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

Across the vertebrates the heart grows steadily more divided, so that oxygenated and deoxygenated blood are kept increasingly apart.

• Fishes have a two-chambered heart, with one atrium and one ventricle. It pumps only deoxygenated blood, which is oxygenated at the gills and sent to the body, giving single circulation.
• Amphibians and reptiles (except crocodiles) have a three-chambered heart, with two atria but a single ventricle. Oxygenated and deoxygenated streams both enter the one ventricle and mix there, so this is incomplete double circulation.
• Crocodiles, birds and mammals have a four-chambered heart, with two atria and two ventricles. The two sides stay separate, oxygenated and deoxygenated blood never mix, and complete double circulation results. …

Every vertebrate has a muscular, chambered heart, but the number of chambers is not the same across the groups, and this difference decides how completely the body can keep oxygenated blood apart from deoxygenated blood. Reading the groups in order shows a clear direction of change.

In fishes the heart has just two chambers — a single atrium and a single ventricle. This heart pumps out deoxygenated blood, which travels to the gills to be oxygenated, is then supplied to the body parts, and finally returns as deoxygenated blood to the heart. Because the blood passes through the heart only once in each complete circuit, this arrangement is called single circulation.

In amphibians and in reptiles other than crocodiles, the heart has three chambers — two atria but only one ventricle. The left atrium receives oxygenated blood while the right atrium receives deoxygenated blood, yet both streams empty into the same single ventricle where they mix, and the ventricle then pumps out this mixed blood. This partial separation is described as incomplete double circulation. …

Method: Tracing an Evolutionary or Comparative Trend Across Groups

Use this approach for any question asking how a structure changes across a sequence of related organisms.

Steps

Step 1: List the groups in their conventional order

Here, that order is fishes → amphibians/reptiles → crocodiles/birds/mammals. Never describe the groups out of this sequence, since the trend only makes sense read in order.

Step 2: State the structural feature relevant to the trend for each group, before discussing function

For heart evolution this means naming the chamber count (two, three, four) for each group first.

Step 3: Connect the structural feature to its functional consequence …