While natural selection is a big part of evolution, the theory now embraces much more than that. One of the big concepts that explains a lot of the pattern of evolution throughout history is called “adaptive radiation.” Adaptive radiation is a process in which environmental changes create new resources, challenges, and environmental niches, enabling rapid diversification of organisms from a single ancestral species.
Adaptive radiation provides a sound explanation that captures the effects of the interactions among organisms on species diversification. However, non-biological effects—the details of how environmental changes interact with species—are not easy to incorporate into this model and have not been extensively explored.
In a recent investigation published in PNAS, a team of scientists developed a method to test how non-biological variables influence the rates of trait evolution within a group of related species. This method was based on a framework that compares evolutionary trajectories, which the scientists validated through intensive simulations.
This model was used to probe the influence of past climate changes on morphological evolution and was compared to classical models of trait evolution. In particular, the researchers focused their investigation on the role of global average temperature over the Cenozoic era, which began roughly 66 million years ago with the demise of the dinosaurs and continues through the present day.
Looking at birds and mammals
The scientific community has found that temperature impacts biological processes at all levels. It directly influences things like metabolic rates, body size, and productivity. However, there’s still debate about how temperature influences trait evolution. To get at this question, the scientists used their method to analyze body-size data for almost all existing birds and mammals (6,110 extant birds and 3,355 extant mammal species).
This analysis revealed that past climate is a primary driver influencing the rate at which bird and mammal body sizes have evolved. Though many in the scientific community might have expected that warmer climates lead to faster rates of evolution, this work suggests that the opposite is true: evolutionary rates are higher during times of global cooling in most groups. This trend is a general feature of the evolution of warm-blooded (endothermic) animals. And it appears to be independent of the ecological or evolutionary history of a species.
This finding was unexpected, so the researchers behind the PNAS paper gave it some thought. They hypothesized that temperature does not influence evolutionary rates through alteration of energy availability or organismal metabolism, which are responsive to selective pressures. Instead, they suggest temperature changes result in modification of the selective pressures that drive evolution.
We already had good reasons to expect that body size would respond to climate, so on some level these results were expected. But finding that body size responds better to cooling is a surprise, and it could be critical to consider in the present-day environment, where rapid climate change is underway.