The body mass of individual beetles of a certain species ranges from 0.8 grams to 2.9 grams. A population of birds is able to catch beetles having a mass of 1.8 to 2.2 grams. The birds removed a majority of these beetles and the population changed. After 300 generations, the individual mass of beetles in the population ranged from 0.8 to 1.4 grams and 2.4 to 2.9 grams. Which is the mechanism for the changes in this beetle population