Describe the evolutionary change in the pattern of the heart among the vertebrates.

The evolution of the heart in vertebrates has been a gradual process that has resulted in a variety of heart structures and functions. Here is a step-by-step description of this evolutionary change:

1. Fish: Fish, the most primitive vertebrates, have a two-chambered heart consisting of one atrium and one ventricle. The heart pumps the blood to the gills where it gets oxygenated, and then it goes directly to the rest of the body.

2. Amphibians: Moving up the evolutionary ladder, amphibians have a three-chambered heart, with two atria and one ventricle. This allows for a small amount of mixing of oxygenated and deoxygenated blood, which is not a problem for these animals as they can also respire through their skin and do not rely solely on their lungs for oxygen.

3. Reptiles: Most reptiles have a three-chambered heart, but it is more advanced than that of amphibians. The ventricle is partially divided which reduces the mixing of oxygenated and deoxygenated blood. Crocodiles, alligators, and birds, which are also classified as reptiles, have a fully divided ventricle, making their heart a four-chambered one.

4. Birds and Mammals: Birds and mammals, the most advanced vertebrates, have a four-chambered heart, with two atria and two ventricles. This allows for complete separation of oxygenated and deoxygenated blood, which is necessary for their high metabolic rates and active lifestyles.

In summary, the evolution of the heart in vertebrates has been a process of increasing complexity and efficiency, from a simple two-chambered heart in fish to a four-chambered heart in birds and mammals.