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Study links speciation to changes in body size in fish

An analysis of nearly 8,000 species of fish found that the speed at which new species emerge is strongly correlated with the speed at which changes in body size evolve.

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This evolutionary tree shows the relationships between nearly 8,000 living species of fish. Red branches denote groups with fast rates of body size change, blue branches indicate slow body size change, and the length of each branch reflects the speed at which groups split into new species.

Illustration courtesy of Dan Rabosky

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Biologists have long observed that groups of organisms with lots of species tend to be more anatomically diverse than groups with only a few species.

For instance, there are just seven species of garfishes, with the smallest, the spotted gar, growing to about two feet as an adult and the largest, the alligator gar, measuring about five times longer.

By contrast, the family that includes salmon, trout, and whitefishes comprises more than 200 species, with the smallest being just five inches long and the largest measuring about 6-1/2 feet, or 15 times longer.

What’s more, groups that have few species tend to remain anatomically stable over time. In “The Origin of Species,” Charles Darwin memorably described the gar, along with lungfishes and platypuses, as examples of “living fossils.”

Evidence for this apparent link between speciation and morphological change has remained mostly anecdotal, however, until now. A study of almost 8,000 species of ray-finned fishes confirms biologists' suspicions: The faster a fish’s lineage splits into multiple species, the faster those species' physical traits diverge.

"The fastest speciating fish typically also had the fastest rate of size evolution," said UCLA biologist Michael Alfaro, one of the study's two lead authors, in a press release. "It didn’t seem to matter whether they were freshwater or marine fish, or lived in cold or warm environments – the correlation was amazingly consistent.”

The study, published this week in the journal Nature Communications, looked at DNA sequence data and size variations of 7,864 species of ray-finned fishes, a class that represents almost all fishes and about half of all vertebrate species. The data set was so large that the researchers had to develop new software just to analyze it.

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