection working in real time, and the textbook example of it involves a moth, some soot, and a scientist willing to spend years marking wings by hand.

What is transient polymorphism? 🔗

A polymorphism, in genetic terms, simply means that two or more forms of a gene exist together in a population. Most polymorphisms are stable for long stretches of time. Transient polymorphism is the exception. Here, a rare allele starts gaining a real survival or reproductive edge, and with each generation it spreads a little further while the once-common allele retreats. Eventually, the rare form becomes the new normal, reducing the older allele to a minority or eliminating it altogether.

The key word is transient. This is not a permanent state of genetic diversity, the way sickle cell trait persists in malaria-prone regions. It is a snapshot of a population caught mid-change, on its way from one genetic equilibrium to another. Given enough time and consistent pressure, the “in-between” stage resolves itself, and the gene pool settles into a new, more uniform composition.

The engine behind the shift: directional selection 🔗

Transient polymorphism doesn’t happen on its own. It’s driven by directional selection, a mode of natural selection where individuals at one extreme of a trait consistently out-survive or out-reproduce everyone else. Over successive generations, the allele responsible for that advantageous trait gradually fixes itself in the population by eliminating the alternative allele.

This is different from balanced polymorphism, where two alleles are kept in a stable ratio because, say, heterozygous carriers do better than either homozygote. Directional selection doesn’t preserve variety. It actively narrows it, favouring one version of a gene until the population looks quite different from where it started. Antibiotic resistance in bacteria works the same way: a resistant strain spreads because it survives a selective pressure (the antibiotic) that kills off its competitors.

Industrial melanism: evolution’s favourite case study 🔗

The phenomenon most commonly used to illustrate transient polymorphism is industrial melanism in the peppered moth, Biston betularia. Before the 1800s, nearly all peppered moths in Britain were a pale, speckled colour called typica, which blended in perfectly against lichen-covered bark. In 1848, Manchester recorded the first sighting of a dark form, later named carbonaria. Over the following century, this once-rare dark form spread rapidly through industrial regions until it dominated local populations, before receding again later in the twentieth century.

One gene, two very different moths 🔗

The difference between the light and dark forms traces back to a single gene locus. British geneticist E. B. Ford spent decades studying this kind of variation and defined polymorphism itself as the occurrence of multiple distinct forms of a species in proportions too high to be explained by recurring mutation alone, a concept he developed while building the field now known as ecological genetics. Ford was convinced that bird predation, not pollution acting directly on the moths, was driving the spread of the dark form, and in 1952 he brought in a young researcher named Bernard Kettlewell to test the idea properly.

Kettlewell’s experiments: putting the theory to the test 🔗

Kettlewell’s approach in the 1950s was refreshingly direct. He used a technique called mark-release-recapture: hundreds of light and dark moths were marked, released into woodland, and recaptured a few days later using light traps. If birds were hunting by sight and camouflage mattered, the moth that stood out more against its background should be recaptured less often, having been eaten.

That’s exactly what turned up. In polluted woodland near Birmingham, a noticeably greater proportion of dark moths were recaptured compared to light moths, meaning fewer of them had been picked off by predators. In unpolluted woodland in Dorset, the pattern reversed, and light moths survived better. Kettlewell backed this up by directly watching birds through binoculars as they hunted moths resting on tree trunks, confirming that predation, not some hidden physiological advantage, was the real selective force at work.

Later researchers pushed the experiment further. A large-scale study using nearly 5,000 released moths found that birds preyed on melanic moths noticeably more often than on the pale form once pollution had declined, with daily selection pressure strong enough to explain the rapid drop in melanic frequency after air quality improved. Other reviews of the data estimated the selective disadvantage of the melanic form against the typical form at roughly ten per cent once bird predation and other factors were combined. That may sound modest, but stacked across dozens of moth generations, it’s more than enough to swing a population from mostly dark to mostly light, or the other way round.

Soot, camouflage, and survival 🔗

The mechanism behind all this is straightforward once you picture the landscape. In heavily industrialised parts of Britain, coal smoke blackened tree trunks and killed off the pale lichen that once coated them. Against that darker backdrop, the pale typica form became conspicuous, while the dark carbonaria form blended in almost perfectly. Birds, hunting by sight, picked off the moth that failed to match its surroundings. In rural, unpolluted areas where lichen still covered the bark, the situation flipped, and the light form kept its camouflage advantage.

This is directional selection in its purest form. The environment itself changed, which changed which allele was “better,” and the population responded by shifting its genetic makeup to match.

Crossing the Atlantic: melanism in the United States 🔗

Britain wasn’t the only place this played out. Researchers studying moth collections in the industrialised belt of the eastern United States, including work carried out using museum specimens at the Carnegie Museum of Natural History in Pittsburgh, documented a very similar rise of melanic moth forms through the mid-twentieth century, followed by a decline once clean air regulations took hold. The fact that two industrialised nations, thousands of kilometres apart, produced nearly identical genetic responses to the same kind of environmental pressure is part of why this case is treated as such a clean demonstration of natural selection rather than a local quirk.

How long does allele replacement actually take? 🔗

Transient polymorphism is temporary, but “temporary” in evolutionary terms can still mean many decades. The carbonaria form took roughly a century to go from rare curiosity to dominant morph in polluted regions of Britain. After clean air legislation reduced pollution from the 1950s onward, the reverse process, a return to the light form, also took several more decades to unfold as lichen slowly recolonised tree bark and selection pressure reversed itself.

Even today, light-coloured peppered moths persist in small numbers in areas where the dark form once dominated, and dark moths still turn up occasionally in cleaner regions. This lingering minority is a reminder that selection, even when strong, rarely wipes out a recessive or disadvantaged allele completely within a short time. Migration between populations, occasional mutation, and simple chance all slow down what looks like a straightforward one-way process. Modern genetic work has since traced the original melanic mutation to a single structural change in a gene called cortex, arising around 1819, just as coal-based industry was taking off, which fits neatly with the timeline naturalists had already pieced together from field observations.

Why this matters beyond moths 🔗

The peppered moth story is memorable, but the underlying principle applies well beyond one species of insect. Any time a population faces a sudden, strong, and one-directional environmental pressure, whether that’s a new predator, a changed habitat, or a human-introduced chemical, you can expect a similar pattern: a formerly rare allele gaining ground, a formerly common one retreating, and a population caught somewhere in between until the pressure stabilises or the shift completes. Recognising this pattern helps make sense of everything from pesticide resistance in insects to antibiotic resistance in bacteria, both of which follow the same basic genetic script.

What do you think? If pollution levels rise again in a given region, do you think peppered moth populations could shift back toward the dark form within our lifetimes, or would other factors slow that process down? And can you think of another species, besides moths or bacteria, where a human-driven environmental change might currently be pushing a population through its own transient polymorphism?

References

1. https://www.biologyonline.com/dictionary/polymorphism
2. https://www.encyclopedie-environnement.org/en/life/genetic-polymorphism-and-selection/
3. https://pmc.ncbi.nlm.nih.gov/articles/PMC3668657
4. https://learningzone.oumnh.ox.ac.uk/peppered-moth-natural-selection-experiments
5. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3391436
6. https://www.nature.com/articles/hdy201292
7. https://carnegiemnh.org/pittsburghs-moths-reflect-environmental-impacts-of-industry/