Scientists Found a Hidden Force That Pushes Life Past the Limits of Evolution

4 min readHere’s what you’ll learn when you read this story:Researchers have found that organisms which reach the peaks of survival in evolutionary “fitness landscapes” still keep evolving, even with no new pressures pushing them.Many different sets of traits can deliver the same peak fitness, a phenomenon called degeneracy, and species can sit at that summit for millions of years.The smallest phenotypic differences can reveal the shape of the landscape a population has traveled, and the direction it’s still drifting. When Darwin famously said “forms most beautiful and most wonderful have been, and are being, evolved” in his On the Origin of Species, many aspects of evolution were still unknown by scientists. Genetics and molecular biology weren’t even scientific fields in 1859. Gregor Mendel was still figuring out the basics of inheritance and wouldn’t publish his findings for another six years. Scientists’ understanding has kept changing ever since—including the long-held assumption that evolutionary change becomes random once selection pressure disappears.One thing, at least, has held as the main consensus for over a century and a half: organisms are more likely to survive and reproduce if they develop specialized advantages suiting their habitat, like traits keeping them from being snapped up by predators. It’s what happens after a species reaches the point of being well-adapted that scientists are now rethinking. Reaching boss level in what some biologists view, not unlike a video game, as a 3D survival “landscape” of peaks and valleys may keep a species from going extinct, but there’s more than one way to get there. While species with different phenotypes—the observable traits produced by the genetic material that makes up an organism’s genotype—all need to adapt to reach that peak, they don’t all take the same route. This is known as degeneracy, the ability of structurally different elements to perform the same function or yield the same result. It’s why different codons can specify a single amino acid, different antibodies can neutralize the same invader, and different sets of traits can deliver identical fitness.When analyzing biological landscapes, biologists Naama Brenner and Razi Fachareldeen of Technion Israel Institute of Technology in Haifa, Israel, noticed that radically different evolutionary trajectories can still lead to the same optimal result. Different species will somehow make it to the same fitness peak despite using different methods to get there. Once a population reached that kind of equal-fitness plateau, the thinking went, any further change would be random. But using a mathematical model, Brenner and Fachareldeen discovered there’s an orderly process at work. Populations keep moving in a consistent direction even when no direction offers a survival advantage. The researchers call it directional drift. Unlike the random genetic drift from textbooks, directional drift is a consistent push in one direction. It happens because the shape of the fitness landscape limits which variations a population can produce, steering it over generations toward the broadest, most level ground, where existing traits are hardest for a mutation to shift.The concept of fitness landscapes was invented by Sewall Wright in the 1930s. It’s a visualization that biologists use to describe fitness in metaphorical terms of peaks and valleys—not literal landscapes. Every direction represents a different trait a population could shift toward, and elevation represents fitness, so a peak is simply a combination of traits that works well. Landscape curvature is how sharply that elevation falls away when a trait shifts a little, and it turns out not to be the same in every direction.“This drift arises from an interaction between population variability and landscape curvature,” the researchers said in a study recently published in PNAS. “Curvature shapes phenotypic variation, which in turn biases evolutionary exploration even when fitness gradients vanish. As a result, evolution exhibits an implicit bias, preferentially selecting flat and robust regions of the degenerate fitness manifolds, without explicit optimization for these properties.”It may seem that degenerate landscapes would not be conducive to further evolution, but change does continue in unexpected ways. Even without fitness obstacles to tackle, selection for the most beneficial mutations still continues, even while potentially harmful mutations are phased out. In other words, there’s still a type of natural selection going on. The direction a population moves in once it reaches a degenerate plateau depends on two things: the variation within that population, and the curvature of the landscape it occupies. Potential variations that would see sharp ascents and descents in the landscape are suppressed by curvature, which allows for other variations in flatter regions. This trajectory that leads towards less curvature protects phenotypes by allowing them to resist being disrupted more effectively.What this means for organisms is that they can continue becoming stronger and more resilient without having to face additional evolutionary pressures. They barely need any effort because this happens like a side effect. While this process might seem like survival entering neutral mode, even the slightest phenotypic variations could reveal what the fitness landscape for a population has looked like over evolutionary time and how that population has navigated it. Mapping traits over generations not only keeps track of traits and how they might further evolve, but reveals how the curvature of the landscape is related to changes in the population as it gravitates towards flatter regions and degeneracy. Phenotypes can still experience variation while the species remains stable. Some organisms might have spent millions of years like this.“Our results provide a complementary geometric and dynamical interpretation of such observations as potentially reflecting gaps in curvature of directions in trait space,” said the researchers. “Further study of natural variability in connection to our model is an important direction for future work.”Elizabeth Rayne is a creature who writes. Her work has appeared in Popular Mechanics, Ars Technica, SYFY WIRE, Space.com, Live Science, Den of Geek, Forbidden Futures and Collective Tales. She lurks right outside New York City with her parrot, Lestat. When not writing, she can be found drawing, playing the piano or shapeshifting.