Time to Mothball the
Peppered Moths
One of the most enduring examples of evolution in action has been the story of the peppered moths in industrial Great Britain, bolstered by Kettlewell’s experiments in the 1950’s. However, over the years a number of critics have challenged the validity of Kettlewell’s results, and more recently, the peppered moth story has been the subject of increased criticism, most notably by Jonathan Wells. Wells has attacked the inclusion of staged photographs in biology texts and argues that the story is often presented to students in a misleading way. Upon closer inspection, it appears that the peppered moth story is fundamentally flawed, insofar as it purports to be an example of evolution at work.
Some evolutionary theorists have recently acknowledged that the peppered moth story may not be an example of evolution per se, but it is still a good example of natural selection, they argue. However, when we examine the facts and assumptions underlying the story, it is not clear that the peppered moth story is even a good example of natural selection. As a result of the story’s failure to live up to its original billing of evolution at work, and its further failure to provide a compelling example of natural selection, I argue that the peppered moth story should be dropped from state science standards regarding evolution. In other words, it is time to mothball the peppered moths.
You are no doubt familiar with the story, and numerous summaries are available on the Internet, so let’s jump directly into the evidence. As the same approach can be used with essentially all stories about evolution, I will outline the essential ingredients of our analysis: on the evidence side we will examine primary facts, secondary facts, and disputed data; on the interpretation side, we will look at the premises (or assumptions) and conclusions.
Primary Facts
As with many observed phenomena, the actual hard data, what I call the primary facts, are often quite limited. In the case of the peppered moths, Biston betularia, the following primary facts appear to be generally accepted:
1. In the 1800’s, Great Britain experienced an increase in pollution near industrial centers due to coal burning factories
2. In the late 1900’s, after adoption of clean air legislation, the amount of pollution subsided significantly
3. In certain areas of Great Britain during the period of pollution, light colored lichens growing on tree trunks either were darkened or were killed, exposing the darker trunks
4. During the period of pollution, there was a significant increase in the percentage of dark moths to light moths
5. After the period of pollution, there was a significant decrease in the percentage of dark moths to light moths, in some cases returning to pre-pollution levels
Note that the primary facts above include certain conclusions (which in turn are based on prior evidence and assumptions). For example, the increase in pollution could have been due to other factors, but it is generally accepted that the coal-burning factories operating in the wake of the Industrial Revolution were the principal factor. As a technical matter, each of the above facts is itself actually a conclusion, based on various observations carried out during such periods. For purposes of the present discussion, we will accept the prior derivatives and start from the above primary facts.
Secondary Facts
Secondary facts are facts that flow necessarily and logically from the primary facts. Secondary facts depend on the validity of the primary facts, but given the primary facts, the secondary facts must nonetheless be viewed as valid. Thus we are only interested in those facts that can be deduced directly and necessarily from the primary facts. In this case, we have the following secondary facts:
1. There is a temporal relationship between the period of pollution, the darkening of the lichen (or exposure of the tree trunks) in some areas of Great Britain, and the increase in the percentage of dark moths
2. There is a temporal relationship between the reduction of pollution and the decrease in the percentage of dark moths
The above is the extent of the primary and secondary facts. Note that if any of the above facts is in dispute, we will have even less to work with. But let’s take the above facts as valid and see how far the story goes.
Disputed Data
There are a host of other propositions that have been put forth and included in versions of the peppered moth story from time to time. Most of these flow from Kettlewell’s experiments, and some of the more prominent propositions are as follows:
1. Birds are important predators of moths
2. Moths rest on tree trunks
3. The percentage of dark moths in polluted and unpolluted areas roughly corresponds to the relative levels of pollution
4. The changes in the percentage of dark moths correspond to changes in lichen cover
Each of the above is the subject of significant dispute, as can be discovered from a cursory search of the Internet. (For one example, see Wells’ article at http://www.trueorigin.org/pepmoth1.asp disputing the latter two statements.) Ultimately, however, these disputed data are not critical to our overall evaluation of the peppered moth story. Nevertheless, they do underscore the challenge of pinning down an explanation for the observed primary facts, and remind us that we should be on the lookout for disputed propositions being presented as undisputed facts.
