Anomaly and Norm

The Adaptability of Life Between Anatomy and Biology

The Anomaly that becomes the Norm

In the first half of the 19th century, the Ancon sheep gained some popularity among English breeders. This breed was different from normal sheep due to the severe shortening of the front legs, which prevented these animals from jumping over fences. It was most likely a case of dwarfism, inherited, which appeared suddenly in 1791. In The Origin of Species (1859), Charles Darwin (1809-1882) referred to this anomaly, favored by humans for easier management of these animals, as an example of what Nature itself can do by continuously selecting the individuals best suited to the environment in which they live.

The theory of evolution thus provides a place for individuals that differ from the norm but may be better suited to survive in a particular environment, which promotes their spread over time. This is what happened to Biston betularia, a species of moth common in England during the 19th and 20th centuries: this nocturnal butterfly spends the day motionless on the bark of birch trees (hence its name), rendered almost invisible by its light coloration.

During the Industrial Revolution, smog and poor air quality covered the birch trees with a layer of dark soot: in just a few years, the moth, once perfectly camouflaged, became a light spot on a dark tree, highly visible. Some anomalous individuals, with very dark bodies (the carbonaria variety), represented a small percentage of the population, but with the environment altered by human pollution, they suddenly thrived.

A disadvantageous anomaly proved to be crucial for survival, and made this variety very common, to the point that by the end of the 19th century, the percentage of dark moths was close to 100%. The anomaly had become the norm: the environment had selected an anomalous trait as necessary, making it common throughout the species in a short period of time.