A common French weed known as Crepis sancta underwent a form of superaccelerated evolution to cope with the difficulties of spreading their seeds in cities. Scientists studying C. sancta discovered that over a period of just twelve years, the plants went from mostly producing "dispersing" seeds that spread on the wind, to producing "nondispersing" seeds that fall to the ground nearby. Why would a plant shift its reproductive cycle so radically and quickly?
Seeds that spread on the wind in cities mostly wind up dead on the concrete, while seeds that fall usually find a spot to grow in the same street plots or concrete cracks where their parents grew. (You can see the two kinds of seeds at left.) Because seeds grew up so close to home, the plants evolved super quickly — sort of an urban Galapagos Islands effect. (One of the ways that Darwin first observed natural selection was on a trip past the tiny, isolated Galapagos Islands, each of which had evolved its own unique types of finches that interbred quickly and in isolation from finches on other islands.)
The Proceedings of the National Academy of Sciences published the researchers' study today, noting:
The authors took Crepis sancta seeds from several locations in the city of Montpellier, France, and grew the plants in a greenhouse, observing what fraction of seeds produced were of the light, easily dispersed type. Compared to plants from the countryside, plants from urban patches consistently produced fewer light seeds. Based on a mathematical model of breeding, the researchers estimate that the current version of urban Crepis sancta took approximately 12 years to evolve. They report that plants in a fragmented urban setting thus become doubly isolated, as reduced seed dispersal would likely lower gene flow and hence chances of species survival.This is just further confirmation that "natural selection" these days doesn't refer to natural environments but rather to built ones.
Rapid evolution of C. sancta [PNAS]