The document discusses bitter taste receptors and how they vary between organisms based on diet. Herbivores tend to have more functional bitter taste receptors to protect against plant toxins, while carnivores like cats have lost sweet taste receptors due to lack of plant material in their diets. Tracking homologous genes between organisms reveals their evolutionary relationships, as advantageous genes like those coding for bitter taste receptors are passed on while disadvantageous ones are removed through purifying selection.