Using the State Curriculum: Science, Grade 8

Organisms within a population have slight differences from each other that may provide some organisms with a survival advantage. Individual organisms do not adapt to a change in an environment, nor do they evolve into new species, based on need or some desire to improve. They are limited to the traits they inherit from their parents. Those traits that provide the organism with a better chance of surviving in a given environment are likely to be passed on to the offspring. The peppered moth in England is an example of how variation within a population enhances its chances of survival.

The peppered moth has two forms: light-colored and dark-colored. Originally, the light-colored moths were more numerous in the population. The moths that were light-colored blended in well with the lighter bark on trees and were not eaten by the birds. The dark-colored moths were easily seen and were eaten. The majority of the moths were light-colored because they survived and passed on the light-colored trait to their offspring. Over time, pollutants associated with the Industrial Revolution coated the bark of trees with dark soot. As a result, the dark-colored moths blended in and birds preyed more frequently on the more visible light-colored moths. Within only 50 years, the population had changed from mostly light-colored moths to mostly dark-colored moths.

In the case of peppered moths, the population did not change color in order to survive a changing environment. Both color forms were always in the population. When the trees had light-colored bark, the light colored moths had a selective advantage. They could easily hide from birds. But when soot from factories coated the trees, the dark moths could hide and the lighter individuals were eaten. Further research has shown that selective pressure from birds was not the only factor affecting the change in the population of peppered moths. The time of day the moths emerged and the places where they rested on the trees were also factors affecting their survival.

Environmental pressure can lead to greater changes within a population over time. The formation of a natural boundary, such as a river or canyon, between members of a population which causes permanent separation may lead to the formation of a new species. If the individuals respond differently to the changed environment and they are no longer capable of interbreeding, they can be classified as distinct species. The process is called speciation.