Tropical birds evolved slowly

Scientists have uncovered a surprising paradox in species evolution: birds in tropical hotspots evolved much more slowly than those in environments with low species diversity.

That means birds in tropical regions such as the Amazon, Central America and the eastern slopes of the Andes Mountains are less likely to form new species than their sparser counterparts in cold, dry environments such as deserts and mountaintops.

The exhaustive study was published in the journal Science by an international team led by Michael Harvey from the University of Texas at El Paso, US.

It suggests “species-rich areas are coldspots rather than hotspots of speciation”, writes Hélène Morlon, from PSL Research University, France, in a related Perspective, “and that the diversity of these areas is explained by their old age and/or low extinction rates”.

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Great Xenops (Megaxenops parnaguae) Bahia,
Brazil. Credit: Ciro Albano

The analysis, she says, “forces reconsideration of traditional evolutionary hypotheses for explaining richness gradients”, a long-standing focus of discussion.

The research derives from four decades of expeditions by field biologists documenting tropical diversity thanks to passionate birders and ornithologists; these have been poorly studied until now because the animal groups are vast and well-sampled studies have been lacking.

Their efforts focused on suboscine birds – passerines, or songbirds – which include more than 1300 species. These represent about a third of bird species in the US tropics and have widely varying characteristics and habitats.

Once the samples reached a critical mass, Harvey and colleagues collected the genetic data needed to examine the birds’ evolutionary history of species diversification.

They sequenced 2400 genes from tissue samples held by natural history museums of virtually all species to determine how each was related to the others and build a phylogenetic tree.

This enabled them to measure the rates that each branch produced new species over time and, combined with knowledge about geography, species distributions and climate, explore factors that might help explain why the birds evolved.

Slender Antbird (Rhopornis ardesiacus) Bahia,
Brazil. Credit: Ciro Albano

The results add to growing evidence in fish, flowering plants and other birds that new species may not evolve prolifically in biodiversity hotspots, says Morlon, raising the question whether they’ve always been “coldspots of speciation” or became that way as diversity grew.

This might occur if environmental conditions, such as resource availability, put a cap on new species evolution, she suggests.

Either way, the discovery underscores the importance of preserving areas with rich biodiversity, Harvey notes. “Species in the hotspots took a long time to accumulate, so we need to protect them because they will be hard to replace.”

The disparities also highlight the need to focus conservation efforts on areas that disproportionately develop new species more rapidly, he adds.

“We hope our results will inspire people to think about the evolutionary processes we need to produce and maintain healthy, diverse environments, and inspire them to protect a range of environments and the different evolutionary dynamics that they support.”

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