Pepper and the Pollution Solution

Kelly’s Field Notes


Common Name: Peppered Moth


Order: Lepidoptera


Family: Geometridae


Genera: Biston


Species: Biston betularia


Description: 


These moths are named for the “peppered” or speckled look on their wings. They are white with black spots on all four wings, and stripes on the ends of all four wings. The peppered appearance varies by the individual. Peppered moth wingspans are around 4.5 to 6.2 cm (1.8 - 2.4 in) in length. They come in several color morphs. 


The larvae, often referred to as “inch worms” in this family, mimic twigs. They can be either green or brown in color, changing to match the twig they are on. In 1887, the British evolutionary biologist, Edward Bagnall Poulton, described them as camouflage using countershading - the first time an animal had been described as such! Additionally, the moths are camouflaged to blend in with lichen on the trees (some color forms better than others) when viewed not only with our eyes but through birds’ eyes (ultra-violet). This was tested by using an ultraviolet sensitive camera.


Life Cycle: 


Female pepper moths lay around 2,000 eggs into the crevices of tree bark. While this is happening the male she mated with guards her until she is finished laying eggs to ensure paternity. Once the eggs hatch the larvae feed on oak, willow, and/or birch leaves until they are large enough to pupate. Pupation happens in the soil, over wintering in the safe and warm soil. They emerge as adults in April and May. While males fly every evening to find females to mate with, females only fly the first night they emerge, relying on pheromones to attract males after.


In North America this species is bivoltine (two generations per year), while in the U.K. it is univoltine (one generation per year).


Color Forms:


Peppered moths have several color morphs - melanic (dark/excess of melanin) and non-melanic. Forms are not subspecies, they are the same species with different melanin levels, they are not genetically distinct enough to be a subspecies. Peppered moths are often used as a classic example of polymorphism and natural selection (more on that in the controversy section). Polymorphism, in biology, is the occurrence of more than one form/morph within a population. This is caused by the expression of alleles (alternative forms of a gene). In the peppered moth there are three types of expression; dark morph (homozygous dominant), the white morph (homozygous recessive), and intermediate (heterozygous). If an allele is homozygous dominant the moth has two copies of the dominant gene, in this case this causes the moth to be black. The white moths are homozygous recessive, they must have two copies of the white allele to be white. The intermediate moths, which are a blend of the two, are heterozygous meaning they have a copy of each of the other two alleles giving the moth an in between morph. 


Color Forms and Pollution:


To add to the interesting variety of peppered moth colors, not only are they blending in with different lichen species on trees, they are blending in with pollution. In the nineteenth century, during the rise of the Industrial Revolution, scientists in England noticed peppered moths began favoring their darker form. Instead of their camouflage matching lichen, which was dying, it started to shift to favor camouflage that matched sooty trees. Moths that stood out against the soot were quickly snapped up by birds. Where once the intermediate form was advantageous, now the dark form took over, hiding resting moths from birds during the day as they blended in with the soot. As pollution continued to advance even the already dark form became darker.


As time went on and pollution standards changed, clearing the trees of soot and inviting the lichen back, so too did our friends the peppered moths shift back to their lighter and intermediate form. This shift led to the term “industrial melanism.”


Peppered moth populations in North America followed a similar trajectory as those in England. Shifting with pollution patterns. However, there were no shifts in populations in Japan because peppered moths are not found near industrialized areas in Japan.


Color Forms Genetics - 


The black form of the peppered moth was first reported in 1848 (a decade before Darwin or Wallace wrote about natural selection). In 2016 researchers discovered which gene was responsible for the industrial melanism in the peppered moth. Scientists tracked all of the genetic differences between the dark form and the typical light form. This led them to the single mutation that caused the dark form. What’s really neat about this is it is a rarer type of mutation - a transposon (a piece of DNA that “jumps” around to different positions in a genome). Transposons can be damaging if they pop into a gene, disrupting its usual function, but in this case it was helpful! Once this was located in the cortex gene, scientists could deduce this mutation happened somewhat recently (because recombination had not happened at a high frequency near that gene). The jumping of this strand of DNA likely happened within a decade centered on 1819. 


