The Peppered Moth and Darwin’s Finches
The Peppered Moth and
Darwin’s Finches
Do the Classic
Textbook Examples Prove Evolution?
Many biology textbooks have long cited two classic examples
as proof of Darwinian evolution. They are the color change of the Peppered Moth
and the beak changes (size and shape) in the Galapagos finches (Darwin’s
finches). These examples continue to be cited even though science has subsequently
shown each to be either greatly oversimplified or completely wrong. Neither
example is a simple nor obvious proof of evolution as claimed. The related scientific literature is
extensive and complicated, but careful reading shows that the examples of the
peppered moth and Darwin’s finches are being misrepresented.
DARWIN’S FINCHS
Birds in the Galapagos Islands now commonly known as
Darwin’s finches are frequently cited as proof of evolution. The claim is that
an original finch evolved into over a dozen different species. This occurred
because various birds were shaped by natural selection to feed effectively on
different types of available nuts and seeds. The birds developed beaks with
different sizes and shapes to feed more efficiently on different foods.
When Darwin visited the Galapagos Islands in the early
1830’s, many animals were collected and taken back to England. Biologists
eventually determined there were 13 or 14 different species of finches. One
very notable difference was the size and shape of the beaks. Each appeared to
be adapted for feeding on a particular seed or nut. Darwin paid little
attention to the finches, and did not refer to them in Origin of Species, but
Lack published a book (Darwin’s Finches) about them in 1947. Lack’s work received considerable attention,
and the variations in the finches have since been widely cited as proof of both
natural selection and Darwinian evolution. Peter Grant devoted even more
comprehensive study to the finch population of the Galapagos Islands and
further established their reputation as a proof of evolution.
More recent research has shown that the beak changes are
indeed caused by natural selection, but not by Darwinian evolution. Cyclical weather causes changes in the food
supply (more nuts and less seeds or more seeds and fewer nuts). As the food
supply changes, the surviving bird population develops beaks more suited to the
available food supply. However, the beak changes are not caused by random
mutations as assumed by Darwinian evolution. Rather, they are simply the result
of inherent genetic variation as discovered by Gregor Mendel.
According to Cornwell, the genetic code that sizes and
shapes the finches’ beaks has been identified and; “Among the ground finches
(Geospiza fortis) of the island Daphne Major, birds that inherited the blunt
version of the gene from both parents had the bluntest beaks, those with one blunt
and one pointy version of the gene had intermediate beaks, and those with two
pointy variants had the sharpest.”
Campas determined that just four variables define all of the observed
beak shapes. This indicates a link between genetic code and various beak
shapes. No mutation is necessary to explain the various shapes. McNew determined that there is no difference
in the genetic code. The beak changes
are associated with epigenetic changes that can be induced by the environment.
They are not mutations.
Not only was the beak change the result of existing genetic
code, but, in many bird populations, the beaks changed back to their original
configuration when the original weather conditions (and food supply) returned.
Obviously, random mutation is not a reversible process, so Darwinian evolution (AKA
Modern Synthesis) cannot be the explanation for the beak changes. Beak changes
are caused by environmental change and natural selection, but not by random
evolutionary mutation. The causes of the changes were already programmed in the
DNA of the birds.
THE PEPPERED MOTH
The darkening color of the
Peppered Moth (Biston betularia) in Great Britain during the Industrial
Revolution is frequently presented as a proof of Darwinian evolution. The moth
species Biston betularia is a member of the family Geometridae (engineer moths)
of the class Lepidoptera (butterflies and moths). Light-colored members of this
species are known as typic, and dark- colored individuals as melanic. As the industrial
revolution polluted the air, the moth population became darker, and, by the
1950s, 90% of the moths in the region were dark.
The British
biologist James Tutt first examined this color change in 1896. According to Tutt,
light colored moths on light-colored lichens growing on tree trunks were not
easily seen by birds. When air pollution killed the lichens, the light colored
moths were exposed on the darker tree trunks. Therefore, natural selection
(predatory birds) caused the moths to evolve into darker colors.
Fifty years later, the
British entomologist Bernard Kettlewell conducted experiments that appeared to
confirm Tutt’s assumption. Scientific American magazine published his results
in an article titled "Darwin's Missing Evidence." Subsequently, the
Peppered Moth became one of the fundamental textbook proofs of evolution.
Although Walton
demonstrated that avian vision can more easily detect melanic moths in an
unpolluted environment, other researchers have shown that Tutt’s assumptions
and Kettlewell’s experiments were fundamentally flawed. Lee observed that the
nocturnal moths avoid birds and do not even tend to rest or sleep on either lichens
or tree trunks. Mikkola, Majerus, Howlett, Holdridge, and others also reported
that the moths seldom rest or sleep on tree trunks. Although Mallet (2003) defended Kittwells research, Mikolla
observed that the moths actually roost on the underside of small, horizontally
growing braches where they are hidden by the leaves. Therefore, their color
makes little difference. Subsequently, Coyne showed that the moths do not even
have any tendency to choose matching backgrounds.
Then, B. Grant showed
that the color change was more closely correlated with increased Sulphur
dioxide in the atmosphere than with any other variable. This provided support
for a theory initially developed by Heslop: that melanism (darkening) in
animals stemmed directly from chemical substances in the air. Heslop reported
that melanism could be produced in several moth species if their larvae were
fed on leaves contaminated with metallic salts.
Lees conducted statistical studies of the occurrence of melanism in two
moth and two butterfly specie in relation to fourteen different environmental
variables. That work showed that melanism is linked to smoke and soot for some
species, but not for the peppered moth.
For the peppered moth, Sulphur dioxide is more important. Lees also showed that melanism sometimes
occurred in unpolluted areas.
