How did farmers observed by Charles Darwin take advantage of natural variation to improve their livestock?

They used artificial selection

Explanation:

Breeding animals who had desirable or good traits. For example, they bred cows who produced the most milk and plants that produced the most vegetables. :))

significant amounts of nitrogen oxides, carbon monoxide, and other pollution

explanation:

passenger vehicles are a major pollution contributor, producing significant amounts of nitrogen oxides, carbon monoxide, and other pollution. in 2013, transportation contributed more than half of the carbon monoxide and nitrogen oxides, and almost a quarter of the hydrocarbons emitted into our air.