Biology 20 – Population Change Case Studies Variations and Adaptations The Peppered Moth: An Evolutionary Case Study Analysing Changes in Species Due to Environmental Variations The Common Peppered Moth (Biston betularia) is an insect prevalent among the Birch tree forests of England. It lives upon the bark of these trees. The moth exhibits wide variations in its general appearance; its overall pigmentation ranges from very dark to very light, including many shades in between. The population of the Peppered Moth was originally, prior to the Industrial Revolution, mostly composed of the lighter phenotypes (called the typica form). The selection pressure within the environment that produced this mainly “light” population was that the bark the moths lived upon: The whitish lichen allowed lighter moths to camouflage from predators, hence increasing its chances of survival. During the Industrial Revolution of the late 18th century, unchecked pollution and excessive carbon-based emissions caused the bark of the Birch trees to darken with soot. It was observed by many scientists during this time that the darker version of the moth (called the carbonaria form) was increasing in its numbers. By 1895, it was reported that 98% of Peppered moths in Manchester were of the carbonaria variety. Scientists theorised that the darker moths were better able to camouflage upon the sooty bark, as compared to lichen covered bark. The light colour of the typica variety had become a disadvantage. It was also observed that Peppered moths in the unpolluted countryside were still predominantly lighter coloured. Later, when the negative effects of pollution were recognised by the general community, the government passed various Clean Air legislations, and the levels of soot upon the bark began to decline. The numbers of dark moths declined rapidly, and many biologists hypothesise that the darker moth may become extinct within a few decades. As the bark of the trees the Peppered moths resided upon became light again, the darker moths became more apparent to birds and were an easier target. As stated in a popular biological study of the Peppered moth: "Differential bird predation of the typica and carbonaria forms, in habitats affected by industrial pollution to different degrees, is the primary influence on the evolution of melanism in the peppered moth." (P. 116, Melanism - Evolution in Action, M. E. N. Majerus, Oxford University Press, New York, 1998). In summary, it can be stated that the main factor in the shift in the colouration of the Common Peppered moth from the light, typica form to the dark, carbonaria form was predation by birds. However, it was not the predator that had changed, but rather the environment they lived in. As the environment changed from lighter trees to darker trees, the appearance of the general population of the moth changed from light to dark. This is how the species can change due to a change in the environment. Questions: 1. What caused the appearance of the first melanic (dark) form? Explain. 2. If the environment is causing the pressure to change (or adapt), what is the actual selecting agent? (What is causing the moth population to decrease?) 3. Explain how the color of the moths increases or decrease their chances of survival depending on the environment. 4. 400 light colored moths and 400 dark colored moths are released into a polluted forest. After 2 days the moths were recaptured, make a prediction about the number of each type of moth that would be captured. 5. Why was the melanic (dark) form of moth been more abundant since the mid 1800s? 6. Why is the dark form still prevalent today? 7. Why is the pepper moth considered a classic example of Darwin’s theory of evolution? (i.e. What is the theory? Explain how it applies to this example.) Biology 20 – Population Change Case Studies Variations and Adaptations The Peppered Moth: An Evolutionary Case Study Analysing Changes in Species Due to Environmental Variations The Common Peppered Moth (Biston betularia) is an insect prevalent among the Birch tree forests of England. It lives upon the bark of these trees. The moth exhibits wide variations in its general appearance; its overall pigmentation ranges from very dark to very light, including many shades in between. The population of the Peppered Moth was originally, prior to the Industrial Revolution, mostly composed of the lighter phenotypes (called the typica form). The selection pressure within the environment that produced this mainly “light” population was that the bark the moths lived upon: The whitish lichen allowed