Selective Pressures Generated by Humans
"Evolution by natural selection" is a familiar phrase, but selective pressures are not always "natural". Darwin used artificial selection, for example pigeon breeding, to understand how traits develop and are fixed in a population. A recent example of "anthropogenic" or human selective pressures is the Tibetan snow lotus. This flower is valued for traditional medicinal purposes and increasingly as a souvenir. The plant grows only at high altitudes, and is largest at the end of its flowering cycle just before it sets seed. Plants bloom only once. The larger plants are harvested preferentially, since they are supposedly more potent and also easier to see. The results are that the average size of the plants has been reduced by about four inches in the past century. In this case, the desired trait is actually selected against in the population, since the larger plants set seed less frequently than the smaller plants. This phenomenon, in which individuals expressing a desirable trait are removed from the population more quickly than other individuals, has had similar effects in salmon populations and other instances. The impact of human selective pressures on the evolution of organisms is a very real and measurable phenomenon.
Shrinking snow lotusShrinking salmon
