Mostly Moths
A moth in England flies at night. During the day, it rests.
Its life, at night, is devoted to reproduction. Lacking reproduction, it dies.
Of course, participating in reproduction does not prevent death. The adult
moth, after surviving a death rate of 90% across its earlier engagement in the
stages of eggs, caterpillars, and pupae, either travels miles to a female or,
if a female, travels a shorter distance and awaits the arrival of a male. She
aids being found by giving off pheromones, odors, so the male can find her. The
encounter between male and female is not brief. A successful encounter can lead
to 2,000 eggs per en copula. If the moths have or have not done their
duty at night, they have nothing to do during the daytime. They rest. That is
assuming they survived their in-flight status. Bats, using their innate sonar,
end the lives of some of the moths. The bats encounter males flying every night
of their lives as they search for females. The females fly for one night. The
adult moths start life at some time from May to August.
These generalities about a particular moth may apply to
other moths. Different kinds of moths, or species of moth, are of a great
number. There are about 160,000 species of moth. One species that has received
much attention over the years was ranked 216th in abundance out of
263 species of moth in Britain. The particular moth of interest, found in
Britain but also in Armenia, China, Europe, Japan, Kazakhstan, the Koreas,
Mongolia, Nepal, North America, Russia, and elsewhere is the commonly called
peppered moth. Its scientific name is Biston betularia. In
Britain, this moth has three “forms”, all of one species. The forms are usually
characterized by their wing color. The typical peppered moth is called typica
and has a white (“off white”) color with some dark spots that are like pepper
or dark freckles. Meanwhile, there is a form called carbonaria that has
the pepper spread out, as it were, so that the moth is dark, sometimes called
“black”. Lastly there is a little discussed intermediate form called the insularia.
It is the dark form of the peppered moth that received the most attention. The
dark form is also called a melanic form.
The representatives of the dark form, or “darks”, became predominant
in the middle of the 19th century in England in the industrialized Midlands.
The darks were presumed to be examples of what was later called “industrial
melanism”. Factories were producing very abundant dark smoke and soot that
carried or were composed of chemicals. The chemicals could be harmful to animals
and plants. In 1848, R.L. Edleston found one of the darks. It was then regarded
as rare. By 1864 Edleston was finding more darks than the typical peppered
moths. It was darkness at noon in Manchester, England. Black soot and smoke were
very thick. Mothers had a difficult time making out their children from across
a narrow street. In short, the pollution was terrible.
Meanwhile, in rural areas of Britain the typical remained
typical. In addition to Edleston, many other lepidopterists, overwhelmingly of
amateur status, were finding more of the darks than the typica in polluted
areas. Also meanwhile, it was thought melanism of the moths could be caused by
the “impressions” of surroundings on moth eggs or moth caterpillars. If reared at
temperatures different from normal or in higher than normal humidity, melanic
adult moths were produced. But even without differing temperature or high
humidity, the “darks” were predominating.
In 1896, J. W. Tutt was maintaining that the increase in the
number of dark forms was due to natural selection (NS). Tutt’s use of the term
“natural selection” made reference to a theory put forward by Charles Darwin.
Darwin in 1859 published his On the Origin of Species by Means of Natural
Selection. This was not yet “evolution”. Darwin used “evolution” in another
book of his in 1871. By 1872, he had put “evolution” into Origin of the
Species. For Darwin, NS was the crux of “evolution”. It could produce
species and end them. It acted quite slowly and could build up small, if not
tiny, differences into structural change. Others were thinking and had thought
species were not stable and definite. Darwin supported this notion. He
marshalled data to help along the idea, but NS was the distinctive feature for
Darwin. This lack of fixity for species was quite enough for some people. A
kind of devotee of the Bible knew that Darwin and his sort were a menace to
their Biblical truth. Moreover, “evolution” was not limited to past action, it
was currently still at work. It operated through sex and death.
