Final answer:
Ray-finned fish (Actinopterygii) and lobe-finned fish (Sarcopterygii) are sister clades due to sharing a common ancestor with fins. Differences in fin anatomy, such as bony spines in ray-fins and fleshy lobes in lobe-fins, define their classification. Lobe-finned fish also have evolutionary significance as the precursors to amphibians.
Step-by-step explanation:
The relationship between ray-finned fish (Actinopterygii) and lobe-finned fish (Sarcopterygii) being classified as sister clades is defined by their descent from a common ancestor with fins. While they currently exhibit different fin morphologies, this divergence stems from their evolutionary paths after splitting from this ancestor. The ray-finned fish have fins supported by bony spines called rays, a characteristic seen in species such as tuna, bass, and salmon. On the other hand, lobe-finned fish have fins that are more fleshy and lobed, supported by bones similar to the bones in the limbs of early tetrapods. Today, the most recognisable lobe-finned fish are lungfishes and coelacanths.Contrasts between the two clades include differences in the underlying skeletal structure of their fins, with ray-finned fishes developing a fan of slender bones, and lobe-finned fishes having a fleshy, bony structure within their fins. Despite their current differences, both groups evolved from bony fishes, and lobe-finned fish are further notable for being ancestral to amphibians.
This evolutionary link showcases the shift from an aquatic to a terrestrial lifestyle as the limbs of tetrapods evolved from the fins of lobe-finned fish.The relationship between ray-finned fish (Actinopterygii) and lobe-finned fish (Sarcopterygii) is defined as sister clades because they share a common ancestor with fins unrelated to rays or lobes. Ray-finned fish have fins supported by bony spines called rays, while lobe-finned fish have fleshy fins supported by bone. The similarities in fin shape and morphology are not the defining factors, but rather the evolutionary consequences of these different fin structures.