Answer:
the mecanism is the selection natural
Step-by-step explanation:
The mechanism of natural selection is based on the fact that the individual that survives is the one that is considered the most suitable for the environment, the adaptations of the most suitable species are through genetic mutations that generate new phenotypic expressions so as not to become extinct. In this case, the mutation that allowed the fish to breathe air and be able to transmit it to that gene through its parents is a genetic acquisition that expresses a favorable phenotype.