Saturday, March 28, 2009
Melanic Moths
On page 52, Carroll gave a brief discussion of the melanic moths and how the lighter populations were slowly declining due to air pollution. At the same time, "there was a dramatic, rapid increase in the frequency of dark, melonic forms of the moth in industrial areas" (Carroll 52). Discuss why it would be advantageous for the moths to appear darker in polluted, industrial areas. Furthermore, discuss the probable changes in the local environment (populations of other species, not necessarily moths but birds or other animals) and possible dangers of this sudden change in nature. If possible, relate to biological themes and outside resources.
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The melonic moth mystery is a very interesting one that has not clear cut answer to it. As the air got more polluted and dirtier in industrial areas; the dark, melonic moth started to become more frequent in the population. As a result of the pollution the lichen on the tree branches started to alter colors; this is very important because this is the peppered moths live. As a result of the change in the coloring of the lichen, the dark moths were adapted or adapted quickly to be able to feed on them. Those few light colored moths that were able to find less altered lichen were able to feed on it and still survive. But as time went on the pollution kept altering the lichen; and less and less lichen that the light colored moths were able to feed on. This explains there 20% average decrease for that 48 year period of 1848-1896. Later with pollution restrictions the light colored peppered moths are coming back because lichen populations are returning to normal colorations.
ReplyDeletePollution is always a factor in the environment and no more than in the last century. Pollution has brought one good thing because the increase production of CO2 has led to the increase in the ability of forests and other photosynthesizes to maximize their population and their ability to have a better chance to survive and reproduce. There has been however negative affects that are much worse. Their destructions of forest that in the process keep more CO2 in the atmosphere and not as beneficial to many animals. It leads to displacing their homes and lead to them dying. Also it alters habitats because each species is adapting and if one cannot adapt to another’s changes they will end up dying. This can be seen with fish populations in England because the sewage system is not good and many of the pollutants in the water are leading to changes of male fish to female fish. This will ultimately lead to problems in mating and damage fish populations. (http://news.bbc.co.uk/2/hi/uk_news/3882159.stm)
Caroll uses the melanic moths as an example of strong selection. The common view is that the peppered moths rest on the mosses of trees. As areas became more polluted, the dark coloration of melanic moths, versus the non-melanic ones, gives them a significant advantage. The reason for this is believed to be that because the rise of pollution caused the darkening and death of the mosses, exposing the dark bark underneath, the melanic moths are camouflaged better in polluted environments. Their predators, the birds, are then able to pick off the lighter non-melanic moths and effectively remove them from the gene pool.
ReplyDeleteHowever, there is much criticism of this view of the natural selection of melanic moths. The strong correlation between the amount of pollution and the dominance of melanic moths shows that pollution does a major factor the natural selection of melanism; this much is not contested. The role of mosses is contested. The first problem comes from the fact that the peppered moths do not typically rest on mosses. Instead, they rest on the underside of small branches. Despite this, the experiments that are typically cited as support for the standard view on melanism in peppered moths all artificially placed the moths onto the mosses on tree trunks. Also, the observation of the decrease in melanism in peppered moths is severely flawed. As the areas became less polluted, melanism was no longer an advantage, and the population melanic moths decreased in direct correlation. However, there was no correlation between melanism and the mosses. Non-melanic moths began recovering their dominance in the absence of pollution before there was any change in the mosses. If the darkening and decay of mosses caused the initial rise in melanism, the melanism should have decreased in direct correlation with the recovery of mosses. Caroll believed that the selective advantage of melanism in peppered moths was an exception to the "challenge of detecting slight selective differences in the wild" (52). Unfortunately, this is only another example of how mutations can cause advantages initially imperceptible to humans and how complicated the subtleties of evolution are.
The impact of industrial pollution is very severe. The destruction of natural resources is also the destruction of many ecosystems of certain organisms, and the mass release of contaminants into the environment causes major instability among different living populations. The result is natural selection. The many organisms that cannot survive in pollution-filled areas no longer contribute to the gene pool. The organisms whose mutations allow for adaptability pass their genetic selective advantage to their offspring. Another example of natural selection due to pollution is the gastropod Cerithium rupestre. The mollusk has naturally selected for resistance against mercury pollution. As a marine organism, mercury pollution, mainly caused by coal-fired power plants, can pose serious threats to its health and very likely kill it. The mutation that allows it to resist mercury has allowed it survive, and so it has become a naturally selected trait.
