Pingelap atoll, or how a human population evolved to full-colour blindness.

The pingelap atoll, is a small island in the federation of Micronesia, with an even smaller population of 250 people. Now what is special about the population of this insignificant atoll is that almost 10% of its population has complete achromatopsia which is an autosomal recessive condition that is generally rare in human populations (BBC, 2015). And so the question has been raised about the possible factors that might have led to
the predominance of such trait.

Achromatopsia is a genetic condition due to the gene CNGA3 in chromosome 2q11 (Sheffield, Val C, 2000), this trait is autosomal recessive and is generally rare (according to the achromatopsia group: achromatopsia affects 1 person for every 33000 people in the united states, giving it a rate of occurrence of 0.003% ). This trait manifests itself “ by reduced visual acuity, pendular nystagmus, increased sensitivity to light (photophobia), a small central scotoma (which is often difficult to demonstrate), eccentric fixation, and reduced or complete lack of colour discrimination” (Sheffield, Val C, 2000). Or in simple words, this condition is characterized by a complete lack of cones
in the retina, making the affected carrier see the world in shades of grey.
The most accepted theory for the predominance of achromatopsia is a population bottleneck due to the Lengkieki typhoon (Sheffield, Val C, 2000), that decimated the population of the Pingelap and neighbouring Mokil Island, thus making all the actual pingelapese descendants of this handful of survivors. It is certain that out of these survivors one element at least has been a carrier of CNGA3 (I.E.Hussels, N.E.Morton, 1972). Through their research, I. E. Hussels and N. E. Morton proved that it might have been Nanmwarki Mwahuele, a prominent survivor of the typhoon that was the carrier of
this gene, which he will then pass to the following generations, as he had multiple children with his three wives. Multiple inbred matings will then favour the prominence of this trait in the pingelapese population (I.E.Hussels, N.E.Morton, 1972). Moreover, a more debatable hypothesis is that such maskuns (the local name given to the colourblind population of the atoll) were advantaged for night fishing thus turning what would seem like a disadvantage, into a clear advantage (BBC, 2015). However, this is debatable as night fishing isn’t as profitable as fishing in the day which is hard for the maskuns as they suffer from light sensitivity due to achromatopsia.

This is a clear example of one of the most common evolutionary force: Genetic drift, which is the effect that random events, that aren’t considered part of natural selection, have on population by sampling them thus giving more predominance to certain alleles that were rare in that population before the event. Such events aren’t just interesting anecdotes, but also living examples of the power evolutionary forces have on the genetics of populations, and that humans make no difference by being affected by such forces of evolution as well.

References:
BBC, The island of colour blindness, 2015
Sheffield, Val C. “The Vision of Typhoon Lengkieki.” Nature Medicine, vol. 6, no. 7,
2000
I.E.Hussels,N.E.Morton, “Pingelap and Mokil Atolls: Achromatopsia “, 1972
I.E.Hussels, “gelap and Mokil Atolls: Historical Genetics “, 1972