Question

Just answer the question in General. Don't need to use the graph

1. Why can you kill the moths off in a few time steps? What does this indicate could happen in an environment to cause a species to go extinct?
2. How could the model be modified so that the proportions of each moth color type would not change through time?

Answer 1

Answer: (1)

Peppered Moths are normally white with black speckles across the wings, giving it its name. This patterning makes it well camouflaged against lichen-covered tree trunks when it rests on them during the day. There is also a naturally occurring genetic mutation which causes some moths to have almost black wings. These black forms (called 'melanic') are not as well camouflaged on the lichen as normal 'peppered' forms and so they are more likely to be eaten by birds and other predators. This means that fewer black forms survive to breed and so they are less common in the population than the paler peppered forms. This is the normal situation observed in the countryside of Britain and Ireland.

However, in the nineteenth century it was noticed that in towns and cities it was actually the black form of the moth that was more common than the pale peppered form. Industrialisation and domestic coal fires had caused sooty air pollution which had killed off lichens and blackened urban tree trunks and walls. So now it was the pale form of the moth that was more obvious to predators, while the melanic form was better camouflaged and more likely to survive and produce offspring. As a result, over successive generations, the black moths came to outnumber the pale forms in our towns and cities. Since moths are short-lived, this evolution by natural selection happened quite quickly. For example, the first black Peppered Moth was recorded in Manchester in 1848 and by 1895 98% of Peppered Moths in the city were black.

In the mid-twentieth century controls were introduced to reduce air pollution and as the air quality improved tree trunks became cleaner and lichen growth increased. Once again the normal pale Peppered Moths were camouflaged and the black forms were more noticeable. Now the situation in urban areas has again become the same as in the countryside, with normal pale Peppered Moths being far more common than the black forms. So natural selection has been seen to work in both directions, always favouring the moth that is best suited to the environmental conditions. The same thing has been observed throughout Europe and the USA.

Sadly, having adapted so well to survive the earlier ravages of industrialisation, this species is now declining overall. Between 1968 and 2002 numbers of the Peppered Moth in Britain fell by almost two thirds, although the causes are as yet unknown.

Answer: (2)

This project models a classic example of natural selection - the peppered moths of Manchester, England. The peppered moths use their coloration as camouflage from the birds that would eat them. Historically, light-colored moths predominated because they blended in well against the white bark of the trees they rested on.

However, due to the intense pollution caused by the Industrial Revolution, Manchester's trees became discolored with soot, and the light-colored moths began to stick out, while the dark-colored moths blended in. Consequently, the darker moths began to predominate.

Now, in the past few decades, pollution controls have helped clean up the environment, and the trees are returning to their original color. Hence, the lighter moths are once again thriving at expense of their darker cousins.

Therefore, the proportions of each moth color type would not change through time.