Tumbleweeds meet modern houses

Cheyenne, Wy., resident Linda Reid tried to walk outside her front door Thursday after high winds caused tumbleweeds to block the entrance.

Over the millennia, tumbleweed (Russian thistle) evolved an effective means of seed dispersal. Rather than have the seeds windborne or carried by animals, selective pressures favored those whose stems broke in the autumn, so that the roughly spherical bush could roll downwind, scattering seed along the way.

Of course as those evolutionary processes were underway, there were no houses in the way...