Evolution of birthing live young at a snail’s pace

Evolution is a gradual process that can result in game-changing innovations, such as flight, vision, and the bearing of live offspring. But we know surprisingly little about how these innovations happen.

New research on the evolution of live-birth shows significant evolutionary changes happen gradually, instead of in dramatic leaps.

“The evolutionary origin of key innovations is important to understand because they can dramatically change the course of evolution, like when live-bearing led to the diversification of mammals or feathers helped birds to evolve flight,” explains Professor Roger Butlin of the University of Sheffield in the UK, senior author of the new paper in Science. “Until now however, there have been few opportunities to study these, mainly because most evolutionary changes happened so long ago.”

The researchers studied a clade of marine snails (Littorina spp.) in which only one species – L. saxatilis – has made a recent shift from egg-laying to live-birth.

A photo of the embryos that developed inside a live-bearing female snail.
A photo of the embryos that developed inside a live-bearing female. Many different stages of development can be observed from just-fertilized embryos, to fully-hatched miniature snails. Credit: Fredrik Pleijel

“Almost all mammals give [birth to live young], and this function has accompanied their evolution for around 140 million years. Yet, in this study, we can investigate how live bearing evolved completely independently, and much more recently, in marine snails,” says lead author Dr Sean Stankowski, a postdoctoral researcher at the Institute of Science and Technology Austria (ISTA) 

Using whole-genome sequences, the team identified that the switch to live-bearing in L. saxatalis evolved gradually through the accumulation of about 50 genetic mutations over the last 100,000 years.

Their results indicate that the ability to give birth to live young evolved progressively in these snails, rather than in a single evolutionary step.

Photographs of many distinct shapes of shells of littorina snails
Live-bearing has allowed Littorina snails to occupy and adapt to a diverse range of habitats. This has led to the evolution many ‘ecotypes’ that vary in their size, shape and behavior. Credit: Fredrik Pleijel

“Most genetic innovations are in fact really old and tangled up on an evolutionary scale which makes it difficult to study their origin. These snails have allowed us to do exactly that, but we have only begun to scratch the surface of what they can teach us about the origins of novelty,” says Stankowski.

The team now aim to map the function of each mutation to understand how each genetic change shaped the snails’ form and function on the way to live-bearing.

“Our results will change the way biologists view major evolutionary transitions, shifting the focus away from big leaps in evolution towards understanding the progressive benefits of small evolutionary steps,” concludes Butlin.

“They will also help others dissect the genetic and historical basis of other adaptive traits, which is important when many organisms are being forced to adapt rapidly to a changing world.”

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