lighter moths to camouflage from predators, hence increasing its chances of survival. During the Industrial Revolution of the late 18th century, unchecked pollution and excessive carbon-based emissions caused the bark of the Birch trees to darken with soot. It was observed by many scientists during this time that the darker version of the moth (called the carbonaria form) was increasing in its numbers. By 1895, it was reported that 98% of Peppered moths in Manchester were of the carbonaria variety. Scientists theorised that the darker moths were better able to camouflage upon the sooty bark, as compared to lichen covered bark. The light colour of the typica variety had become a disadvantage. It was also observed that Peppered moths in the unpolluted countryside were still predominantly lighter coloured. Later, when the negative effects of pollution were recognised by the general community, the government passed various Clean Air legislations, and the levels of soot upon the bark began to decline. The numbers of dark moths declined rapidly, and many biologists hypothesise that the darker moth may become extinct within a few decades. As the bark of the trees the Peppered moths resided upon became light again, the darker moths became more apparent to birds and were an easier target. As stated in a popular biological study of the Peppered moth: "Differential bird predation of the typica and carbonaria forms, in habitats affected by industrial pollution to different degrees, is the primary influence on the evolution of melanism in the peppered moth." (P. 116, Melanism - Evolution in Action, M. E. N. Majerus, Oxford University Press, New York, 1998). In summary, it can be stated that the main factor in the shift in the colouration of the Common Peppered moth from the light, typica form to the dark, carbonaria form was predation by birds. However, it was not the predator that had changed, but rather the environment they lived in. As the environment changed from lighter trees to darker trees, the appearance of the general population of the moth changed from light to dark. This is how the species can change due to a change in the environment. Questions: 8. What caused the appearance of the first melanic (dark) form? Explain. THE APPEARANCEOF THE FIRST MELANIC FORM WAS CAUSED BY THE INDUSTRIAL REVOLUTION. POLLUTION AND SOOT CAUSED THE TREES TO DARKEN MAKING THE WHITE TYPE MORE PROMINENT AND CAMOUFLAGING THE DARK TYPE. 9. If the environment is causing the pressure to change (or adapt), what is the actual selecting agent? (What is causing the moth population to decrease?) THE SELECTING AGENT WOULD BE THAT PREDATORS ARE NOW ABLE TO SEE THE WHITE FORM VERSUS THE DARK FORM. THIS LOWERS THE POPULATION OF THE WHITE FORM, LESS OFFSPRING IN FUTURE GENERATIONS, AND A LOSS OF THE WHITE FORM IN THE GENE POOL. 10. Explain how the color of the moths increases or decrease their chances of survival depending on the environment. Biology 20 – Population Change Case Studies 11. 12. 13. 14. THE MOTHS RELY ON BLENDING IN WITH THEIR SURROUNDINGS TO PREVENT BEING EATEN. DARK MOTHS HAVE A BETTER CHANCE AT SURVIVAL IN A DARK ENVIRONMENT. 400 light colored moths and 400 dark colored moths are released into a polluted forest. After 2 days the moths were recaptured, make a prediction about the number of each type of moth that would be captured. IN A POLLUTED FOREST, THE LIGHT COLOURED MOTHS WOULD BE EASIER TO SEE AND RECAPTURE, PROVIDING A LOT WERE NOT LOST TO PREDATION. THE DARK COLOURED MOTHS WOULD BE DIFFICULT TO SEE AND THEREFORE, DIFFICULT TO CAPTURE. Why was the melanic (dark) form of moth been more abundant since the mid 1800s? THE MELANIC FORM OF MOTH HAS BEEN MORE ABUNDANT BECAUSE THEY ARE MORE PROMINENT IN THE GENE POOL. IT IS AN ADVANTAGE FOR THE DARK FORM TO BE PRESENT IN A POLLUTED ENVIRONMENT. Why is the dark form still prevalent today? WE STILL LIVE IN AN INDUSTRIAL TIME WITH POLLUTION PRESENT. IT WOULD STILL BE ADVANTAGEOUS FOR THE DARKER FORM. ALSO, THEY ARE MORE COMMON IN THE GENE POOL, GREATER CHANCE THAT EACH SUBSEQUENT GENERATION IS DARK AS WELL. Why is the pepper moth considered a classic example of Darwin’s theory of evolution? (i.e. What is the theory? Explain how it applies to this example.) IT SHOWS CHANGE OVER TIME. DARWIN SAID THAT THOSE ORGANISMS THAT ARE NATURALLY SELECTED WOULD GO ON TO PRODUCE OFFSPRING WITH CHARACTERISTICS THAT WILL ALLOW THEM TO SURVIVE. THE DARKER MOTHS WERE MORE “FIT” TO SURVIVE THE INDUSTRIAL REVOLUTION, WHICH MEANT THEIR NUMBERS INCREASED WHILE THE LESS FAVOURABLE LIGHT COLOURED MOTHS WEREN’T NATURALLY SELECTED.
