February 2024

The word evolution is nearly synonymous with change. One species diversifies into many. A disaster triggers a mass extinction among marine life. A microbe becomes resistant to our drugs. Each of these changes, large or small, is a classic example of evolution in action. But what about lack of change? Most of the species we observe around us today look about the same as they did in our grandparents’ time. And the fossil record includes many species that seem hardly to have changed at all for millions of years. How does this conspicuous stability square with evolutionary theory? New research supports one explanation.

Where's the evolution?

The new research focuses on the evolution (or lack thereof) of the four Anolis lizards shown below. Each species has distinct traits that help it thrive on different parts of trees. Over two and a half years, researchers studied these four species on a Florida island, measuring the physical traits of individual lizards and documenting which lived and which died to figure out how those traits affected their survival.

The researchers used their data to construct a landscape of evolutionary fitness – a graph showing which combinations of traits promote or hinder survival and reproduction. In the hypothetical example below, lizards with relatively large bodies and relatively small toepads have the highest fitness – the best odds of surviving and reproducing. But having extremely large bodies and extremely small toepads (marked on the fitness landscape with an asterisk) doesn’t further boost fitness; it lowers it. In this case, we’d expect natural selection to cause a population of lizards to evolve the body and toepad sizes that match the position of the red peak; this is where survival and reproduction are optimized.

The hypothetical example on the left shows a fitness landscape with a bulge or hill that represents stabilizing selection – a form of natural selection in which selection acts against traits that are too extreme in either direction (e.g., too big and too little, too heavy and too light). It’s easy to imagine traits that might experience stabilizing selection. For example, birth weight in humans: very light babies might face challenges after birth and very heavy babies might lead to risky deliveries. If both very heavy and very light babies have lower chances of survival, stabilizing selection will keep birth weight at a happy medium. Stabilizing selection, which pushes species towards an optimum middle ground, is one potential explanation for why some species stay the same over time.

The scientists behind the new research wanted to know if the four distinct Anolis species were kept that way by stabilizing selection. Over the course of the study, the patterns of survival and death among the lizards produced the fitness landscape shown below. The four peaks mean that there are four different combinations of traits that give the best odds of survival.  And it turned out that each species matched up with one of those peaks. For example, a typical crown specialist lizard has traits that match up with the leftmost peak below.

The four bulges in the fitness landscape are hallmarks of stabilizing selection. Individuals that happen to have traits too far off their fitness peak are unlikely to survive and contribute their gene versions to the next generation, keeping each species relatively the same over time.

However, when the researchers looked at shorter time periods within the two-and-a-half-year study, they saw a different pattern. While the overall pattern resembled stabilizing selection, in any single season, stabilizing selection was rare. Instead, the lizards were likely to experience natural selection pushing them towards one extreme (e.g., towards overall bigger toepads or overall smaller bodies). But because the direction of this selection changed from one season to the next (e.g., big toepads might be favored in winter and small toepads in summer), the overall result was evolutionary stability.

The new research could help explain a paradox in evolutionary research: the fossil record contains many examples of long-term stability, but studies of the modern species around us often document rapid evolutionary change in one direction (e.g., towards antibiotic, pesticide, and herbicide resistance, away from large body sizes, towards tusklessness) and rarely find evidence of stabilizing selection. Perhaps the explanation for the macroevolutionary pattern of stability is not stabilizing selection, but instead quick shifts in the direction and nature of microevolutionary processes like natural selection that wind up cancelling each other out over geologic timescales. Perhaps when it comes to evolution, the more things change in the short term, the more they stay the same in the long.