Flexi Says: The evolution of species is not a straight line where one type of animal evolves into the next and the original disappears. Rather, all living organisms continuously evolve at the population level. This leads to the diversification of species to fill new or changing environments and roles (niches). In the case of fish and amphibians, certain populations of fish evolved into new species that had traits of early amphibians. The formation of a new species happens when a population of an existing species becomes isolated from other members of the species. For example, if a population of fish became trapped in an area where the water dried up seasonally, that population may evolve traits to help them survive in air. However, all of the other populations of fish living in deeper water would continue to evolve adaptations to their own environments. Thus, there are many thousands of fish species adapted to different water environments. Likewise, when the earliest ancestors of modern amphibians left the water, they moved into new environments on land and many new species evolved.
