Origin & Evolution of Life

Biologists Uncover The Secrets Of Evolutionary Change

By Keith Cowing
Press Release
University of Sheffield
January 5, 2024
Filed under ,
Biologists Uncover The Secrets Of Evolutionary Change
Topology weighting reveals genomic regions associated with reproductive mode. (A) For each genomic window, we inferred a tree for all haplotypes and then classified 10,000 “subtrees” by randomly picking one haplotype per group. Topology weights are the proportions of each topology among all subtrees. Windows were plotted in a ternary plot based on the weights. (B) Simulated distributions of weights. A greater opportunity for lineage sorting (i to iii) biases the distribution toward the topology that matches the demographic history. Incomplete lineage sorting yields genealogies that are a better fit to one of the discordant trees, but the distribution is always symmetrical between the left and right half-triangles. Additional factors, including gene flow, create a bias toward one discordant genealogy (iv to vi). (C) Possible topologies and the empirical distribution of weights for the 154,971 100-SNP windows (“C,” compressa; “A,” arcana; “I” Iberian saxatilis; “N,” northern saxatilis). Hexagonal bins are colored by window count. (D) Counts of windows in the left and right half-triangles, with asymmetry quantified using DLR. Further division into subtriangles reveals left-right asymmetry throughout the distribution. Asterisks indicate significant asymmetry between corresponding left- and right-sided subtriangles. (E) Distributions of weights >0.7. — Science

Significant evolutionary changes happen gradually as opposed to in dramatic ‘monster’ steps, biologists have discovered, answering the long-debated question as to how game-changing innovations like flight, vision, and the bearing of live offspring came to be.

Evolution is usually a gradual process, taking place over small, incremental steps, but occasionally producing striking new functions, like feathers that eventually allowed 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 they 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.

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, are now able to settle the debate for at least one example.

The study used new methodology to discover whether this new shift in birthing style happened rapidly or gradually, findings which could then be applied to help explain other dramatic shifts in evolution.

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

Littorina snails are common on the rocky shores of Europe, the UK, and the East Coast of the USA CREDIT Daria Shipilina

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 the way marine snails give birth, we’re now able to understand these major changes and apply our methods to many other evolutionary shifts.”

He added: “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. 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 now plan to study the functions of the genes they have identified, in order to understand the series of 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.

The genetic basis of a recent transition to live-bearing in marine snails, Science


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