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How do shark and dolphin body shapes illustrate convergent evolution?

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A diagram showing the tentative evolutionary relationships among hominid species over the past four million years, with skull images and labeled species names connected by solid and dotted lines to indicate proposed evolutionary pathways. — phylogeny Tentative phylogenetic scheme for the evolution of the human lineage. Solid bars indicate the time ranges during which species are thought to have existed, based on fossil evidence. Dotted lines signify evolutionary relationships between hominin species that have been proposed on the basis of the fossil evidence.

The similarity in body shape between sharks and dolphins is a classic example of convergent evolution. This phenomenon occurs when unrelated or distantly related species independently evolve similar traits or features because they occupy comparable environments or ecological niches.1

While sharks are cartilaginous fishes and dolphins are mammals, both have evolved a streamlined, spindle-shaped body plan that is highly efficient for movement through water. This shared morphology is not the result of a common ancestor possessing these specific traits; rather, it is an adaptive response to the physical demands of swimming in an aquatic environment.21

In biology, this type of functional similarity is known as analogy. Unlike homologous structures—which share a common evolutionary origin—analogous structures like the streamlined bodies of sharks and dolphins represent independent evolutionary solutions to the same functional challenge: achieving speed and efficiency in a fluid medium.31