Image credit: Martina Pepiciello

A collaborative study from the University of Sussex and the Max Planck Institute for Evolutionary Biology shows that the age of a gene determines how fast they adapt, demonstrating how gene evolution occurs as an “adaptive walk” through time.

New species emerge and evolve because individuals accumulate mutations in their genomes, some of which have no effect. Others lead to changes that give their bearers striking competitive advantages.

In 1932, Sewall Wright introduced a metaphor that inspired decades of theoretical and experimental research in evolutionary biology, aiming at characterising the process of adaptation. Wright described the "fitness landscape" model that depicts evolving populations as "walking" toward a fitness peak, just like a hiker slowly making its way to the top of the mountain. In 1998, Orr demonstrated that this “adaptive walk” follows a simple rule of diminishing returns, where the further away a population is from its fitness peak, the larger the steps it makes.

One prediction of this theory is that recently evolved, or "young", genes tend to accumulate more adaptive mutations with larger effects than older genes because they are further away from their fitness peak. It was this hypothesis that Professor Adam Eyre-Walker and Dr Ana Filipa Moutinho in the School of Life Sciences, together with Dr Julien Dutheil from the Max Planck Institute in Evolutionary Biology, set out to test.

Testing this hypothesis proved to be quite challenging. The historical record of the mutations accumulated in a gene is usually unavailable, and their fitness effects are largely unknown. Furthermore, other properties of genes, such as their length, can act as confounding factors to the effect of gene age. Thus, the authors proposed a new approach to test the adaptive walk model of gene evolution.

Dr Ana Filipa Moutinho, a Research Fellow in the Department of Evolution, Behaviour and Environment, said: “First, we used population genetic models that can assess the variation of the fitness effect of mutations. We accomplished this by comparing the genomes of several individuals in a population and measuring the rate of adaptive evolution in different categories of genes.

“We then took advantage of the fact that all genes within a genome do not have the same age: some are young, shared by very few closely related species, while others are more ancient, shared by species that separated millions of years ago. Finally, we used the distribution of mutations among genes of different ages to understand how adaptive mutations spread across time.”

Using two distinct species, the fruit fly Drosophila melanogaster and the small flowering plant Arabidopsis thaliana, this study showed that a gene's age significantly impacts the rate of molecular adaptation and that mutations in young genes tend to have larger effects. These results provide the first strong empirical evidence that molecular evolution follows an adaptive walk model over a deep evolutionary timescale and adds a new layer of evidence to the fitness landscape theory proposed almost 100 years ago.

Strong evidence for the adaptive walk model of gene evolution in Drosophila and Arabidopsis’ is published in PLOS Biology.

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