Don't believe in evolution? Just look to the weeds in the sidewalk:

Like other members of its family, Crepis sancta produces two types of seeds. Heavy seeds fall into the grass below the plant, whereas lighter seeds with feathery tails drift in the wind to new habitats. Ecologists have long known that plants in patchy habitats, such as islands, for example, produce more heavy seeds than light seeds, presumably because wind-swept seeds tend to get lost in the ocean. But controlling for environmental changes has been difficult.

A good study system turned up in Pierre-Olivier Cheptou's backyard. The population ecologist at CNRS, the French national research agency in Montpellier, France, and colleagues realized that they could do experiments on the Crepis sancta that invaded grassy patches around trees planted along the sidewalks in the city. After all, it stands to reason that a seed that lands on concrete would be as doomed as a seed that falls in the ocean; thus, it would be more advantageous for city plants to produce heavy seeds. So the team designed experiments to test whether the city environment had spurred evolution.

The fate of the city seeds clearly depended on their type. By counting the various kinds of seeds on the plants and then counting the seeds that fell on a sticky surface just below the plant, the team found that heavy seeds always landed on ground beneath the plant whereas light seeds had only a 45% chance of sticking around. Then, the researchers took seeds from city weeds and countryside weeds and grew them under similar conditions in the lab, comparing heavy seeds to light seeds. Whereas the urban dwellers yielded roughly 15% heavy seeds, the country plants produced closer to 10%. Using genetic models, the researchers confirmed that the changing pattern of seed dispersal took place in the past 12 years, about the time that the sidewalks in the city were constructed. "I was surprised that evolution can go so fast," Cheptou says.

Here is the paper. The authors estimate that this evolution occurred in about 5-12 generations.

Crepis sancta incidentally look sort of like dandelions.