Interpreting the Data
According to the classic version, the pollution darkened the lichen (or killed the lichen, exposing the darker trunks), the moths rested on the trunks in sight of predatory birds, which proceeded to snack on the unfortunate light moths leaving a greater number of dark moths unscathed. Some of the moths successfully adapted to the changing environment, and the dark moths went from being a significant minority of the population to being a significant majority. Thus we see an example of natural selection, and more broadly, evolution, in action. At least that is the story.
In order to explain the increase in the percentage of dark moths during the period of pollution (Primary Fact #4) the classic story uses the following:
· Primary Fact #1 (pollution)
· Primary Fact #3 (darkened or killed lichen)
· Secondary Facts #1 and #2 (contemporaneous nature, implied)
· Disputed Data #1 (birds are a significant predator)
· Disputed Data #2 (moths rest on tree trunks)
To these data points are added certain assumptions, in order to reach the ultimate conclusion:
· Assumption #1: the pollution caused the darkened lichen or killed the lichen
· Assumption #2: the extrapolated predation numbers of Kettlewell can be applied across the local population
· Assumption #3: the predation numbers obtained by Kettlewell with high concentrations of moths in small areas during daylight hours can be extrapolated to lesser concentrations in broad areas during nighttime hours
· Assumption #4: the extrapolated predation numbers of Kettlewell can be applied across the broad population at large (for example, throughout Great Britian)
Although not watertight, it seems that Assumption #1 may be appropriate, due to the contemporaneous nature of the primary facts regarding pollution and lichen. Assumption #2 may also be a reasonable assumption, based on general agreement between Kettlewell’s numbers and those of other observers. For purposes of our discussion, we will accept these two assumptions.
More problematic are the inclusion of Assumptions #3 and #4 and the use of Disputed Data #1 and #2 (also used, expressly or implied, in most retellings of the story are Disputed Data #3 and #4). These are the areas where most of the challenges to the validity of Kettlewell’s experiments have been raised. These are the principal points that Wells has focused on in his reviews of the peppered moth story.
I should interject here that in addition to Wells’ substantive criticisms, he has also been vocal about the staged photographs accompanying the story used in biology textbooks. This criticism has generally been countered by proponents of evolutionary theory in one of two ways: either by (1) acknowledging that the photographs are misleading, but that this is a minor defect in an otherwise valuable scientific story; or (2) acknowledging that the photographs are staged, but that this is common practice and is not misleading. An example of the second approach can be found at http://www.nmsr.org/text.htm#moth, in a response by Ian Musgrave.
I think Wells may have overplayed the significance of the staged photographs, but only time will tell whether biology texts are updated to correct this deficiency. If not, he may have been justified in his vocal criticism. In any event, the existence of staged photographs is definitely not the principal criticism Wells has raised, though it may be the most inflammatory. Responses such as those by Dr. Musgrave miss Wells’ larger substantive point, which is that the peppered moth story taken as a whole is misleading.
The Story Evolves
Due to the challenges raised regarding Kettlewell’s experiments, and perhaps due as well to recent criticism, more sophisticated, and thankfully in some cases more factually accurate, versions of the peppered moth story have been put forth. For example, the California Department of Education 2003 Science Framework for California Public Schools contains the following version:
“An example of natural selection is the effect of industrial ‘melanism,’ or darkness of pigmentation, on the peppered
moths of Manchester, England. These moths come in two varieties, one darker
than the other. Before the industrial revolution, the dark moth was rare;
however, during the industrial revolution the light moth seldom appeared.
Throughout the industrial revolution, much coal was burned in the region,
emitting soot and sulfur dioxide. For reasons not completely understood, the
light-colored moth had successfully adapted to the cleaner air conditions that
existed in preindustrial times and that exist in the
region today.
