Another example is rye ("Secale cereale"), a grass which is derived from wild rye ("Secale montanum"), a widely distributed Mediterranean species. Rye was originally just a weed growing with wheat and barley, but came under similar selective pressures to the crops. Like wheat, it came to have larger seeds and more rigid spindles to which the seeds are attached. However, wheat is an annual plant, while wild rye is a perennial. At the end of each growing season wheat produces seeds, while wild rye does not and is thus destroyed as the post-harvest soil is tilled. However, there are occasional mutants that do set seed. These have been protected from destruction, and rye has thus evolved to become an annual plant.