When did whales and dolphins become adapted to life underwater?

How cetaceans evolved key genetic adaptations for their aquatic lifestyle after they separated from their close relatives, the hippos.

Whales and dolphins make up the cetaceans, a special group of mammals adapted to life underwater. Their transition from dry land to an aquatic environment is one of the greatest examples of evolutionary adaptation.

Georgia Tsagkogeorga and her colleagues at Queen Mary University of London were interested in how cetaceans evolved in comparison with their closest relatives. In particular, they wanted to know if the molecular adaptations related to aquatic lifestyle appeared before or after whales and dolphins split from the hippos.

These analyses would have taken months to complete without access to parallel computing.
Georgia Tsagkogeorga

Tsagkogeorga used GridPP’s computing resources to run large-scale molecular evolution analyses. Specifically, “we looked for evidence of Darwinian selection in over 11,000 genes across five lineages of aquatic and semi-aquatic mammals,” Tsagkogeorga explains.

To test for selection, the team ran a series of probabilistic analyses using codon models of evolution. For that, they needed to fit two models onto the data: one representing the null neutral evolution hypothesis and a second model assuming selective pressures acting on the gene. “We compared the likelihoods of the null vs. alternative models to infer the selective regime of the gene in the lineage of interest,” Tsagkogeorga adds.

In total, they performed about 110,000 computations. High-Throughput Computing gave a helping hand. “These analyses would have taken months to complete without access to parallel computing,” says Tsagkogeorga. “Our study benefited greatly from the 3,000+ CPU cores of QMUL GridPP cluster, by speeding up the analyses to only a few weeks or even days.”

The conclusions, published in Royal Society Open Science, show that the most significant molecular adaptations to aquatic life, for example the evolution of underwater sensory perception or their ability to dive, appeared in cetaceans after the split with the hippos, 55 million years ago.

Reference

Tsagkogeorga et al. 2015 A phylogenomic analysis of the role and timing of molecular adaptation in the aquatic transition of cetartiodactyl mammals. Royal Society Open Science doi: 10.1098/rsos.150156

(Full Text, Open Source)