Post-Darwin support for Darwin had come from Tutt. But John
W. Heslop Harrison did not agree with Tutt about melanism in moths. Around 1926
Harrison had reasoned chemicals in the polluted environment, such as in leaves
that the moth caterpillars fed on contained “metallic salts”, and could alter
the adult moths. Harrison did not use the peppered moth. The larvae of other
moths that he did use were fed foliage covered with lead nitrate and manganese
sulfate. Mutation occurred. There were dark forms of the moths. But others
could not get his results. In other later activities he was found out as a
fraud. His earlier scientific work became adversely back-colored by those
fraudulent practices.
Harrison wasn’t the only one to doubt the efficacy of NS as
exemplified by the peppered moth. Nevertheless, it was thought that the dark
peppered moths offered a proof of the action of NS. It was simply too good to
pass up. Consider: few darks before industrial soot and smoke, and the typicas rested
during the day, it was said, on lighter-colored bark of the trunks that were
coated with light-colored lichen. Then along came the pollution and tree trunks
became dark, and the lichen died. The dark moths couldn’t be seen against the
dark background. Meanwhile the poor lighter-colored moths were more easily
found out. What was seeing the moths? What was finding them out? The assumption
was that more typicals were killed more often than the darks were being killed.
The agent of death? It must be birds. What else could it be? The deaths of the typicals
was due to “predation” – this being a general term – but specifically because
birds could more easily find the typicals against dark, industrially polluted,
background tree trunks sans lichen.
Scientists at Oxford, great supporters of NS, thought the
dark vs. lighter moths presented a neat opportunity to confirm NS as real and
actually in operation even as they spoke about it. At Oxford was E. B. Ford,
who recruited a non-scientist, Bernard Kettlewell, to undertake field
experiments to bolster the notion of how important NS was to promoting
evolution. Kettlewell entered the English countryside in 1953 on a mission to
find out why the darks prevailed, with none of his field notes having ever been
found. If he could know the why of such prevalence, some perceived he could
rescue evolutionary biology as exemplied by NS, in short, “evolution”, from its
lack of support in the field. “Evolution” was a theory, but where was the
experimental data? The theory and the experiment(s) should intertwine.
Otherwise, “evolution” was history – a part of an historical effort to place
organisms in the context of Earth’s activities. Predating the 1950s were the
faithful many who found large scale mutation and genetic random drift, among
other devices, as being the only way or the almost only way “evolution” worked.
But E.B. Ford felt Kettlewell had a good shot at scoring a big hit for NS.
Perhaps it could be said Ford tutored or mentored Kettlewell about NS in
relation to peppered moths. In any event, Kettlewell was brought by Ford into
Ford’s Oxford School of Ecological Genetics. At best it could be said Ford
wanted Kettlewell to investigate the appropriateness of referring to the
occurrence of dark vs. typical peppered moths as being a result of “evolution”
in action via NS.
Perhaps it could be said Ford got what he wanted in
Kettlewell –difficult to tell since Ford destroyed all his papers. Kettlewell
was a moonlighting lepidopterist past his youth, with a lackluster MA in
zoology and a medical degree. He was well versed in field knowledge, but he was
not a scientist. He knew a thousand things found only by trial and error. He
could “hand-pair” moths – pushing them into copulation position. He had the fanatical
enthusiasm of an amateur. He insisted you had to know the “private lives” of
the insects under study. Kettlewell decided to find out if birds ate more darks
(of Biston betularia) in unpolluted woods and if birds ate more typicals in
polluted woods. He would use the “mark-release-recapture” procedure. Marked by
a dot of paint, released, and perhaps (if not eaten by birds) captured. They
were to be caught in light traps by being attracted to the traps. Both darks
and typicals were to be released in the polluted area and both darks and
typicals were to be released in the unpolluted area. More darks were expected
to be recaptured in the polluted area and less so in the unpolluted area.
The summer of 1953 would be the season for the release of
the moths. Kettlewell had to tend to thousands of moth larvae in 1952. He was
at it all hours of day and night protecting the larvae and operating mercury
vapor light traps at night to record the counts of darks and typicals in the
area. All of this activity was on behalf of a species of moth that Kettlewell
didn’t know very well. He assumed the mark-release-recapture procedure would
recapture a worthwhile number of moths, be they dark or typical, in the light
traps. It was not known if the Biston betularia, among moths having a reputation
for not being weak in the power of their wings, might fly away entirely.
