A farmer applies a new pesticide to a field, killing 98% of a specific insect pest population. However, after a few generations, the insect population rebounds to its original size, and the pesticide is now largely ineffective. Which statement best explains this phenomenon from an evolutionary perspective?

A population of island birds has very low genetic variation, and all individuals possess thin, delicate beaks perfectly suited for cracking small, soft seeds from a single type of plant that is abundant on the island. A new, invasive plant species is introduced, and its seeds are large and hard-shelled, quickly outcompeting the native plant. What is the most probable outcome for this bird population in the face of this environmental change?