The pressure to keep billions of humans fed can have a transformative impact on amimal populations. Overharvesting that targets the largest animals can result in reduction of the average size of species, as seen in Caribbean conch snails. And sport-fishing pressure on large mouth bass can winnow out the most agressive in the gene pool, resulting in a "lazier," more passive remnant population. Martha Foley and Curt Stager talk about the human factor in animal evolution.