Members of a species of red fox have teeth of varying sharpness. Foxes with very sharp teeth are able to kill large prey for food, while foxes with very dull, strong teeth are able to crush eggs and small animals. Foxes with teeth of medium sharpness, however, cannot get food and many die before they are able to reproduce. Over time, the fox population shows a greater proportion of individuals with either very sharp or very dull teeth. Which type of natural selection best describes this situation?