Fish, the most biodiverse vertebrates in the animal kingdom, present evolutionary biologists a conundrum: The greatest species richness is found in the world’s tropical waters, yet the fish groups that generate new species most rapidly inhabit colder climates at higher latitudes. A new study helps to explain this paradox. The researchers discovered that the ability of fish in temperate and polar ecosystems to transition back and forth from shallow to deep water triggers species diversification. Their findings suggest that as climate change warms the oceans at higher latitudes, it will impede the evolution of fish species.Fish, the most biodiverse vertebrates in the animal kingdom, present evolutionary biologists a conundrum: The greatest species richness is found in the world’s tropical waters, yet the fish groups that generate new species most rapidly inhabit colder climates at higher latitudes. A new study helps to explain this paradox. The researchers discovered that the ability of fish in temperate and polar ecosystems to transition back and forth from shallow to deep water triggers species diversification. Their findings suggest that as climate change warms the oceans at higher latitudes, it will impede the evolution of fish species.