Because Kettlewell was uncertain about some particulars regarding Biston betularia,
he had been considering experiments with Gonodontis bidentata, also a
species of moth having a dark form.
(Ford could not abide by the idea of random drift –
alteration of reproductive persistence not brought about by being “fit”, but by
chance, by a chance change in genetic frequency. Ford wanted “evolution” to be
progressive adaptation. There was then purposeful change for organisms. It must
be so, no random drift in any aspect was acceptable, nor was any other means of
evolutionary transformation.)
Before Kettlewell entered the woods to give the theoretical
fray a resolution, Cain and Sheppard thought the shell patterns of snails, Cepaea
nemoralis, showing many bands or colors, must mean something and such
banding or colors could be related to the snail backgrounds on which they were
seen by birds that ate the more conspicuous (in terms of banding and color)
snails. So it was NS at work. No random drift here, they declared. Also,
Drosophila and human blood groups showed selection at work. But was NS at work
in all instances of “evolution”? Weren’t Ford and his allies searching for an
instance to definitively show the power of NS? Concerning the snails: Lamotte
said genetic drift and mutation and migration and NS were at play, though NS by
predators was exceedingly small in effect. The supporters of NS wanted only NS
involved. The snails had differences in colors and number of bands on their
shells. If NS was to prevail, then one would expect a prevalence of some colors
and bands but the various colors and bands now being displayed had been in
existence since the Pleistocene. Prior to selectionism becoming dogma in the
late 1940s, the snails were thought to be examples of nonadaptive variation.
Lamotte stressed multiple causal factors were at work in biology.
Most definitely in the selectionist camp was Kettlewell who needed
to show that he recaptured significantly (statistically) more darks in polluted
areas and more typicas in unpolluted areas. In addition, there needed to be
adequate experimental controls, and a demonstration that birds ate darks or the
typicas as the case might be. Kettlewell had the female moths’ scents in
profusion in his experimental wooded area to keep the male moths from going too
far away from his traps. That the birds would eat moths, any moths at all, was
in need of verification, odd that this was so after all those years, but then
there had been many, many organisms for study and very, very few people
interested in any of the organisms. Kettlewell had two birds released in the
presence of posed peppered moths. Day 2 had shown the birds ate the moths more
readily. Kettlewell thought he had created a bird feeder. On day 3 Kettlewell
put in local moths and insects with the peppered moths. The more evident
peppered moths were eaten. Kettlewell also worked out a scale to indicate how
hidden a moth appeared, which involved pacing away from the moths and seeing
when a moth was hidden or at least was not very conspicuous.
Kettlewell released his first batch of moths on June 25,
1953. He shook a moth from a box onto a place he had selected, which was one of
33 boughs or tree trunks. It being daytime, the moths were sluggish though they
moved somewhat, though not far, to apparently get a good hiding place on a
bough or tree trunk. He didn’t let a moth get established in a sunlit spot or
it would have been “cooked”. After 5 days, Kettlewell was getting very poor
recapture rates. Ford wrote Kettlewell on July 1 about the low recapture rates,
calling them disappointing but noting the results would be worthwhile. Then
Kettlewell’s recapture rates improved. Not only were the recapture rates as a
whole as was wanted, but the numbers of darks vs typicals were improving. Some of
the statistics that were worked out on his data ended in being very close,
perhaps too close, to the theoretical expectation. It was known what the
destination, in theory, should be and upon arrival there was mild surprise that
they got where they had wanted to go. Could it have been there were windy days
during the low recapture rates and less so for the higher recapture rates? In her book, Of Moths and Men, Judith
Hooper checked weather reports for those days of low and high recapture rates
and found steady weather, little wind. Wind with or without, Kettlewell was
doing it all alone. He set up the experiments, conducted the experiments, and
gathered the data. As it turned out, he
could not do the unpolluted part of the experiments in the summer of 1954
because viruses had ravaged the moth pupae. He appealed to collectors to supply
him with eggs.
