Natural Selection among Biston Betularia

Peppered moths (Biston betularia) have long been used as an example of natural selection leading to changes in populations. These white and dark colored moths existed mainly in light colored forms prior to the industrial revolution. Once sooty air started to invade their environment in Europe, the populations were more likely to be dark colored, a process referred to as industrial melanism. The light colored moths then came back after clean air laws and regulations became commonplace. The theory has been that the darker colored moths were camouflaged against soot colored trees and overlooked more often by predatory birds. This simulation of selection pressures based on the color of the moths may make the concept more clear.

Some have argued that the shift in the color of the peppered moth was not evolution. Evolution requires some genetic pressure that permanently alters a population or creates a new population from the original species. In April 2011, evolutionary biologists identified the genetic region underlying the color switch, suggesting that, yes, the moths evolved due to natural selection.

As reported by Jef Akst of The Scientist, the study published in Science Express on April 14 by a research team led by an ecological geneticist at the University of Liverpool provides the long missing molecular evidence of what most biologists figured all along. A single genetic mutation in the genome of the peppered moth appears to be responsible for the dark color of the ‘carbonaria’ form. Specifically, they identified a 200 kb portion of the genome that is homologous to chromosome 17 in the silkworm. The researchers sampled moths from 80 different locations in the United Kingdom. They found the same genetic fingerprint in all of them using BAC-FISH gene mapping (see image at Physorg).

Based on patterns of linkage within the chromosome, the researchers suspect it was a relatively recent alteration from the lighter ‘typica’ form of the moth (which is considered the wildtype form), corroborating timing with the industrial revolution.

However, several questions remain. Most importantly, exactly which gene is it? They were able to isolate the chromosomal region, but not the gene itself. The same genomic region was previously found to affect wing patterns in the related Heliconius butterflies, making the find even more intriguing to evolutionary biologists. Another question is what is the exact mutation that causes the two colors of moth. Especially in the context of the gene’s involvement in complex wing patterns, something that should include multiple genes, the research findings beg the question – Is there a more complex pattern humans are missing?