The strategy of Noah's Ark will fail. In the most acute sense, this is the conclusion of an unprecedented study that illuminates which marine species may have the ability to survive in a world where temperatures are rising and the oceans are becoming acidic.
Remainders of two by two, or even moderate in size, may have little chance of persisting on a planet that changes the climate. Instead, for many species, "we will need large populations," says Melissa Pespeni, a University of Vermont biologist who led the new research examining how hundreds of thousands of sea urchin larvae have responded to experiments in which their water of the sea was moderate or extremely acidic.
The study was published on June 11, 2019, in the Proceedings of the Royal Society B.
Pespeni and his team were surprised to discover that a rare variation in the DNA of a small minority of hedgehogs was highly useful for survival. These rare gene variants are "a bit like having a winter coat among fifty lightweight jackets when the weather hits twenty down in Vermont," says Pespeni. "It's that coat that allows you to survive." When water conditions became extremely acidic, these rare variants increased in frequency in the larvae. These are the genes that allow the next generation of hedgehogs to alter the functioning of various proteins – such as the ones they use to make their hard shells, but easily dissolved, and to manage the acidity in their cells.
But keeping these rare variants in the population – in addition to other necessary genetic variations that are more common and allowing the response to a range of acid levels in the water – requires many individuals.
"The larger the population, the more variation you'll have," says Reid Brennan, a postdoctoral researcher at Pespeni's UVM lab and lead author of the new study. "If we reduce population size, then we will have less fodder for evolution – and less chance of having rare genetic variation that can be beneficial."
In other words, some organisms may persist in a climate-altered world because they are able to change their physiology – think of sweating more; some may migrate, perhaps further north or up. But for many others, their only hope is to evolve – rescued by the potential for change they are expecting in rare stretches of DNA.
The purple sea urchins that the UVM team studied in their Vermont lab are part of the natural populations that stretch from Baja, California, to Alaska. Found in rocky reefs and seaweed forests, these thorny creatures are a favorite snack of sea otters – and a key species in shaping life in inter-tidal and sub-tidal zones. Because of their large numbers, geographic reach and the changing conditions in which they live, hedgehogs have high "permanent genetic variation," the scientists note. This makes purple urchins probably survive in the difficult future of an acidified ocean – and good candidates to understand how sea creatures can adapt to rapidly changing conditions.
It is well understood that the rise in global average temperature is a key factor for the imminent extinction faced by a million or more species – as recently noted by a UN biodiversity report. But it's not just the rising averages that matter. It can be the hottest – or more acidic – moments that test the limits of an organism and control its survival. And, as the UVM team writes, "the genetic mechanisms that allow rapid adaptation to extreme conditions are rarely exploited."
CURRENCY IN THE CURRENT SEA
The new study used an innovative "single generation selection" experiment that began with twenty-five adult sea urchins caught. Each female produced about 200,000 eggs, from which scientists were able to extract DNA from pools of about 20,000 surviving larvae living in different water conditions. This very large number of scientists gave scientists a clear view that purple hedgehogs have a genetic inheritance that allows them to adapt to extremely acidic ocean water. "This species of sea urchin will look good in the short term. They can respond to these low pH conditions and have the genetic variation needed to evolve," says Reid Brennan of UVM. "As long as we do our part to protect their habitats and keep their populations large."
But overcoming the fierce challenge of rapid climate change can be costly. "We expect evolution to happen – and it is surprising and exciting that these rare variants play such a powerful role," says Melissa Pespeni, an assistant professor in the biology department at UVM and a specialist in ocean ecosystems. "This discovery has important implications for the long-term persistence of species. These rare variants are a kind of coin hedgehogs have to spend," she says. "But they can only spend once."
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