(Meanwhile, P.B.M. Allan, an editor of Entomologist’s
Record reviewed Ford’s Moths and especially its chapter on what
Kettlewell had been doing in 1953. Allan doubted birds ate moths in large
numbers nor that they did so from tree trunks too fast to record. Additionally,
Allan mentioned that a lack of moths in the traps could mean that they weren’t
eaten by birds; other explanations could account for their absence. Ford stated
birds were engaged in selective elimination of moths on a large scale. This
occurred with great frequency, so said Ford. Ford said Kettlewell saw moths
destroyed in large numbers by robins and sparrows. Back when science was
science these statements would have been called lies. In post-science they
could be called exaggerations. Allan did note that Ford had “given a new
orientation to our conceptions of the behavior of both the robin and the
hedge-sparrow”.)
In Dorset for the unpolluted part of the experiment, Kettlewell
got a reversal of the numbers for captured moths, that is, more typicals than
darks. He cut out results from several days. Kettlewell knew results could be
affected by differing life spans of the forms or one form might fly better than
the other, or would be more attracted to a light trap or more affected by
female scent. Nevertheless, for Kettlewell only birds as eaters of the moths
could explain the recapture numbers. Most observers thought birds were not
natural predators of peppered moths. Doubts persisted so Kettlewell got Niko
Tinbergen to film birds devouring moths. (Kettlewell wanted to “cine” birds
eating moths.) Kettlewell set out a dense group of moths on tree trunks. Birds
ate the moths, favoring the darks. To the unaware, viewing the film was
conclusive.
Too little was as conclusive as Kettlewell and others wanted
it to be. This was according to Judith Hooper in her book, Moths and Men.
I had been tagging along in this book for its “Moths” and less for its “Men”.
However tempting was the novelistic approach to the “Men”, I wished to confine
events to “facts” – those that could be described as scientific – to ultimately
arrive at the truth the “Men” gave or did not give to their involvement with
“Moths”. She has a rebuttal, more a refutation, of Kettlewell and Ford and
others aided and abetted by Ted Sargent late in her book. Some points are repeated, some new, for
example, Kettlewell set out too many moths, and they were placed to facilitate
obtaining recapture rates that would be statistically significant. Kettlewell
had, in effect, created a bird feeder. The pinned specimens in the famous
photographs were pinned, that is, dead. If the forms of the moth preferred
differing backgrounds to rest on, why didn’t the forms do equally well in
polluted or unpolluted areas in that you still had lots of dark places? If the
background was so important then, since you would always have dark backgrounds,
and then you would always have the dark forms, no pollution needed. All the
concern was about seeing moths that we see; however, birds see in the UV. What
other species of moth in polluted areas had the darks? Kettlewell himself scored the degree of
camouflage the moths enjoyed and he put out the moths and he did the counts.
Somewhere or everywhere along the line he should have had assistants blinded to
the task at hand. He stopped counts for a time. He changed the number of moths
per tree at Dorset. He had differing densities of moths in Dorset and
Birmingham, but recapture rates as he wanted in each area. Kettlewell had
stuporous moths in unnatural resting places in unusual densities. It
constituted unnatural predation, an artifact of the experiments. Kettlewell had
both lab-bred moths and wild moths. In Dorset he had more of the lab-bred
typicals and in Birmingham; he had more of the darks from the lab. Kettlewell
knew bats probably got 90% of the flying moths, but he assumed the bats were not
engaging in differential predation. In Britain, clean air laws were enacted in
1956. Pollution by sulfur dioxide and smoke declined; as did the number of
darks. Clean air acts were in place in the USA in 1963. The number of darks
declined. Rise and fall of darks in Michigan occurred without change in the
lichens, or if the lichen disappeared, the darks came back before the lichens
did. In West Virginia, the darks are few in areas that are heavily polluted.
There are no darks in Japan, no matter what the conditions.
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