When the small ponds or lakes that the fish inhabited started to dry up, these fish had a possible escape from the increasingly inhospitable environment. If they were able to travel a short distance over land, they might be able to find a more favorable environment.
Pressures came from increased competition for dwindling food resources, increased population, and from increased predation as habitat was reduced.
The species with the ability to breath could remove themselves from this environment, while aquatic-bound species could not.
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