The moth species Biston betularia has two color morphs, or variants. One has a speckled white and black color, while the other is all black. In the early 1800s in England, the distribution was 10,000 speckled morphs to 1 black morph. That distribution changed dramatically between 1900 and 1950. By 1950, the distribution of the speckled morph had declined to less than 3% of the population and the black morph distribution increased to 97%. Which statement most likely explains this change?

A. A predator of the moth underwent steep decline, which reduced selective pressures on the moth.
B. A chance mutation occurred in one black individual, which gave an advantage to the black morph's ability to survive.
C. The changes are due to normal random fluctuations that might be observed in any population over time.
D. Changes in the environment changed each morph's ability to evade predation in different ways.

The moth species Biston betularia has two color morphs, or variants. One has a speckled white and black color, while the other is all black. Over the period of time, the distribution of moths dramatically changed between 1900 and 1950 in such a way that black morph became dominant but speckled morphs became very much declined in abundance. The reasons behind this change was heavy industrial pollution in UK between 1900 and 1950.

When the environment became very polluted, dark and blackish, the speckled moth had more chanced of getting eaten by predators thats why they declined in number as compared to black morphs. On the other hand, backgrounds of grime and soot made black morphs less visible to the predators and they survived well. This is an excellent example of natural selection that only those specie better survive that have the ability to cope better with adverse environmental fluctuations.

Therefore, option D is best option.

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