Seasonal polyphenism in butterfly wings is the ability of some butterfly species to develop distinctly different wing colours and patterns depending upon the season. This is most common in tropical species like Bicyclus anynana of Africa, the wings of which resemble large eyes during the wet season but not during the dry season.
In biology, seasonal polyphenism refers to the ability of multiple seasonally determined phenotypes to arise from a single genotype. What that means, in layperson's terms, is that a single set of genes in the butterfly's DNA can give rise to more than one type of colour pattern on its wings.
That contrasts with, for instance, human hair and eye colour - in those cases, one set of genes gives rise to only one possible colour. However, it does resemble the coat thickness and colour changes which some mammals undergo between summer and winter, except that insects like butterflies are only alive for a single season. An individual butterfly's wings will not change colour patterns during its lifetime, but its genes carry all the necessary information for subsequent generations to express different colours. This ability for an organism to alter its phenotypes, or physical traits, in response to changes in its environment is also known as phenotypic plasticity. Interested readers can find more information about seasonal polyphenism in butterfly wings in research papers published by biologists like Scott F. Gilbert, Tony Frankino, and Paul Brakefield.
Most butterfly species which exhibit seasonal polyphenism can be found in the tropics. Although there are several such species, the most well-known and the most extensively researched is Bicyclus anynana, also known as the squinting bush brown butterfly, which is found throughout southern and eastern Africa. According to Frankino, who is part of an international research group studying this particular species, B. anynana looks strikingly different between the wet season and the dry season. During the wet season, the butterflies have "large, conspicuous, concentric rings (eyespots)" on their wings, but "these markings are reduced or nearly absent" during the dry season. Pictures of the wet season and dry season wing patterns of B. anynana may be found on the website of Swarthmore College.
Biologists still aren't sure exactly how one set of colour pattern genes can lead to different colour patterns on butterfly wings, depending simply on during which season the butterfly develops. One theory is that the eye-spots are useful for scaring off potential predators during the wet season, when the butterflies are particularly active and therefore potentially exposed. However, in the dry season, the butterflies might be better off avoiding hungrier predators altogether, in which case their nondescript brown wings might provide useful camouflage.
Breeding many generations of butterflies in the laboratory, scientists have discovered that it is possible to affect how B. anynana develops its wing colour patterns by adjusting the temperature and environmental conditions, and they have also discovered that it is possible to alter the wing colour patterns through selective breeding. However, even if that's true, they're still not sure how these genes really work, and research is ongoing.