Study of marine snails helps clarify debate over how to visualize major evolutionary transitions

Significant evolutionary changes occur gradually rather than in dramatic “monstrous” steps, biologists have discovered, answering the long-debated question of how revolutionary innovations such as flight, vision and living offspring came to be.

Evolution is generally a gradual process, occurring in small, incremental steps, but sometimes producing amazing new functions, like the feathers that ultimately allow birds to fly.

Until now, it has been difficult to understand how these important evolutionary changes occurred, partly because many of them occurred so long ago and partly because it is difficult to imagine intermediate stages. Some have suggested that they occur in large stages, when mutations of great effect give rise to “hopeful monsters”; others have argued that innovations are built gradually, with natural selection favoring intermediate stages.

By obtaining and studying whole genome sequences from a group of marine snails, which recently transitioned from egg-laying to live birth, scientists from the University of Sheffield and their collaborators from the University of Gothenburg and The Institute of Science and Technology Austria is now able to settle the debate on at least one example.

The study used a new methodology to discover whether this new change in birthing style occurred quickly or gradually, findings that could then be applied to explain other dramatic changes in evolution. The research was published in Science.

Scientists were able to identify 50 genes perfectly associated with the mode of reproduction, as well as estimate the time of their origin. The results showed that they accumulated gradually, spreading at different times in the past. This demonstrates that innovation can evolve incrementally, rather than in a single evolutionary step.

Professor Roger Butlin, from the School of Biosciences at the University of Sheffield, said: “It is important to understand the evolutionary origin of key innovations because they can radically change the course of evolution, such as when reproduction living led to the diversification of mammals or that feathers helped. birds to evolve in flight, but until now there have been few opportunities to study them, mainly because most evolutionary changes occurred so long ago.

“By discovering and studying the recent evolutionary change in the way marine snails give birth, we are now able to understand these major changes and apply our methods to many other evolutionary changes.”

He added: “Our results will change the way biologists view major evolutionary transitions, shifting the focus from large evolutionary steps to understanding the incremental benefits of small evolutionary steps.” They will also help others dissect the genetic and historical bases of other adaptive traits. , which is important when many organisms are forced to adapt quickly to a changing world. »

The team now plans to study the functions of the genes they identified, in order to understand the series of evolutionary steps that led to live birth. They also hope their methods will be applied to other types of adaptation, including things like thermal tolerance, which must evolve if some species are to survive climate change.