Genetic Researchers Reconstruct Evolutionary
In a new study, an international team of scientists used 22 newly-sequenced genomes from 18 extant species of penguins to reconstruct the history of their diversification and adaptation. The authors found that ancient penguins diverged in the early Miocene epoch in Australia and New Zealand and identified the emperor and king penguins, the two largest species, as the sister group to all other extant penguins. Their results appear in the Proceedings of the National Academy of Sciences.
Penguins, the only extant family of flightless diving birds, currently comprise at least 18 species, distributed from polar to tropical environments in the southern hemisphere.
The history of their diversification and adaptation to these diverse environments remains controversial.
“We are able to show how penguins have been able to diversify to occupy the incredibly different thermal environments they live in today, going from 9 degrees Celsius (48 degrees Fahrenheit) in the waters around Australia and New Zealand, down to negative temperatures in Antarctica and up to 26 degrees (79 degrees Fahrenheit) in the Galápagos Islands,” said senior author Professor Rauri Bowie, a researcher at the University of California, Berkeley and curator in the Museum of Vertebrate Zoology at Berkeley.
“But we want to make the point that it has taken millions of years for penguins to be able to occupy such diverse habitats, and at the rate that oceans are warming, penguins are not going to be able to adapt fast enough to keep up with changing climate.”
In the study, Professor Bowie and colleagues gathered blood and tissue samples from 22 penguins representing 18 species and then sequenced and analyzed their whole genomes.
The researchers found that penguins arose in the cool coastal regions of Australia and New Zealand, not frigid Antarctica, as many scientists thought, and they pinpoint the origin of penguins at about 22 million years ago.
The genetic evidence indicates that the ancestors of the king and emperor penguins soon split off from the other penguins and moved to sub-Antarctic and Antarctic waters, respectively, presumably to take advantage of abundant food resources.
This scenario is consistent with the contested hypothesis that these two species are the sister group to all other penguin lineages.
“It was very satisfying to be able to resolve the phylogeny, which has been debated for a long time,” Professor Bowie said.
The other penguins diversified and spread widely across the southern oceans, after the Drake’s Passage between Antarctica and the southern tip of South America fully opened about 12 million years ago.
The opening revved up the clockwise-moving Antarctic Circumpolar Current allowed these flightless birds to swim with the ocean currents throughout the southern ocean, populating both the cold sub-Antarctic islands and the warmer coastal areas of South America and Africa, where they populated to coastlines and remote islands with cold, upwelling, nutrient-rich water.
The scientists also determined that several groups of penguins have interbred over the course of their evolutionary history.
Through exchange of genetic material, penguins may have shared genetic traits that facilitated the diversification of penguins across the steep thermal and salinity gradients encountered in the southern oceans.
The most hybridized are the rockhopper penguins and their close relatives, which experienced at least four introgressions over the course of millions of years.
The authors also pinpointed genetic adaptations that allowed penguins to thrive in new and challenging environments, including changes in genes responsible for regulating body temperature, which allowed them to adapt to subzero Antarctic temperatures, as well as tropical temperatures near the equator; oxygen consumption that permitted deeper dives; and osmoregulation, so they could survive on seawater without the need to find fresh water.
New analytical tools helped the team to infer the sizes of ancient penguin populations going back about 1 million years.
Most penguin species increased to their greatest numbers as the world cooled 40,000 to 70,000 years ago during the last glaciation — many species prefer to breed on snow and ice — and some had a bump in population during the previous glaciation period 140,000 years ago.
Two species — the gentoo and the Galápagos — seem to have been declining in populations for at least the past 1 million years.
“We saw, over millions of years, that the diversification of penguins decreased with increasing temperature, but that was over a longtime scale,” said first author Dr. Juliana Vianna, a scientist at the Pontifical Catholic University of Chile.
“Right now, changes in the climate and environment are going too fast for some species to respond to the climate change.”
Juliana A. Vianna et al. Genome-wide analyses reveal drivers of penguin diversification. PNAS, published online August 17, 2020; doi: 10.1073/pnas.2006659117