In terms of biological evolution , and for almost a century, one of the examples that has best illustrated the mechanism of natural selection has been the industrial melanism observed in the birch moth ( Biston betularia ).
It was the evolutionary biologist John BS Haldane , one of the most prominent scientific popularizers of his time, and one of the first scientists to formulate a feasible theory of the abiogenesis process, who indicated for the first time, in 1924, that after the phenomenon of melanism industrial there should be a selective pressure . However, creationist pressure groups have always opposed this interpretation of the event, arguing that it is an acclimatization and not a true evolutionary process.
In order to know if the industrial melanism suffered by birch moths is really an evolutionary process or not, we must understand what industrial melanism is and what we mean by “evolutionary process”.
What is industrial melanism?
One of the characteristics that allow the birch to be identified even in winter, without leaves, is its bark. The birch is a tree with a peculiar trunk, with white bark with small gray or black spots, in the form of horizontal cuts, called lenticels . The birch moth does not get its name for nothing, as it is a common animal in birch forests; in fact, its scientific name, Biston betularia , refers to these trees belonging to the Betula genus.
The birch moth is an insect that camouflages itself very efficiently when it perches on tree trunks. Its coloration, white with dark spots, is very similar to birch bark, so that, at rest, it is very difficult to observe.
But if the human being has proven to be good at something, it is in altering the environment. When the Industrial Revolution began, tons of ash and soot were released into the atmosphere as a result of burning coal. That dust ended up deposited on all surfaces, including tree trunks. Birch trees, once white, were stained black blight and moths, once exceptionally camouflaged, now became perfect targets for predators, especially birds.
However, some moths managed to hide successfully in the sooty trunks: they were black instead of white . Before long, most of the moth population was black in color. The dark coloration caused by accumulation of melanin is called melanism , and in this specific case it was called industrial melanism .
In 1958, evolutionary biologist Henry Bernard D. Kettlewell experimentally confirmed Haldane’s hypothesis: the main driver for industrial melanism in the birch moth was selective pressure, specifically the pressure caused by predatory birds on the moths. But was this simply acclimatization or was it really a case of biological evolution?
Acclimatization versus adaptation
The concept of biological evolution refers to cumulative heritable changes in a population due to selective pressure .
Undoubtedly, what happened in the moth population was due to selective pressure, but to consider it ‘evolutionary change’ it was necessary to verify that the changes were heritable. Cumulative evolutionary changes that provide advantages to the population are called adaptations .
The alternative to an evolutionary explanation is that it is acclimatization . Acclimatizations are physiological modifications that occur in an organism in response to changes in the environment. acclimatizations are not heritable traits .
One of the best examples of acclimatization is the height of cabbages. When a cabbage grows in permeable soil, the surface of which remains relatively dry, the stem grows short and wide, and its leaves grow close together. But if it grows in very humid soil, the lower leaves tend to rot, which induces a stem growth in the plant, which becomes longer and thinner; its leaves are thus separated from each other along the stem. If you take a seed from a long-stemmed cabbage and plant it in dry soil, you will get a short-stemmed cabbage, and vice versa. Cabbage length is therefore a non-inheritable acclimatization.
What is the case of industrial melanism in birch moths? Is it the soot in the environment that somehow turns the moths black, acclimating to the new environment? Or is there really a genetic trait that gives moths that melanism? Is it acclimatization or evolutionary adaptation?
The evolutionary adaptation of industrial melanism
Several experiments indicated that there was no acclimatization mechanism that induced melanism in moths. The food or habitat of the caterpillars did not in any way influence the coloration of the adults , which depended directly on the coloration of their parents. Furthermore, the melanism trait was observed to follow a purely Mendelian inheritance . All this pointed to the fact that it was indeed an evolutionary adaptation.
But it was necessary to wait until the 21st century to verify the existence of a specific gene that encodes the color trait in birch moths. Specifically, it is a transposon , or transposable genetic element, a gene with the ability to move from one part of the genome to another. Mutations can occur in the transposition process, and one of them is the cause of melanism. Recent genetic studies indicate that the transposition that gave rise to the industrial melanism event occurred around the year 1819 . The subsequent natural selection, caused by the predation of birds, which saw the white moths better than the melanistic ones on the black trunks, was what favored the generalization of the trait in the population, establishing itself as an evolutionary adaptation.
In short, despite what creationists try to say, the industrial melanism of Biston betularia remains a perfect example to illustrate the evolutionary process in an understandable way.
References:
Hof, A. E. van’t et al. 2016. The industrial melanism mutation in British peppered moths is a transposable element. Nature, 534(7605), 102-105. DOI: 10.1038/nature17951
Kettlewell, H. B. D. 1955. Selection experiments on industrial melanism in the Lepidoptera. Heredity, 9(3), 323-342. DOI: 10.1038/hdy.1955.36
Walton, O. C. et al. 2018. Avian vision models and field experiments determine the survival value of peppered moth camouflage. Communications Biology, 1(1), 1-7. DOI: 10.1038/s42003-018-0126-3
