Biologists have discovered that significant evolutionary changes happen gradually instead of in dramatic ‘monster’ steps.
This discovery helps to answer long-debated questions about how evolutionary changes such as flight, vision, and the bearing of live offspring came about.
Evolution is usually gradual, taking place over small, incremental steps. However, it occasionally produces striking new functions, like feathers, that eventually allow birds to fly.
Until now, it has been difficult to understand how these significant evolutionary changes have happened, partly because many of them took place so long ago and partly because it is hard to imagine intermediate stages.
Some have suggested that evolutionary changes occur in big steps when large-effect mutations give rise to ‘hopeful monsters’. Others have argued that innovations are built gradually, with natural selection favouring intermediate steps.
The study, ‘The genetic basis of a recent transition to live-bearing in marine snails,’ is detailed in Science.
Settling the debate about evolutionary change
By obtaining and studying whole-genome sequences from a group of marine snails, which have made a recent shift from egg-laying to live birth, scientists at the University of Sheffield and their collaborators at the University of Gothenburg and Institute of Science and Technology Austria have settled the debate for at least one example.
The study used new methodology to discover whether this shift in birthing style happened rapidly or gradually, findings which could then be applied to help explain other dramatic shifts in evolutionary change.
Professor Roger Butlin, from the University of Sheffield’s School of Biosciences, said: “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.
“Until now, however, there have been few opportunities to study these, mainly because most evolutionary changes happened so long ago.
“By discovering and studying the recent evolutionary shift in how marine snails give birth, we’re now able to understand these major changes and apply our methods to many other evolutionary changes.”
How will the study aid future research?
“Our results will change how biologists view major evolutionary transitions, shifting the focus away from big leaps in evolution towards understanding the progressive benefits of small evolutionary changes,” explained Professor 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.”
The team plans to study the functions of the genes they have identified to understand the evolutionary steps that led to live birth.
They also hope that 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.
