Bacteria, butterflies, Covid-19: accelerated evolution – Sciences et Avenir

This article is from the magazine Les Indispensables de Sciences et Avenir n°208 of January/March 2022.

When a butterfly reveals its dark side

The birch moth (Biston betularia) is considered the emblematic example of evolution in action. This small butterfly usually has light-colored wings decorated with darker spots that form a mosaic (typical shape). But in 1848, an entirely black form of the insect was discovered in England near Manchester, where it became increasingly abundant over time. We are then in the midst of the industrial revolution, and the factories in the north of the country are spitting out their coal soot in a continuous stream. In 1896, a few years after the publication of Darwin’s theories, a naturalist committed to evolutionary ideas, James William Tutt, argued that the dark forms – called carbonaria – were favored because they went unnoticed over logs blackened by pollution. .thus escaping the sight of birds of prey.

In 1955, the British zoologist Bernard Kettlewell developed an experiment to test this hypothesis of natural selection. He proceeds to release light and black moths, in one contaminated and one uncontaminated area, before recapturing them and counting the survivors. It seems that the black form is much more conserved in polluted regions, while it is the preferred target of birds in uncontaminated areas, where its dark color contrasts with the light trunks. A perfect example of selective advantage to illustrate the theory of evolution.

Too perfect… Because the Kettlewell experiment raises methodological criticism. It is criticized for numerous biases, in particular for having used very few insects and for having unrealistically placed them on the trunks (this would not be the main natural refuge of the moth). For many creationists, the little butterfly becomes the shining symbol that the theory of evolution is a farce! However, in the 1990s, the British biologist Michael Majerus showed that Kettlewell’s conclusions should not be disqualified.

This article is from the magazine Les Indispensables de Sciences et Avenir n°208 of January/March 2022.

When a butterfly reveals its dark side

The birch moth (Biston betularia) is considered the emblematic example of evolution in action. This small butterfly usually has light-colored wings decorated with darker spots that form a mosaic (typical shape). But in 1848, an entirely black form of the insect was discovered in England near Manchester, where it became increasingly abundant over time. We are then in the midst of the industrial revolution, and the factories in the north of the country are spitting out their coal soot in a continuous stream. In 1896, a few years after the publication of Darwin’s theories, a naturalist committed to evolutionary ideas, James William Tutt, argued that the dark forms – called carbonaria – were favored because they went unnoticed over logs blackened by pollution. .thus escaping the sight of birds of prey.

In 1955, the British zoologist Bernard Kettlewell developed an experiment to test this hypothesis of natural selection. He proceeds to release light and black moths, in one contaminated and one uncontaminated area, before recapturing them and counting the survivors. It seems that the black form is much more conserved in polluted regions, while it is the preferred target of birds in uncontaminated areas, where its dark color contrasts with the light trunks. A perfect example of selective advantage to illustrate the theory of evolution.

Too perfect… Because the Kettlewell experiment raises methodological criticism. It is criticized for numerous biases, in particular for having used very few insects and for having unrealistically placed them on the trunks (this would not be the main natural refuge of the moth). For many creationists, the little butterfly becomes the shining symbol that the theory of evolution is a farce! However, in the 1990s, the British biologist Michael Majerus showed that Kettlewell’s conclusions should not be disqualified.

Between 2001 and 2008, he released nearly 5,000 butterflies in the same region to observe the effects of natural selection. The data, exploited after his death, confirm the hypothesis of selection by predation… to the benefit of the lighter butterflies. Because since Kettlewell’s time, pollution has decreased and the black form has almost disappeared in nature…

In 2018, two researchers from the University of Exeter (UK) modeled the vision of a predatory bird, the blue tit, and showed that butterflies of typical shape were less easily identified by the bird because they were more easily confused with proboscises. . Despite this body of evidence in favor of evolution, the humble butterfly is still hotly debated. Scientific, but above all ideological.

The birch moth has been studied since the 19th century. It was then that along with the light form of this butterfly, at that time the only one known, a black version spread in the areas darkened by the smoke from the chimneys. Credit: ALEX HYDE/NATUREPL/BACCEGA

Longest lab evolution experience

One day in the summer of 1988, American biologist Richard Lenski took twelve samples of the same strain of Escherichia coli bacteria and placed them in twelve vials filled with a minimized, low-glucose nutrient medium. Evolution’s longest experiment in the lab has just begun: it involves observing how quickly different strains mutate and multiply. To do this, students ritually take 0.1 milliliters from each flask every day and re-grow it in a new flask filled with the same nutrient medium.

More than 70,000 generations of bacteria have occurred in the laboratory to date. “We found that all twelve colonies have fairly similar evolutionary paths around the world. They adapted effectively to their poor environment. It took an hour for the original strain to multiply. After 30,000 generations, it only takes forty minutes. Today, this speed of multiplication has reached a point, but bacteria continue to mutate and adapt. They are constantly evolving. This is undoubtedly an intrinsic property of living organisms”, emphasizes Dominique Schneider, from the TIMC (Translational Research and Innovation in Medicine and Complexity) laboratory in Grenoble, partner of the American Experiment for which he performs molecular biology analyses.

Around generation 30,000, a surprise awaited the researchers: in addition to glucose, one of the twelve colonies began to absorb citrate, a molecule intended solely to maintain the pH of the culture medium. The likely result of a series of rare mutations over a long period of time. “In bacteria, in the natural environment, this ability to use another nutrient and to coexist without competition is the first step for the appearance of a new species”, points out the researcher from Grenoble.

The experiment has another fascinating goal: to “reproduce” evolution. To do this, the team froze samples of the original strain and then brought them back to life to introduce, in varying order, mutations that appeared thousands of generations later. “Result: the order in which mutations appear influences selection. This confirms Darwin’s hypothesis that evolution is gradual”, explains Dominique Schneider.

Today, six populations out of twelve have a very high mutation rate, up to 100 times that of the original strain. “Over time, bacteria evolve towards greater complexity, they restructure their metabolism and present more mutations, beneficial or not, on which the selection process will be exerted”, summarizes Dominique Schneider. In other words, evolution – it even evolves!

For nearly thirty-five years, biologist Richard Lenski has been growing bacteria under identical conditions to observe their evolutionary behavior. Credit: CHARLOTTE BODAK

Covid-19: predicting variants one day

Alpha, beta, gamma, delta, omicron… The variants of the SARS-CoV-2 virus responsible for Covid-19 appear according to a typically Darwinian evolutionary mechanism. When it multiplies, the virus copies its genetic material, which consists of RNA, in countless copies. During this process, copy errors occur. Most are repaired by the virus machinery, but not all: it is then a mutation. The sequencing of the SARS-CoV-2 genome has identified thousands of them. They refer mainly to the genes for the Spike protein, which allows it to attach itself to human cells in order to infect them. But only a few give it a selective advantage: greater virulence, or a more contagious character, for example, which explains its rapid spread. Today, population genetics, which studies the distribution and spread of different forms of genes (alleles), is able to predict whether one variant will become dominant, as was the case with alpha and delta variants. On the other hand, there is still no evolutionary model that can predict, from a viral genome, the appearance of a mutation that would give rise to a new variant. The large amount of data accumulated during the pandemic should facilitate the development of these types of tools.