Peppered moth evolution
Industrial melanism is a variant of melanism, that is a particularly pronounced intercalation of dark pigments - especially of melanin - the skin. The name came about after a drastic change of its external appearance was observed in the peppered moth ( Biston betularia ), a moth, the end of the 19th century English industrial areas: Had previously the individuals of a light morph dominated, were then predominantly dark-colored individuals on so-called carbonaria - morphs.
The industrial melanism is considered a prime example of how environmental changes can influence the course of phylogeny strongly within short periods of time. A genetic analysis published in 2011 concluded that all carbonaria - morphs on the same allele ( " singular origin" ) feature and that this allele has emerged only recently.
The historical background
Moths are typically brightly colored and therefore (if they sit on the equally bright bark of birch trees ) from the ground to distinguish itself from close barely. This camouflage - a protection against predators - failed as a result of his time dramatic air pollution from factory chimneys of the birches increasingly dark colored soot and settled on them lichens have been killed. 1848 was observed in the vicinity of Manchester for the first time, a larger number of dark-colored specimens. In 1895, 98 percent of all moths were dark colored in the industrial area in Manchester. Also, in at least 70 different species of butterflies, the frequency distribution of its appearance changed in a similar way.
Since the 1960s, the share of melanistic variants in the English industrial regions - in parallel to reduce air pollution - again considerably. The compared to the baseline situation in the 19th century, we still slightly higher proportion of dark shape at the beginning of the 21st century by researchers attributed to the fact that the birches are many places still sparsely populated because of the still existing air pollution caused by exhaust gases from lichens and the bright individuals therefore still do not find the optimal living conditions.
Doubt as to the interpretation of the observations
A common misconception is that the moths can be found mainly on the bark of tree trunks during the day. In fact, he usually hides in the light on the underside of small branches and under their foliage. The error was provoked showed photographs in various textbooks of ecology and evolutionary biology, the white copies on dark tree trunks and dark copies on light tree trunks. However, these photos were not from free-living specimens, but they were descended from certain experiments that have been repeated several times since 1953. In fact, the proportion of moths during the day resting on tree trunks is, roughly about a third: in a study over 6 years 135 dormant individuals were found, of which at tree trunks ( the excellently camouflaged moths are also for experienced biologists hard to find) 48.
Were first published in such experiments to industrial melanism in 1955 by Bernard Kettlewell, a zoologist at Oxford University, who reviewed a 1896 formulated by the butterfly expert James Hutt hypothesis. 1953 and 1955 Kettlewell had exposed in the highly polluted place Rubery in Birmingham as well as in the unloaded tracts of land from Dorset in the early hours each hundreds of moths of both color forms on tree trunks, where the animals usually immediately took their rest position. Evening Kettlewell determined with the help of butterfly traps the distribution frequency of the two color forms and began to notice that he was able to capture in Birmingham twice as many dark as bright variants, while in Dorset, the ratio was exactly reversed.
Under colleagues pushed his study on unbelief, since neither Lepidopterologen still ornithologists birds saw as relevant predators of moths. Therefore Kettlewell asked the behaviorist and experienced wildlife photographer and videographer Nikolaas Tinbergen, to document a repetition of the experiments. Then, in 1958, actually published by Tinbergen a documentary, which demonstrated convincingly that bright butterflies are frequently discovered and consumed on dark bark of birds as dark individuals.
Only in 1998 had Michael Majerus, a geneticist at the University of Cambridge, pointed out that these experiments were not convincing proof of the natural processes that had led to a shift in the ratio of light to dark individuals: The ecological relationships ( the lodging places of the moths ) were not sufficiently followed precisely and been promoted inappropriately by an oversupply of moths that prey behavior of the birds. In addition, the relativization of the selection pressure was not sufficiently incorporated by diurnal birds by the non- selective nocturnal hunting by bats in the studies.
Majerus ' critique of Kettlewell's approach was sharpened in 2002 by a journalist in a popular science book: You threw Kettlewell scientific fraud. The evolutionary biologist Jerry Coyne ( University of Chicago) dismissed the insinuation in the journal Nature while back immediately, but Majerus ' professional criticism and the popular science book by creationists as evidence for a major evolutionary dizziness was used.
A long-term study
In 2002, Michael Majerus began a five-year field study to test the hypothesis again and this time close to nature, according to which the frequency of the light and dark peppered moth form is influenced by the predation of birds .. This study was designed explicitly as to cover all hitherto put forward criticisms of Kettlewell's experimental design. In Madlingley Wood west Cambridge were exposed to different parts of 103 bright and lichen -covered birch trees from wild type and melanic moths in the same frequency ratio, which they had at that time in the wild population (initially about 12 percent melanistic ); at the end of the test series were still being present 97 ( storm losses). All exposed animals, about 800 per year, came from the region in which they were released, their density was in the test area do not face excessive normal conditions, they were only exposed to their natural period of activity. Males and females, animals bred and released wild catches were analyzed separately. The animals were released at dusk in large cages so that they could choose a resting place at sunrise, the cage then removed. The resting moths were visited again after four hours, and existing and due to lack of predation by birds recorded ( the clamps are inactive in daylight and do not move ). The result of this experiment was very complex: From the melanistic form of a share of 29.2 per cent on average were eaten by the bright form 21.2 percent. This results in a selection coefficient of 0.22 against the melanistic form. The frequency of melanistic form declined in the investigation period (2002 to 2007) in the same area in parallel, from 12 percent to just over one percent, this would result in the equivalent of a selection coefficient of 0.29 against the melanistic form. The differences between these coefficients are statistically significant. Thus it can be seen as evidence that predation by birds is sufficient as a reason to explain the observed decline in the melanistic form on the back light trees. In addition, during the experiment, a number of birds were observed directly in the attack on resting moths, including Robin, Tree Sparrow, Great Tit, Blue Tit, Blackbird.
However, It is unclear, influence what additional factors, the frequency distribution of both peppered moth forms. In East Anglia the dark form has always been relatively common, although there never a particularly high air pollution was recorded.
The controversies surrounding the causes of the Industrial Revolution moths are when used again and again by creationists to provide the mechanisms of evolution in question. A partial explanation of the reasons for the frequency of occurrence of colors in the peppered moth, however, in this context, no probative value, because a change of environmental factors ( so-called environmental factors ) and will be made even by the critics of the theory of industrial melanism in accounting.