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| In the last 500 years, the people of Cabo Verde have evolved at a breakneck pace -- thanks to a genetic variant, inherited from their African ancestors, that protects against the ravages of malaria. Credit: F. Mira |
Imagine if you will, you are setting out to prove Darwin's Theory of Evolution by Natural Selection. First off you fix in your mind the definition of evolution - change in allele fequency in a poulation over same - an allele being a variation of a gene.
According to Charles Darwin's theory, this happens when the environment differentially selects the allele in question for 'favour' so more descendants end up with the favoured allele, compared to those with the less favoured allele. The hypothesis you are testing then is that a population will, given enough time, have a greater frequency of the given allele than it started out with and this will be because of something natural in the environment. To check this latter requirement, you need to show that when that something in the environment is missing, there is no change in allele frequency in the population.
So, imagine if you could take large enough number of people, a few with the gene that gives an advantage in the given environment, and a lot with the 'normal' gene and put them on different islands in an archipelago, one with an environement which favours one allele, and the rest with an environment that doesn't favour any of them. Then you leave this setup for nature to take its course and then go back and measure the frequency of the different alleles on the different islands in, say, 500 years, if that were possible.
The hypothesis is that on the island where the favoured allele gave an advantage, there would, after enough time, be a greater frequency of it in the population gene pool than on the islands where there was no advantage to be had.
And this is precisely what the Portuguese colonialist who discovered the then uninhabited islands off the west coast of Africa, unintentionally set up. And 500 years later we have the results of this natural 'experiment'.
Robin A. Smith in the Duke University press release explains:
Malaria is an ancient scourge, but it’s still leaving its mark on the human genome. And now, researchers have uncovered recent traces of adaptation to malaria in the DNA of people from Cabo Verde, an island nation off the African coast.In other words, the environment on the island of Santiago, where malaria was prevalent, produce a population where the frequency of the allele which produced resistance to the malaria parasite, Plasmodium vivax, compared to islands where malaria was less common. All the islands had been populated initially by the same people with the same frequency of the allele conveying the resistance.
An archipelago of ten islands in the Atlantic Ocean some 385 miles offshore from Senegal, Cabo Verde was uninhabited until the mid-1400s, when it was colonized by Portuguese sailors who brought enslaved Africans with them and forced them to work the land.
The Africans who were forcibly brought to Cabo Verde carried a genetic mutation, which the European colonists lacked, that prevents a type of malaria parasite known as Plasmodium vivax from invading red blood cells. Among malaria parasites, Plasmodium vivax is the most widespread, putting one third of the world’s population at risk.
People who subsequently inherited the protective mutation as Africans and Europeans intermingled had such a huge survival advantage that, within just 20 generations, the proportion of islanders carrying it had surged, the researchers report.
Other examples of genetic adaptation in humans are thought to have unfolded over tens to hundreds of thousands of years. But the development of malaria resistance in Cabo Verde took only 500 years.
“That is the blink of an eye on the scale of evolutionary time,” said first author Iman Hamid, a Ph.D. student in assistant professor Amy Goldberg’s lab at Duke University.
It is unsurprising that a gene that protects from malaria would give people who carry it an evolutionary edge, the researchers said. One of the oldest known diseases, malaria continues to claim up to a million lives each year, most of them children.
The findings, published this month in the journal eLife, represent one of the speediest, most dramatic changes measured in the human genome, says a team led by Goldberg and Sandra Beleza of the University of Leicester.
The researchers analyzed DNA from 563 islanders. Using statistical methods they developed for people with mixed ancestry, they compared the island of Santiago, where malaria has always been a fact of life, with other islands of Cabo Verde, where the disease has been less prevalent.
The team found that the frequency of the protective mutation on Santiago is higher than expected today, given how much of the islanders’ ancestry can be traced back to Africa versus Europe.
In other words, the chances of a person surviving and having a family thanks to their genetic code -- the strength of selection -- were so great that the protective variant spread above and beyond the contributions of the Africans who arrived on Santiago’s shores. The same was not true elsewhere in the archipelago.
The team’s analyses also showed that as the protective mutation spread, nearby stretches of African-like DNA hitchhiked along with it, but only on malaria-plagued Santiago and not on other Cabo Verdean islands.
In other words, evolution has occurred over a period of 500 years exactly as Darwin's theory predicts.
The teams findings were published open access in elifesciences:
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| Enrichment of West African ancestry at the DARC locus in Santiago, Cabo Verde. (A) Map of Cabo Verde islands and sample sizes for number of individuals from each island region. (B) The distribution of West African-related local ancestry proportion across the genome by SNP (n = 881,279) by island, with the DARC locus marked by vertical red lines. Local ancestry was estimated using RFMix (see Materials and methods). The DARC locus is an outlier for high West African-related ancestry in Santiago, but not Fogo or the Northwest Cluster. |
So there we have it! An experiment inadvertently set up 500 years ago by Portuguese colonists and allowed to run its course for 500 years has produced exactly what Darwinian evolution by natural selection would be expected to produce - an increase in allele frequency in a population over time in response to a selective environment, all neatly controlled.Abstract
Humans have undergone large migrations over the past hundreds to thousands of years, exposing ourselves to new environments and selective pressures. Yet, evidence of ongoing or recent selection in humans is difficult to detect. Many of these migrations also resulted in gene flow between previously separated populations. These recently admixed populations provide unique opportunities to study rapid evolution in humans. Developing methods based on distributions of local ancestry, we demonstrate that this sort of genetic exchange has facilitated detectable adaptation to a malaria parasite in the admixed population of Cabo Verde within the last ~20 generations. We estimate that the selection coefficient is approximately 0.08, one of the highest inferred in humans. Notably, we show that this strong selection at a single locus has likely affected patterns of ancestry genome-wide, potentially biasing demographic inference. Our study provides evidence of adaptation in a human population on historical timescales.Hamid, Iman; Korunes, Katharine L; Beleza, Sandra; Goldberg, Amy
Rapid adaptation to malaria facilitated by admixture in the human population of Cabo Verde
eLife 2021;10:e63177 DOI: 10.7554/eLife.63177
Copyright: © 2021 The authors. Published by eLife Sciences Publications Ltd.
Open access
Reprinted under a Creative Commons Attribution 4.0 International License (CC BY 4.0)
All that remains now is for creationists to explain why this isn't an observed case of rapid human evolution. I won't hold my breath!
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