I hope your walking shoes and Mackintosh are at hand, for we are off on a ramble through the English countryside. We have jumped back to the nineteenth century, and as we head out, we see green hues burst forth from crops and weeds, grasses and shrubs, and trees and marsh. Birds and insects seem to be flying, buzzing, or crawling most places we look. We can even see a tall, tweedy fellow with a net chasing butterflies at the edge of the woods. Generations of farmers have encouraged this diversity of plants by dividing fields, plowing some and leaving others fallow, growing several types of crops, and planting hedgerows.

Now let us jump forward to today. The landscape looks very different. Large fields have replaced small fields, hedgerows and woods have disappeared to make tractor plowing easier, manufactured fertilizers have substituted for plant and animal manures and fallowing, single crops grow over large areas, grasslands are closely cropped, herbicides have annihilated weeds, and combine harvesting has left few seeds behind for avian gleaning. Monotony has squeezed out diversity. Mostly these changes have been ecological, altering the distribution and abundance of organisms, but some of the changes have been evolutionary, changing the traits of organisms.