Three- spined stickleback fish Gasterosteus aculeatus) show substantial heritable variation in gill- raker length. Fish caught in open water tend to have longer gill rakers and eat different foods than those caught in shallower water. Longer gill rakers appear to function better for capturing open- water prey, while shorter gill rakers function better for capturing shallow- water prey. Which of the following types of selection is most likely to be found in a large lake open water in middle and shallow water around the sides) with a high density of these fish?