Natural Selection Controversy -


Bernard Kettlewell, of Oxford University, the original scientist to conduct an experiment on bird predation on different color forms of peppered moths came under fire for his research. He claimed this was an excellent example of natural selection. In this case, birds forcing the color form selection of moths to avoid predation. This is a form of directional selection, where an extreme of a trait is selected for. 


Kettlewell released tagged moths of the dark and typical forms in both polluted and unpolluted forest to see which was predated on by birds in greater numbers. He found that typical forms were preyed upon more in polluted forests and dark forms were preyed upon more in unpolluted forests. 


This experiment was replicated by Theodore David Sargent in the 1960’s in which he did not get the same results. He claimed birds showed no preference. He went so far as to accuse Kettlewell of training the birds.


In the 1990’s the experiment was critiqued by Michael Majerus in his book Melanism: Evolution in Action and by Jerry Coyne. Both called for more experimentation and that this is not a great example of natural selection. Additionally, Judith Hooper went so far as to claim his experiment was fraudulent because his notes could not be found. She also claimed the photos of moths were staged - dead moths on logs. This got scientists so rattled some demanded the removal of this classic example from textbooks. Her allegations have since been found to be without merit.


To make matters worse, creationists jumped on board the Kettlewell hate train in the late 90’s early 00’s. Phillip E. Johnson claimed the moths were glued to tree trunks and never rest on tree trunks anyway so the entire study must be a scam. This is not true at all, as peppered moths do rest on tree trunks (as well as tree branches and under twigs).


Back to Michael Majerus! He decided to replicate Kettlewell’s experiment to put this whole thing to rest, one way or another. He created the largest ever study of industrial melanism - observing 135 moths. He found 37% were in fact resting on tree trunks even if a greater percentage were resting on the lower half of branches. This immediately answers the criticisms of the creationists. He also observed birds preying upon the moths, with the majority in unpolluted forest praying upon the dark form of the moths. This vindicated Kettlewell’s original study and the natural selection theory. Sadly, Majerus died before he could finish writing his findings, but his works were later published in 2012 by a team of his colleagues. Subsequent studies have continued to support the link. The peppered moth has remained an example of natural selection in textbooks today. 



Peppered Moth Super Powers:


Peppered Moths in Culture:


References:



Asami, T. and Grant, B. (1995). Melanism has not evolved in Japanese Biston betularia (Geometridae). Journal of the Lepidopterists’ Society, 49: 88-91.




Eacock, Amy, et al. "Adaptive colour change and background choice behaviour in peppered moth caterpillars is mediated by extraocular photoreception." Communications biology 2.1 (2019): 286.


Grant, Bruce, and Rory J. Howlett. "Background selection by the peppered moth (Biston betularia Linn.): individual differences." Biological Journal of the Linnean Society 33.3 (1988): 217-232.


Hof, Arjen E. van’T., et al. "The industrial melanism mutation in British peppered moths is a transposable element." Nature 534.7605 (2016): 102-105.


Keep, Stephanie. “What’s the Problem with Peppered Moths?” National Center for Science Education, 2 Sept. 2014, ncse.ngo/whats-problem-peppered-moths.


“Peppered Moth.” Wikipedia, 8 Apr. 2024, en.m.wikipedia.org/wiki/Peppered_moth. Accessed 27 Apr. 2024.


“Peppered Moth Evolution.” Wikipedia, Wikimedia Foundation, 6 Oct. 2019, en.wikipedia.org/wiki/Peppered_moth_evolution.


Walton, Olivia C., and Martin Stevens. "Avian vision models and field experiments determine the survival value of peppered moth camouflage." Communications biology 1.1 (2018): 118.


‌Webb, Jonathan. “Famous Peppered Moth’s Dark Secret Revealed.” BBC News, 1 June 2016, www.bbc.com/news/science-environment-36424768.


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