Crossing the valley
Pupfish shed light on how new species evolve when the steps to get there reduce survival.
One of the main drivers of evolution is natural selection, which is when organisms better adapted to their environment are more likely to survive and reproduce. A common metaphor to explain this process is a landscape covered in peaks and valleys: the peaks represent genetic combinations or traits with high evolutionary fitness, while the valleys represent those with low fitness.
As a population evolves and its environment changes, it moves among these peaks taking small steps across the landscape. However, there is a limit to how far an organism can travel in one leap. So, what happens when they need to cross a valley of low fitness to get to the next peak? To address this question, Patton et al. studied three young species of pupfish that recently evolved from a common ancestor and co-habit the same environment in the Caribbean.
Patton et al. sequenced whole genomes of each new species and used this to build a genotypic fitness landscape, a network linking neighboring genotypes which each have a unique fitness value that was measured during field experiments. This revealed that most of the paths connecting the different species passed through valleys of low fitness. But there were rare, narrow ridges connecting each species.
Next, Patton et al. found that new mutations as well as genetic variations that arose from mating with pupfish on other Caribbean islands altered genetic interactions and changed the shape of the fitness landscape. Ultimately, this significantly increased the accessibility of fitness peaks by both adding more ridges and decreasing the lengths of paths, expanding the realm of possible evolutionary outcomes.
Understanding how fitness landscapes change during evolution could help to explain where new species come from. Other researchers could apply the same approach to estimate the genotypic fitness landscapes of other species, from bacteria to vertebrates. These networks could be used to visualize the complex fitness landscape that connects all lifeforms on Earth.
Originally published at https://elifesciences.org/digests/72905.
