Pepsin enzymes can then digest dietary in the lumen of the stomach

Pepsin enzymes can then digest dietary proteins in the lumen of the stomach. Pepsin in a protease digesting the proteins into smaller peptides. It is produced by the chief cells of the stomach in the form of a proenzyme called pepsinogen. A proenzyme or a zymogen is basically inactive and needs a biochemical change to become an active enzyme. This biochemical change is brought about by the acidic environment of the stomach by the secretion of the hydrochloric acid by the parietal cells in the stomach lining.

if the type of the food available changes, then the frequency of beak types will change, because birds with beaks more suited to the available food will be more successful over time.

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).