However, the light-colored moth appears to have lost its
survival advantage during times of heavy industrial air pollution. One early
explanation is that when soot covered tree bark, light moths became highly
visible to predatory birds. Once this change happened, the dark-peppered moth
had an inherited survival advantage because it was harder to see against the
sooty background. This explanation may not have been the cause, and an
alternative one is that the white-peppered moth was more susceptible to the
sulfur dioxide emissions of the industrial revolution. In any case, in the
evolution of the moth, mutations of the genes produced light and dark moths.
Through natural selection the light moth had an adaptive advantage until
environmental conditions changed, increasing the population of the dark moths
and depleting that of the light moths.”
(California
2003 Science Framework, p. 241; see Part 5)
Although the
authors of this version of the story acknowledge that Disputed Data #1 and #2
(and implicitly, Disputed Data #4) are in dispute and that the cause of the
change in the peppered moth population is not understood, they do not hesitate
in making some key assumptions and jumping to the same conclusion as the
classic story. Specifically, the authors
make the following statements:
·
The reason for the
high percentage of light-colored moths in pre-industrial times was that the
“light-colored moth had successfully adapted to the cleaner air conditions.”
·
“In the evolution of
the moth, mutations of the genes produced light and dark moths.”
·
Natural selection gave
the light moth “an adaptive advantage until environmental conditions
changed.” The implied counterpart is
that natural selection gave the dark moth “an adaptive advantage” once
environmental conditions changed.
Let’s examine each of these in turn.
You will recall that our set of facts did not include any information that would allow any conclusions to be drawn about the light moths prior to the industrial period. Thus the first statement above is simply a proposal that the light moths must have successfully adapted to the pre-industrial environment, otherwise they would not have been around in such numbers. Given that the whole point of the peppered moth story is to prove the effect of natural selection acting upon adaptations, this statement amounts to adopting as a premise the very conclusion the authors are trying to reach. This is another example of natural selection falling into that ever-frequent logical fallacy, tautology.
The authors’ second statement above assumes that mutations produced light and dark moths. You will recall that our set of facts did not have any information about mutations. Indeed, we do not have any information about how the dark moths and light moths came on the scene in the first place. The authors state that at the beginning of the peppered moth story, there were both light and dark moths. Thus, the supposed “fact” that the mutation of genes resulted in light and dark moths is nothing more than a hypothesis based on the assumption that such changes must arise from mutations. However, there is an even larger problem in the present case.
The peppered moth story appears in the California 2003
Science Framework under the heading entitled “Evolution (Speciation)” and under
the subheading “Evolution is the result of genetic changes that
occur in constantly changing environments.”
Thus, the whole point of the section is to provide evidence supporting
the idea that gene mutations (“genetic changes”) result in evolution. Apparently, the only “evidence” the authors
could come up with to prove this point was to throw in a sentence at the end of
the peppered moth story asserting that “in the evolution of the moth, mutations
of the genes produced light and dark moths” (emphasis added). The reader will recognize that the authors
have again fallen into the logical fallacy of asserting as their premise the
very conclusion they are trying to reach.
The third statement regarding natural selection is more subtle,
and leads back to the discussion of what the peppered moth story is really
about.
How Did We Get So Many Dark Moths?
A number of explanations have been proposed over the years to
explain the increased percentage of dark moths during the industrial period:
1. The moths became outwardly darkened or dirty in appearance as a result of the pollution in the air. Under this theory the moths just needed a good washing. (This theory was proposed early on, but has generally been rejected and will not be discussed further.)
2. The light moths continued to reproduce normally, but as more and more of the light moths were eaten they reproduced less and eventually declined in numbers. The dark moths reproduced more (the happy result of being eaten less) and increased in numbers.
3. Dark moths enjoyed a physiological advantage over the light moths in polluted environments that was independent of coloration or camouflage. This is similar to #2, in that the light moths would end up reproducing less.
