The Triumph of the Peppered Moths and the Failure of Creationism

The peppered moth, Biston betularia, is a species of nocturnal moths that can be found across the Northern hemisphere. In Britain, the two most common morphs are called typica (white with dark irregular patches and spots) and carbonaria (dark melanic). There is also an intermediate morph that is known as insularia.

Before the early 1800s, the melanic morph was unknown. About a hundred years later, this morph completely dominated the population. This coincided with the industrial pollution of forests, thereby depriving the white morph of its good camouflage and allowing the melanic morph to better hide in the surroundings, since trees had been darkened by soot and a lot of lichens died due to sulfur dioxide. This became known as industrial melanism. From the 1960s and to the present, this trend has been reversed. As the air and the environment became less polluted, the melanic morph fell in prevalence and the white morph rose again.

Research of the prevalence of different morphs at different places and times can be found in many scientific papers, such as Kettlewell (1956), Grant, Owen and Clark (1996) and Cook and Saccheri (2003).

Why are peppered moths a case of natural selection in the wild?

This is a fantastic example of natural selection in the wild, because there was a shift in the prevalence of the two forms due to selection from the environment and this occurred both during increased and decreased pollution. This has also been observed in many locations in both Europe and North America, further solidifying the case.

Later research showed that the major mechanism behind this change turned out to be bird predation. Before the pollution, white morphs had a good camouflage against light trees. During the strong pollution era, these were easier to spot by birds against the blackened background, whereas now the melanic morphs had a better ability to hide. As the environment improved, the situation was reversed again.

Creationists try to spread misinformation about these mechanistic details and how frequent these moths rests on different parts of the trees, but whatever they claim, their assertions do not disprove the fundamental fact that natural selection caused the shift in the prevalence of different morphs over time.

What did Kettlewell’s experiments find?

Kettlewell (1955, 1956) carried out classic selection experiments in the Christopher Cadbury Bird Reserve in Birmingham and Deanend Wood in Dorset. He marked moths of different morphs with cellulose paint, released them into the wild and recaptured them later. He then estimated how much birds had predated on the different morphs and reached the conclusion that moth of the white morph were more vulnerable in polluted woods, whereas the melanic morph was more predated in cleaner environments. Kettlewell and Tinbergen videotaped bird predation on moths, showing that birds were indeed affected by the camouflage of the different moths in the different environments just like Kettlewell predicted (Rudge, 2005).

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What limitations did Kettlewell’s experiments have?

The research carried out by Kettlewell was impressive (since it even included live video capture) and it supported the fact that natural selection had affected the moths as the environment around them changes and became more polluted.

However, it was not unassailable at the time. Questions emerged regarding some of the more technical aspects of the experiment. Kettlewell had released the moths during the day, but they are nocturnal, released them at high densities untypical for the natural situation and it was not clear that these moths really rested on trunks. However, these limitations were patched successfully in modern-day replication experiments, most notably by Michael Majerus, who carried out a six-year long experiment between 2001 and 2008, published in Cook, Grant, Saccheri, and Mallet (2012) after the untimely death of Majerus.

How did the Majerus replication experiment verify the mechanism behind peppered moth example?

Majerus, a critic of some aspects of the work done by Kettlewell, released almost 5000 moths during the six years the experiment ran. He released moths in the night instead of the day, used reasonable density, used netting sleeves instead of placing them on trunks and used several locations to compare bird and bat predation.

Two findings stand out. First, Majerus found that a considerable proportion (36%) of wild moths (not part of the experiment) did, in fact, rest of trunks. A majority of them rested on lateral branches (52%), which also allows camouflage. Second, Majerus predation experiment in an unpolluted forest showed a substantial selection against the melanic morph, with a strength of about s = 0.1 per day.

The paper concludes triumphantly:

Nonetheless, with this new evidence added to the existing data, it is virtually impossible to escape the previously accepted conclusion that visual predation by birds is the major cause of rapid changes in frequency of melanic peppered moths. These new data answer criticisms of earlier work and validate the methodology employed in many previous predation experiments that used tree trunks as resting sites. The new data, coupled with the weight of previously existing data convincingly show that ‘industrial melanism in the peppered moth is still one of the clearest and most easily understood examples of Darwinian evolution in action’.

Despite this victory, creationists continue to spread nonsense about the peppered moths in a feeble attempt to undermine modern evolutionary biology and its conclusions.

How do creationists abuse the peppered moths example of natural selection in the wild?

