The correct answer is: altitude.
Evolution is driven by the environment. In other words, a plant that grows in two different geological zones will develop distinct characteristics because the environment is significantly different. In a new environment, the operation of natural selection means that individuals who possess characteristics that are adaptive to the new conditions will be the ones that survive. In plants, this usually means they find favor with local pollinators, who are critical to their survival. This seduction campaign involves not only how attractive the flowers are – this is a popular research topic – but also the fragrance they emit. One of the rare studies done on this latter characteristic was conducted in the Swiss Alps by a group of Zurich scientists. It showed that differing selection pressures in the plains and the mountains resulted in regional differences in floral fragrance.
The researchers studied more than a thousand individual plants of the orchid species Gymnadenia odoratissima over two seasons in three mountain and three plains regions, at various altitudes. They found that the flowers’ fragrance varied considerably and was stronger at lower altitudes. Other characteristics such as the plants’ height or the flowers’ color did not appear to exhibit significant variation due to altitude.
The researchers also studied populations of the orchids’ pollinators (primarily flies, butterflies, mites and beetles) in various locations, as they are considered the main driver behind this selective evolution, and found that they were different in the two geographical zones. The pollinators’ activity is thus likely a function of the volatile fragrances emitted by the flowers. For the scientists, these results suggest that this selection criteria is one of the evolutionary forces that contribute to regional differences in floral chemical signaling.
Source: PLOS One