4. The light moths began to reproduce dark moths as a result of mutations.
5. The light moths continued to reproduce normally, but pollution in the environment affected the larvae, which resulted in melanism in the developed moths.
There may be other potential explanations (such as “thermal melanism”) and we may never know exactly what caused the increased percentage of dark moths during the industrial period. Nevertheless, let’s see whether any of the above explanations offered to date provide a good example of evolution or natural selection in action.
The light moths continued to
reproduce normally, but as more and more of the light moths were eaten, they
reproduced less and eventually declined in numbers. The dark moths reproduced more and increased
in numbers.
This explanation depends largely on the assumption that coloration and camouflage determine predation. Specifically, this explanation assumes Disputed Data #1 and #2, which are probably the two pieces of data that have been the subject of most dispute. Nevertheless, this explanation corresponds somewhat with the classic version of the story, and thus merits examination. What does it show about evolution? The authors of the California version state that we had both light and dark moths before, during, and after the industrial period. Under this explanation, light moths have not turned into dark moths (except as a result of mutations, which we will discuss under the next proposed explanation). Fair enough, you say, but isn’t the different survival rate an example of natural selection in action? Let’s see.
The concept of natural selection as proposed by Darwin was the driving force that allowed a creature that had in some way become more adapted to its environment to out-compete its siblings and thus be more likely to survive and reproduce. In other words, it was adaptive change of an organism within an environment, coupled with natural selection, which led to new and better organisms. Darwin a number of times downplayed the importance of the environment, but ultimately returned time and again to the environment as an important triggering force for change.
In the case of the peppered moth (under explanation #2), there is no adaptive change in the organism taking place in response to the environment. Rather, there was a radically-changed environment thrust upon the organism in a very short space of time. This distinction is subtle, but needs to be borne in mind.
For example, if a flood were to wash through a stadium during a playoff game, it is likely that, on average, strong swimmers would tend to be spared over non-swimmers. But would there be any adaptive change taking place? No. It would simply be an example of a catastrophic event killing a large number of individuals. Darwin recognized and addressed this distinction in The Origin of Species and declined to link natural selection to abrupt environmental catastrophes. To do so would be to water down the concept of natural selection to include virtually every instance of survival.
Because there is no evidence of
adaptive change in the facts of the peppered moth story, the authors of the
California 2003 Science Framework helpfully supplied the following assertion: “mutations of the genes produced light and dark moths.” But as we have seen, this statement is not
based on any evidence. In addition, the
authors acknowledge the existence of dark moths prior to the period of pollution. Thus, this statement is logically problematic
in the context of natural selection, because it proposes that the adaptive
change (becoming dark) occurred prior to the triggering environmental event. Such a result could be nothing but sheer coincidence.
Thus, under
explanation #2, the peppered moth story is not an example of adaptive change
and natural selection leading to better organisms. Rather, it is an example of an abrupt (on the
evolutionary timetable) environmental change leading to a significant loss of
life among the light moths.
But what if the polluted environment had persisted for a longer period, might not the light moths have died out completely, resulting in a very different population than we started with? Might this not be viewed as an example of the evolutionary process? Surely, if this is not an example of macroevolution, then it is at least an example of microevolutionary changes within the population?
This might sound attractive at first blush: we witness change in the population, therefore there must be some kind of evolution taking place, the thought goes. But is this really an example of adaptive natural selection moving toward evolution? If the light moths had died out completely, this would not be evolution; it is called extinction. The result would be a decrease in the natural variety that previously existed in the population, a decrease in the overall genetic information of the population. In formulating his theory, Darwin recognized the important role played by extinction, but it should not be confused as a surrogate for the creative process of adaptive change leading to the evolution of the organism.
Dark moths enjoyed an
independent physiological advantage over the light moths in polluted
environments.
This is essentially the same situation as the previous explanation. There is no evidence of any adaptive change taking place. And if the change in the population is carried out to the extreme, all we have is extinction, not evolution.
