Pingelap Atoll is a small atoll in Micronesia, just under 2 km in size2with a unique characteristic in the world: approximately 5% of the population is affected by achromatopsia, a very rare genetic disease of the retina which prevents the perception of colours. Cases of achromatopsia on the island began to slowly increase starting in 1775, when a catastrophic typhoon hit Pingelap, decimating its population. Following that event, a phenomenon called “bottleneck” – a form of evolution independent of natural selection and driven by chance – favored the spread of mutated copies of the gene responsible for achromatopsia, which spread one generation after another until Pingelap was famous as the “island without colors”. Today the atoll, located in the middle of the Pacific Ocean, has a population of approximately 360 inhabitants.
A paradise without colors
In the heart of the Pacific Ocean, about 14,000 kilometers as the crow flies from Italy, there is a small atoll in southern Micronesia called Pingelap. It is a true earthly paradise, where the green of the coastal woods is reflected in the blue of its crystalline waters, separated from each other only by long stretches of golden beaches. In short, one of those places that, at the sight of just one photo, makes you want to spend at least a week’s holiday there. Or, in the most extreme cases, to be reborn there, far from the stress of the Westernized world.
Too bad that if we were born on Pingelap, we probably wouldn’t be able to enjoy all the wonderful shades of color that the island has to offer every day. In fact, approximately 5% of the population suffers from achromatopsia, a very rare genetic disease that affects the cones, the cells of the retina that allow us to see colours, causing strong sensitivity to light (photophobia), reduced visual acuity, but, above all, the complete inability to distinguish colours.
The first cases of achromatopsia after the typhoon of 1775
The geographical location and climatic conditions make Pingelap particularly vulnerable to extreme natural disasters. One of the most devastating occurred in 1775, when typhoon Lengkieki hit the island, exterminating around 90% of the inhabitants and leaving only 20 survivors alive. However, as the old saying goes, ighey never come alone. Thus, while over the next two centuries the few survivors tried to repopulate the island, achromatopsia began to spread rapidly among the newly born.
According to local myths, the person responsible for this disaster must have been Isoahpahu, an evil nocturnal spirit who, having fallen in love with some women on the island, impregnated them through deception, leading to the birth of children who were comfortable in the darkness, just like him. Today, however, thanks to the work of anthropologists, ethnographers and geneticists, we know that the explanation is entirely scientific.
At the origin of Pingelap’s achromatopsia
By going back four generations, for a period of about 200 years, the family tree of the approximately 3,000 inhabitants of the island, some scholars in the 1970s managed to trace the “father” of all Pingelapese affected by achromatopsia: The Nanmwarki Mwahuele.
In 1775, Mwahuele was not just any Pingelapese, but the Nanmwarki, or the highest political and religious authority of the community. Although he was not affected by the disease, he carried, without his knowledge, a mutated copy of the gene for achromatopsia, which however manifests itself clinically only in recessive homozygosis (i.e. in the presence of two altered copies). That is, he was what we now call a “healthy carrier”.
After the devastating typhoon, Nanmwarki Mwahuele contributed significantly to the repopulation of the island, marrying three women, with whom he had numerous children. Some of them inherited a copy of the altered gene. And so do their children’s children. With the passing of generations, and in the absence of contact with the outside world, marriages between blood relatives became inevitable, thus favoring the birth of children with two mutated copies of the same gene, and therefore affected by achromatopsia.
A particular case of evolution
At this point, a question arises spontaneously: why have the “defective genes” of achromatopsia spread so much, instead of being eliminated by natural selection? After all, Darwin taught us that for a trait to spread in a population it must confer an advantage in its environment. And achromatopsia, without a doubt, has none. Cases like that of Pingelap are called “bottlenecks” and represent particular examples of evolution in which those who are advantaged are not the most suitable, but the luckiest.
In Pingelap, chance – and not environmental pressures, as happens in natural selection – meant that Nanmwarki Mwahuele survived the typhoon of 1775, a healthy carrier of an extremely rare gene in the world population, who without that catastrophic event would have slowly disappeared from the island. But in a suddenly decimated population, devoid of reproductive exchanges with the outside world and with inbreeding, that gene found fertile ground to spread, taking away, one generation after another, the colors from the island of Pingelap.