Melanism in the Peppered
Moth was also observed to occur (and recede) independently in industrialized
areas of Britain, Europe, and America. Grant BS 1996 observed parallel
melanism in Britain and America, and Sergiu observed it in Romania. That
suggests it was not the result of rare, random, evolutionary mutations. Rather, existing variation in the moth’s
genes enabled a rapid color shift in the population.
Stephen showed that adaption due to sudden environmental
change can occur very rapidly without mutation.
He stated that “Evolutionary
adaptation after sudden environmental changes can occur very rapidly. The
mechanisms facilitating rapid adaptation range from strong positive directional
selection leading to large shifts in the allele frequencies at a few loci
(selective sweeps) to polygenic selection causing small changes in allele
frequencies at many loci. In addition, combinations of these two extreme
mechanisms may also result in fast evolution.”
Melanism has been
observed in other animals as well, and even in mammals. Nachman observed
colonies of pocket mice that lived in rocky environments. Mice living among dark colored rock tended to
develop darker fur, apparently to minimize predation by eagles. He found that different colonies in different
areas did not depend upon the same genetic variations to achieve melanism. Furthermore, one single colony had four
different mutations of a single gene that established melanism.
Saenko observed that melanic adaptations are widespread in
both arthropods and vertebrates. Melanism in animals that experience
metamorphosis is especially interesting because the larval and adult stages
live in differently colored environments.
Therefore, they display different colors and patterns to avoid predation.
Noting that various populations of animals in, isolated
environments independently develop similar traits, Uy wondered if that was the
result of identical or different mutations in the same genes or unique
mutations in different genes. He therefore studied the genetic basis of similar
plumage color, including melanism, in small, isolated populations of birds
(flycatchers) in the Solomon Islands. He
found that different populations had different genetic variations that produced
similar plumage colors in isolated populations. In particular, melanism on
different islands was not caused by the same genetic variations.
Van’t Hof (2019)
studied two other moth species that experience industrial melanism. He found that melanism in those species was
not caused by the same genetic code as in the peppered moth.
Bastide demonstrated that melanism is not necessarily due to
predators. He studied flies that got darker at higher elevations for UV
protection. Furthermore, he showed that all populations did not use the same
genetic variation to achieve melanism.
Since the validity of the initial experiments was often
challenged, Majuris conducted additional experiments to verify Kittwell’s
original work. Majuris died before the
results were published, but Cook (2012) reviewed the data and concluded that it
did validate Kitwell’s conclusions.
Cook 2013 summarized much
of the work that has been published on the peppered moth. He showed that the
complete body of work is inconclusive at best. Many factors effect color
changes in the moths. The accumulated evidence does not conclusively establish
what, if any, significance bird predation has relative to the many other variables
that may be causing color changes.
Finally, Clarke
showed that the color change was reversed when the air pollution was eliminated following legislation mandating stricter emission
controls. Cleanly, the information for
color changes is in the existing genes of the moths. Observed color changes were not the result of
mutation.
Saey reported on research conducted by Ilik Saccheri at
the University of Liverpool in England.
By studying the DNA of hundreds of peppered moths, and eliminating many
mutations that did not cause melanism, Saccheri eventually determined that
melanism in the peppered moth is caused by a transposable element (jumping
gene). However, that gene is not in the
DNA that encodes the protein causing melanism. Instead, it is in an intron, a
stretch of DNA that gets cut out after the gene is copied into RNA and before
the melanized protein is made. Schrader
(figure 3 in his paper) illustrates the regulating effect of the intron on the
gene linked with wing color.
Mallet (2016, last page) also discussed the intron that
produced melanism in the peppered moth and reported that the very same intron
is also involved in color pattern development in other moths and butterflies.
Ravindan reviewed the discovery of jumping gene in maize and
the slow acceptance by the scientific community. Previously, it was thought that
DNA was fixed except for mutations.
Van’t Hof (2016) reviewed the details of jumping gene discovery.
Because the many peppered moth studies are dispersed in the
literature and not always easy to find, Cook (2018) reviewed and compiled data
from past studies to form a resource that can facilitate future studies.
Wells discussed the serious problems with the continuing use
of the pepper moth example in textbooks, arguing that it should be replaced
with a less problematic example. Cook (2020),
published a rebuttal to such criticism.
DISCUSSION
Mendel was a contemporary of Darwin, but Darwin was never
aware of his work. Mendel lived from
1822 to 1884 and published his conclusions about genetic variation in 1866.
However, his work received little attention during his lifetime. It did not
become common knowledge until it was rediscovered in 1900. Darwin lived from
1809 to 1882 and published Origin of Species in 1859. Darwin did not understand
quantitative genetic variations such as plant height or beak size. If he had,
Origin of Species may have been a very different book. Darwin was ignorant of
the biological mechanism of genetic variation, and thought that the environment
could cause genetic changes. Darwin published his erroneous hypothesis of “pangenesis”
in 1868. It was completely discredited when Mendel’s work was rediscovered by
others decades later.
CONCLUSIONS
The color change in the Peppered moths may have been caused
(at least partially) by natural selection, but it was not caused by evolution
(random mutation). The mechanism(s) for
the color change is programmed into the DNA of the moths, and is reversible.
The beak changes in Darwin’s finches were caused by natural
selection, but not by a random process of evolutionary mutations. The DNA of
the finches contains the information required to make the beak changes, and the
process is reversible.
Since a series of random mutations introduced into DNA is
obviously not a reversible process, neither the color changes of the moths nor
the beak changes of the birds prove Darwinian evolution. Biology textbooks should not claim that they
do. They should only claim that they demonstrate the processes of genetic
variation and natural selection.
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