Evolution Of Live Birth Helped Fish Diversify

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Some fish lay eggs, others give birth directly to tiny wiggly babies. According to a newNature Communicationsstudy, giving birth to live young increased the diversity of ray-finned fishes like guppies.

How often lineages accumulate new species depends on the balance between speciation and extinction. But diversification rates vary a lot. The coelocanth genus has produced just two species that we know of in the past 80 million years, for example, while cichlids in Lake Victoria have produced as many as 500 species in as few as 15,000 years. Species richness may be influenced by the evolution of novel traits that promote diversification.

Two traits in particular are thought to enable diversification into new habitats: live birth (or viviparity) and one-year life cycles (called annualism). Viviparity originated about 150 times in vertebrates, and possessing an annual life cycle (as opposed to a multi-year life cycle) allows for the colonization of seasonal ponds. Both traits have arisen in the fish order Cyprinodontiformes, which includes about 1,250 ray-finned fish that live mostly in Africa and the Americas. It includes killifish, mollies, and guppies. About 27 percent of the order are viviparous, and 25 percent are annuals.

Each of the two traits arose independently five times in Cyprinodontiformes, but only viviparity played a role in shaping patterns of diversity. In fact, viviparity triggered a burst of diversification each time it evolved.

The team dont know for sure how viviparity stimulates diversification, but they think it might have to do withincreased colonization rates. Viviparous females carry their fertilized embryos with them, and that means a single, pregnant, viviparous female can colonize a new watershed. More frequent colonization of geographically isolated areas helps facilitate speciation. Not to mention, the young of viviparous species have higher survival rates than the young of egg-laying ones.

Image in the text: The annual killifish Austrolebias charrua, which lives in seasonal ponds in South America. Tom Van Dooren

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