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The Model of Selection can be Applied to Peppered Moth

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    The form of Light peppered was always found until the 18th Century in Britain, But a dark Melanic form was first recorded in 1848. The Melanic forms increases up to 90% in polluted areas. In un polluted areas the Light form Remain common. Clean air laws were passed in the mid 20th century and the frequency of Melanic form decreased.

    So we will be looking forward to see how natural selection operates in the fitness of melanic and light peppered form. In what way it is acting either through bird predation or by other factors

    The Light form has genotype cc and the melanic form represented as dominant homozygous CC.

    So three methods to find how natural selection operates and the fitness of melanic and light form, in this case, is below

    Estimation of Fitness By Using the rate of change in Gene Frequency:

    The first estimate of fitness was made by Haldane in 1924. for the first method, we need measurements of frequencies of the different color forms at least 2 times. we can then estimate the gene frequency from genotype frequency. The melanic form was first seen in 1848 but it was also present before in a less proportion. This low frequency is because of Mutation selection balance. Mutation selection balance means that the gene is disadvantageous, and exit as low frequency and determined by a balance between being formed by mutation and being lost by selection.

     Typical mutation rates for genes are about 10 raise to power -6 and a selective disadvantage of about 10% for the melanic mutants in preindustrial times may be approximately correct. By using these figures and calculation we get the frequency of the melanic C gene that is 10 raise to power -5 up to the year 1848  By 1898, the frequency of the light-color genotype was 1–10% in polluted areas 

    The change over the 50 generation is estimated and showed that the survival rate for peppered moth is 2/3 after 50 generations.

    The second estimate of the Fitnesses from the survivorship of the different genotypes in mark-recapture experiments

    Kettlewell (1973)measured survival rates by mark-recapture experiments in the field. The gene frequency change was (and still is) thought to be produced by survival differences between the two forms of moth in nature, rather than differential fertility. 

    He released melanic and light-colored peppered moths in known proportions in polluted and unpolluted regions, and then later recaught some of the moths (which are attracted to mercury vapor lamps). He counted the proportions of melanic and light-colored moths in the moths recaptured from the two areas.

    Birmingham (polluted) and DeanendWood, an unpolluted forest in Dorset, UK. The proportions in the recaptured moths are as we would expect: more light-colored moths in the Deanend Wood samples and more melanic moths in the Birmingham samples. In Birmingham, melanic moths were recaptured at about twice the rate of light-colored ones, implying s =0.57. This is a higher fitness difference than the s=0.33 implied by the change in gene frequency.

    both estimates are uncertain; it could have a number of causes. Possible causes include sampling error in the mark–recapture experiments (the numbers in Table 5.6 are small) and errors in the assumptions of the estimate from gene frequency changes

    Bird predation factor:

    Bird predation is the agent for natural selection. he light-colored form is better camouflaged in unpolluted woods and therefore less likely to be eaten by visually hunting birds. But smoke pollution killed the lichens that covered the trees, after which the melanic form was better camouflaged (Figure 5.4).Several lines of evidence support Kettlewell’s explanation. Bird predation is not supported widely by everyone however.

    We looked above at fitness estimates from gene frequency changes and from mark-recapture experiments. Kettlewell and others also estimated fitnesses by pinning out dead moths of the two forms on tree trunks in polluted and unpolluted areas. He then measured how many moths of each form disappeared over time. These experiments were particularly criticized after it was discovered in the 1980s that peppered moths do not naturally settle on tree trunks but on the higher branches and twigs of trees. Other criticisms were also made. However, Kettlewell’s case does not depend on these pin-out experiments.

    Migration and inherent factor:

    Cook in 2000 also worked on these 2 forms of moths. Evidence has also been put forward for other factors, in addition to bird predation. Migration is one extra factor.

    The geographic distribution of the two forms does not exactly Ft Kettlewell’s theory. The melanic form had a frequency of up to 80% in East Anglia, where pollution is low. And in some polluted areas, the dark form did not seem to have a high enough frequency.

    Migration may explain why melanic moths are found in some unpolluted areas such as East Anglia and why light-colored moths persisted in polluted areas where they were less well camouflaged.

    A second additional factor is that the two forms may differ in fitness independently of bird predation. Creed et al. (1980) collected all the measurements that had been made on survival to adulthood in the laboratory They analyzed the results of 83 broods, containing 12,569 offspring; They concluded that melanic homozygotes are more and have an Inherent advantage and fitness advantage. and melanic moths replace light moths even in bird predation. In unpolluted areas, light-colored moths may remain only because birds eat more of the conspicuous melanic moths some biologists also disagree with inherent advantage and rule it out in their case study.

    In conclusion, the industrial melanism of the peppered moth is a classic example of natural selection. It can be used to illustrate the one-locus, two-allele model of selection. The model can be used to make a rough estimate of the difference in fitnessbetween the two forms of moth using their frequencies at different times; the fitnessescan also be estimated from mark-recapture experiments. Good evidence exists that bird predation is at least partly the agent of selection, but some biologists suggest other factors are at work too.

    Reference ; Book of Evolution Mark Ridley.

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