http://www.arn.org/docs/wells/jw_pepmothshort.htm
http://www.discovery.org/a/1263
http://www.ncbi.nlm.nih.gov/pubmed/3996551
The melanic moths rapid growth in response to increasing pollution of environmental areas has been a favorite example of natural selection and (micro) evolution. As such, it has received many attacks by critics and many incongruities have been found. But overall, the rapid rise and fall of the melanic moths remains as a model for the effects of natural selection. But in the classic model, the darker colored melanic moths were better camouflaged in the polluted areas around England; therefore, the a great increase of frequency of these melanic moths were found with typicals less frequent. In original conditions typical light colored peppered moths were better camouflaged to their settling positions in trees as their peppered patterns fit with the speckled pattern of crustose lichen on common branches that are visible with the ultraviolet vision of birds.
ReplyDeleteBernard’s Kettlewell’s experiments with (melanic moths) carbonara and (lighter moths) typica has been used as one of the most important evidence supporting the theory of industrial melanism. In the experiment Kettlewell released typical and melanic moths onto tree trunks in a polluted woodland near Birmingham, England. He observed himself that visually the typicals were more conspicuous and less camouflaged on the surrounding tree trunks. At the end of the day he was only able to collect 27.5% of the melanic moths and 13.0% of the typicals, demonstrating that in the immediate surrounding environment melanic moths had better survived predation most likely from birds. He repeated this experiment in an unpolluted woodland in Dorset, England and found the opposite results. His results correlated with the rise of carbonara during the Industrial revolution of England and the increasing pollution that exposed the dark bark of trees as well as place soot and other darkened materials on the trees, then with more anti-pollution laws, the decline of carbonara. It also reinforced the J.W Tutt’s hypothesis of differential bird predation as the selective force of the peppered moths.
There are some errors in the experiment such as the errors with releasing moths into unfamiliar environments. Normally, as Nikil stated, Howlett and Majerus experimented and concluded that of the 135 moths examined, over half were on tree branches, mostly on the lower half of the branch, 37% were on tree trunks, mostly on the north side, and only 12.6% were resting on or under twigs. When releasing the moths unto tree trucks, they possibly did not have adequate time to adapt to the surroundings and chose the best hiding spots on the tree. As well, the moths most likely have adapted to the environments they were found in that could have considerable differences to the relocation. Corresponding from information above, the lighter peppered moths are not well camouflaged with foliose lichen of tree trunks. Another discrepancy are experiments conducted in Michigan and Pennsylvania. While data I Michigan resembles that of England in regards to the declining melanic moth populations, there has not been any noticeable or visual changes in the environment in Michigan to provoke these changes. However, air purity did increase during the data collection periods. As well, when searching for moths in other darker areas such as coal mining regions, near a steel industry, farmlands, and mountains, the corresponding melanic moth populations were similarly low in every case with continuing decline.
As well, melanic frequencies mores strongly correlated with sulfur dioxide concentrations than with smoke so it’s possible that this factor may have more noticeable effects in Michigan and Pennsylvania. One opposing hypothesis is that of Heslop Harrison. He stated that mutation pressure and not the differential bird predation hypothesis was the cause of increasing frequencies of melanic moths. Using Salts of lead and manganese, which were present in the airborne pollutant particles of England‘s Industrial Revolution, he suggested that these caused the mutation of genes for melanin production but of no others. He fed moth larvae with leaves that had incorporated these salts and melanics subsequently appeared. While Hughes McKenney in 1932 and Thomasen and Lemche in 1933 failed to replicate the results, it is still a viable hypothesis.
But overall, the classic example of the melanic moths is still a good example of natural selection and microevolution. While some concessions must be made, such as other factors contributing to the rapid rise of melanic moths, the idea of differential bird predation has been supported by most experiments.
http://jhered.oxfordjournals.org/cgi/content/full/93/2/86
http://en.wikipedia.org/wiki/Peppered_moth_evolution
http://www.arn.org/docs/wells/jw_pepmothshort.htm
http://en.wikipedia.org/wiki/Peppered_moth
http://books.google.com/books?id=Kb2cACRfI54C&pg=PA56&lpg=PA56&dq=Industrial+revolution+birds&source=bl&ots=XFx108OXs7&sig=zybYYTBD3ma6uxeQnYOqceNGHV4&hl=en&ei=ZGflSaCCNaXlnQeJxrG0CQ&sa=X&oi=book_result&ct=result&resnum=4#PPA57,M1