The Peppered Moths in England: A Fascinating Example of Natural Selection

A famous and fascinating example of natural selection is the case of peppered moths (Biston betularia) in England. These moths are found in two colour forms: speckled white and black .The colour form of the moth is determined by a gene called carb-TE. Historically, almost all the moths were white, with only a small percentage of the population being black.

The main danger faced by the moths is predation by birds. The white moths were effective at avoiding predators by camouflaging against the light-coloured, speckled backdrop of tree bark and lichen.When the Industrial Revolution began in London, urban peppered moth faced a new challenge. Soot covered all the tree trunks throughout the city, killing lichen that grew on the bark and turning tree trunks from speckled light grey to black.

The speckled white moths that once blended into the lichen-covered bark, now stood out as easy targets for predators. Those that were darker in colour gained a competitive advantage as they blended into the soot and concrete surfaces.

Over several decades, the proportion of colour forms shifted so that almost all the urban peppered moths were black, with only a few speckled whited moths.

1. Explain why natural selection occurred by describing evidence (from the text above) that each of Darwin's four postulates was met'
   
   
2. Even during the height of the Industrial Revolution, in rural areas there was much less soot, and the speckled white form remained the most common. Suppose that there was gene flow between the rural and urban pepper moth populations. What effect would you expect this gene flow to have on the phenotypic frequencies of the two populations? Why?
   
   
3. Would gene flow between these populations increase fitness, decrease fitness, or have a neutral effect? Justify your choice.

GRADING RUBRIC (9 marks)