Creationists, faced with the clear example of natural selection that is industrial melanism, has to find some way to attempt to discredit it. They typically make use of two dishonest tactics:

(1) They overemphasize the methodological limitations of the original research carried out by Kettlewell. This is done by e. g. citing a 18-year-old book review by Coyne (1998), while ignoring the replication research by Majerus. They also confuse mechanism with fact, trying to undermine the observation that the different moths changed in frequency (evolution) by pointing to perceived limitations with research investigating the precise mechanism (bird predation).

(2) They accuse Kettlewell of fraud by pointing to illustrations that involve dead moths pinned to tree trunks. However, this has nothing to do with the experimental data itself and modern data confirms that moths spend over 1/3 of their time on trees resting on trunks.

There are many refutations of the creationist misuse of peppered moths, such as Isaak (2006), NCSE (2008), Padian and Gishlick (2002), Gishlick (2003), and even Coyne (2012), who called the peppered moth example of natural selection in the wild “solid”.

What are the genetic changes behind the melanic morph?

In another lethal strike against creationist, researchers recently uncovered the probably genetic change that led to the melanic morph (Editors of Nature, 2016). After extensive mapping efforts that narrowed it down to 13 genes, van’t Hof et al. (2016) showed that morph change was caused by the insertion of a mobile genetic element called a transposon into the first intron of the gene cortex. The genetic alteration increases the transcript abundance of the corresponding transcript, and the protein is known to be involved in early development of wings.

In another case of independent verification, the researchers traced this genetic event back to 1819, which fits nicely with the time period for the documented appearance of the melanic morph in Britain.

As always, a lot of research remains to pin down the precise molecular mechanism by which this genetic alteration causes the melanic morph during wing development, but this is yet another nail into the coffin of creationism, and in particular, their rabid abuse of the peppered moths example of natural selection in the wild. Will this now finally end the creationist obsession with spreading falsehoods about peppered moths? If history is any guide, the answer is a resounding no. But, like before, proponents of mainstream science will be there to counter them at every step.

References:

Cook, L. M., Grant, B. S., Saccheri, I. J., & Mallet, J. (2012). Selective bird predation on the peppered moth: the last experiment of Michael Majerus. Biology Letters, 8(4), 609-612.

Cook, L. M., & Saccheri, I. J. (2013). The peppered moth and industrial melanism: evolution of a natural selection case study. Heredity, 110(3), 207-212.

Coyne, J. A. (1998). Not black and white. Nature, 396(6706), 35-36.

Coyne, J. (2012). The peppered moth story is solid.. Why Evolution is True. Accessed: 2016-06-05.

Editors of Nature. (2016). Dark satanic wings. Nature, 534(7605). 5.

Gishlick, A. D. (2003). Icons of Evolution? Why much of what Jonathan Wells writes about evolution is wrong. NCSE. Accessed: 2016-06-05.

Kettlewell, H. B. D. (1955). Selection experiments on industrial melanism in the Lepidoptera. Heredity, 9(3), 323-342.

Kettlewell, H. B. D. (1956). Further selection experiments on industrial melanism in the Lepidoptera. Heredity, 10(3), 287-301.

Kettlewell, H. B. D. (1958). A survey of the frequencies of Biston betularia (L.) (Lep.) and its melanic forms in Great Britain. Heredity, 12(1), 51-72.

Kevin P., & Gishlick. A. D. (2002). The Talented Mr. Wells. The Quarterly Review of Biology, 77(1), 33-37.

NCSE. (2010). Critique: Exploring “Explore Evolution”. Accessed: 2016-06-05.

Grant, B. S., Owen, D. F., & Clarke, C. A. (1996). Parallel Rise and Fall of Melanic Peppered Moths in America and Britain. Journal of Heredity, 87(5), 351-357.

Isaak, M. (2006). An Index to Creationist Claims. Accessed: 2016-06-05. (search for “peppered moth” to read refutations of about 10 or so creationist myths)

Rudge, D. W. (2005). The Beauty of Kettlewell’s Classic Experimental Demonstration of Natural Selection. BioScience, 55(4), 369-375.

van’t Hof, A. E., Campagne, P., Rigden, D. J., Yung, C. J., Lingley, J., Quail, M. A., . . . Saccheri, I. J. (2016). The industrial melanism mutation in British peppered moths is a transposable element. Nature, 534(7605), 102-105. doi:10.1038/nature1795