Final answer:
Mammals and birds have compact myocardium, whereas many fishes and amphibians have spongy myocardium. The complexity of heart chambers increases through evolution, supporting the different metabolic needs of these animals.
Step-by-step explanation:
Mammals and birds have compact myocardium while many fishes and amphibians have spongy myocardium. This difference is reflective of the evolutionary changes in the anatomy of the heart and its requirements to support different lifestyles and metabolisms. The more efficient four-chambered hearts of mammals and birds, which completely separate oxygenated and deoxygenated blood, supports their warm-blooded, active lifestyle by improving the efficiency of double circulation. In contrast, fishes with their two-chambered hearts and amphibians with three-chambered hearts exhibit a less complex and less efficient circulatory system.
The four-chambered heart of mammals and birds is an example of convergent evolution, where similar biological traits evolve independently in different lineages. In mammals, the heart has specialized fibers called the sinoatrial node which regulates the heart rate, and mammalian red blood cells are anucleated, differentiating them further from other vertebrates.