What happened to these fish as they adapted to living exclusively in freshwater? a) Over many generations, populations of fish changed in many different ways, including in their skeletons.
b) They acquired new characteristics by mating with fish that lived in these lakes.
c) The new environments caused individual fish to change within their lifetime.

the two answers are a, the population of birds with large beaks will increase on the island, and c, the trait for large beaks will be passed on to future generations through natural selection. the reason for that is because if the birds that have beaks developed for eating small seeds lose all of their food, they will go hungry and eventually starve because they physically cannot eat the large seeds, therefore making the population of large beaked birds higher. also, if the trait of large beaks are dominant on the island over small beaks, then over time the large beaked birds will reproduce more and more, while the smaller beaked birds will reproduce less and less because they can't survive with the food provided. that leads to the trait of small beaks disappearing and the trait of large beaks taking over the island.