The light moths began to
reproduce dark moths as a result of mutations.
Although there appears to be little evidence for this explanation, it nevertheless should be addressed. After all, if the light moths changed as a result of mutations, and then after the period of pollution the dark moths changed back to light moths as a result of mutations, we might have two wonderful independent examples of evolution in action.
We might be justified in asking the authors, however, just exactly what wonderful evolutionary event we have witnessed. Dark moths were already present in the population at the beginning, and thus there is no evidence that new novel genetic information was introduced into the population. And once the environmental stimulus was removed, the population tended to revert to its original state. Thus, after two significant environmental events we are left with very little, if any, net change in the population. This does not sound much like a creative evolutionary process. In fact, it sounds a lot like resistance to change, or to use a well-known term for the lack of evolution: stasis.
The light moths continued to
reproduce normally, but pollution in the environment affected the larvae, which
resulted in melanism in the developed adults.
This explanation was bolstered by observations of other insects with increased melanism during the industrial period and experiments with larvae fed on polluted food sources, although the latter experiments were subsequently criticized.
However, even if correct, this explanation does not teach us much about evolution in action either. Once the pollution in the environment subsided, the population tended back toward its pre-pollution state. Thus, the explanation of environmental pollution affecting the larvae is not an example of a creative evolutionary process, but rather is an example of the organism’s ability to meet immediate environmental conditions, while ultimately resisting fundamental physiological change.
Summary of Explanation
Certainly, the peppered moth story has no basis as an example of macroevolution, despite the misleading placement of the story in the California 2003 Science Framework. Upon examination of the proposed explanations for the increase in the percentage of dark moths, one also has to wonder whether it even makes sense to refer to the peppered moth story as a meaningful example of microevolution or natural selection.
In both the bird-predator explanation and the independent physiological characteristic explanation, we have an abrupt environmental event taking the life of many organisms, while the overall population remains surprisingly resistant to change in the long run. If one posits a more extreme turn of environmental events, the story becomes not a story of evolution, but of extinction. Additionally, in the explanations of adaptive mutation or affected larvae, the result appears to be the organism’s ability to temporarily adapt, while ultimately remaining resistant to change.
Thus, the peppered moth story is likely not an example of microevolution or natural selection either, unless we define the concepts so broadly as to encompass virtually every change that takes place within a population, whether novel or not, whether permanent or not, in which case the concepts come perilously close to the edge of the tautology precipice and lose much of their meaning. Indeed, upon closer examination of the facts and assumptions surrounding the peppered moth story, perhaps the main lesson that can be drawn is that through significant environmental changes the peppered moth population showed its ability to adapt, while ultimately remaining resistant to fundamental change.
Conclusion
Over the years, the story of the peppered moths has been touted as one of the most prominent examples of evolution in action. However, upon closer inspection, the facts underlying the story cannot support the weight of the assumptions and conclusions piled upon them. I agree with Jonathan Wells’ suggestion that the peppered moth story can be a valuable educational tool, not to teach evolution, but to teach students about the scientific process and how this process has revealed flaws in the long-accepted story. I would add that the peppered moth story can also be used as a good introduction for students to learn how to critically evaluate scientific data.
Given the recent example of the California Department of Education in its 2003 Science Framework, however, it unfortunately may be the case that the story has accumulated too much evolutionary baggage over the years to be used effectively across the board in the public schools. I would of course encourage individual instructors to guide students through a critical analysis of the peppered moth story. But if the state (and the local district, to the extent relevant) cannot propose a more thorough examination of the facts, and if the state continues to repeat the unsupported evolutionary assumptions and conclusions year after year, then perhaps it is time to eliminate the story altogether. Certainly, insofar as the learning standards handed down by the state are concerned, it is time to mothball the peppered moths.
Eric Anderson
September 4, 2003
(This page is subject to corrections and has not yet been finalized.)