1.10 A classic example of natural selection is the change in relative populations of the
peppered moth that occurred during the industrial revolution in England. After the
industrial revolution in 1870s, booming industrial cities started releasing tons of black
soot, blackening tree trunks and rocks and creating an environment where moths with
light colours were easily predated by birds. Peppered moths captured in 1850 and 1900
reveal that two industrial cities (X and Y) had the following percent distributions.
Year 1850 1900
Moths Light coloured Dark coloured Light coloured Dark coloured
X 66 34 22 78
Y 65 35 27 73
Pollution control rules were introduced in 1902 and were properly followed in city Y, but
not in city X. If all other factors remain constant, then what could be the approximate
expected distribution of light and dark coloured moths in 1950?
(A) Light X = 8, Y = 28 and Dark X = 92, Y = 72
(B) Light X = 8, Y = 62 and Dark X = 92, Y = 38
(C) Light X = 18, Y = 23 and Dark X = 82, Y = 77
(D) Light X = 18, Y = 50 and Dark X = 82, Y = 50

To understand the expected distribution of light and dark-colored peppered moths in 1950, we need to consider the impact of environmental changes due to industrial pollution and subsequent pollution control measures. The classic case of the peppered moth illustrates natural selection, where the coloration of the moths affects their survival based on their environment.

Background on the Peppered Moth

The peppered moth (Biston betularia) has two primary color variations: light and dark. Before the industrial revolution, the light-colored moths were more common because they blended in with the lichen-covered trees, making them less visible to predators. However, as pollution increased, tree trunks became darkened with soot, favoring the survival of dark-colored moths.

Population Changes Over Time

In the data provided, we see the distribution of moths in two cities, X and Y, at two different points in time: 1850 and 1900. The significant shift in the population from light to dark moths in both cities is evident:

• City X:
  • 1850: Light 66%, Dark 34%
  • 1900: Light 22%, Dark 78%
• City Y:
  • 1850: Light 65%, Dark 35%
  • 1900: Light 27%, Dark 73%

Impact of Pollution Control

After 1902, pollution control measures were implemented in city Y, which would lead to a gradual recovery of the lichen on trees and a more favorable environment for light-colored moths. In contrast, city X continued to experience high pollution levels, maintaining the advantage for dark-colored moths.

Estimating Future Populations

To estimate the expected distribution in 1950, we can assume that the trends observed from 1900 will continue. In city Y, where pollution control is effective, we can expect an increase in the light-colored moth population. Conversely, in city X, we anticipate that the dark-colored moths will continue to dominate.

Let's analyze the data:

• City X:
  • Continued pollution suggests a further decline in light moths. If we assume a similar trend, we might estimate around 18% light and 82% dark moths.
• City Y:
  • With pollution control, we can expect an increase in light moths. A reasonable estimate could be around 50% light and 50% dark moths.

Final Distribution Estimates

Based on these assumptions, the expected distribution of moths in 1950 would be:

• City X: Light 18%, Dark 82%
• City Y: Light 50%, Dark 50%

Now, looking at the answer choices provided, the closest match to our estimates is:

• (D) Light X = 18, Y = 50 and Dark X = 82, Y = 50

This choice aligns well with our predictions based on the environmental conditions and the effects of natural selection on the peppered moth populations in both cities.