Ichthyosaurs, now extinct, were aquatic reptiles with dorsal fins and tails, similar to those of fish. Their most recent ancestors were terrestrial reptiles that had neither dorsal fins nor aquatic tails. The dorsal fins and tails of ichthyosaurs and fish are ________. homologous structures adaptations to different environments adaptations to a common environment and examples of convergent evolution adaptations to a common environment and homologous structures

Convergent evolution is a process by which two independent and phylogenetically separated organisms, get to develop similar structures or behavior to survive. These structures make possible a better performance in a similar environment. These organisms are morphologically or ethologically similar in their whole bodies or just in parts.

Convergent evolution occurs in different phylogenetic branches independently from each other. It results from the need of different organisms to adapt to a similar environment and to solve a problem from that particular ecological niche. Ecological pressures acting on them might be similar, modeling the organisms´ phenotype. Each evolutive branch ends by developing the same structures, which they both find effective in their environment.

In the exposed example, Ichthyosaurs (reptile) and fishes are separated species on evolutive time, corresponding to different phylogenetic lineages. Both of them inhabit the same aquatic environment. Pressures of the habitat acted on them and modeling their phenotypes. These species need to adapt to living in water, moving, feeding, scaping, chasing, etc. So both species arrived at the same solution. They developed dorsal fins and tails, which are important to stabilize the body in the water while swimming, making it more hydrodynamic and getting to propel better.