Professor Mark Pagel | Evolution’s Gradual Path: New Model Reveals How Mammals Changed Over Time

A groundbreaking statistical model developed by Professor Mark Pagel and colleagues at the University of Reading demonstrates that even dramatic evolutionary changes in mammals can be explained through gradual Darwinian processes, resolving a long-standing puzzle in evolutionary biology and challenging our understanding of how species change over time.

The Great Evolution Debate

For over 150 years, scientists have grappled with an apparent contradiction in evolutionary theory that has sparked heated debates and led to competing explanations for how species change over time. Charles Darwin proposed that species evolve through a series of small, incremental changes in a process referred to as gradualism. According to this theory, the remarkable diversity of life arose through countless tiny modifications accumulating over millions of years, driven by natural selection.

However, researchers examining the fossil record or comparing modern species often find evidence of seemingly abrupt changes in animals’ features or long periods where species appear to remain unchanged. These observations have led some scientists to propose that evolution might sometimes proceed through sudden leaps or ‘quantum jumps’ rather than gradual changes. Others have suggested that evolution might be constrained by various factors, leading to long periods of stasis interrupted by brief bursts of change.

This apparent conflict between Darwin’s theory and real-world observations has been particularly challenging in macroevolution (the study of how species change over long periods and how new species arise). While scientists have developed various mathematical models to explain these patterns, each model has typically focused on just one aspect of evolutionary change, potentially missing the bigger picture.

A Revolutionary New Approach

A team led by Professor Mark Pagel at the University of Reading in the UK has developed an innovative statistical model that shows how even dramatic evolutionary changes can be explained through conventional Darwinian processes. Their work represents a significant advance in our understanding of how evolution operates over long time scales.

Professor Pagel and his colleagues created what they call the ‘Fabric’ model – a sophisticated statistical tool that separates two distinct types of evolutionary change. The first type involves directional changes that shift a species’ characteristics in a particular direction, such as becoming larger or smaller. The second type involves changes in ‘evolvability’ – how easily a group of related species can explore different possible forms.

‘Previous models failed to distinguish between these two kinds of change, attributing both to changes in evolvability. Our approach allows us to separate the directional from evolvability changes, and this allows us to see that the sort of incremental and cumulative processes Darwin proposed can explain what appear to be large, even dramatic events of directional change – no change to the underlying evolvability is needed’, Professor Pagel explains.

The Power of Big Data

To test their model, the team examined an enormous dataset comprising body size measurements from 2,859 mammal species, representing virtually every major group of mammals alive today. This included everything from tiny shrews weighing just a few grams to enormous whales weighing over 100 tonnes. By mapping these measurements onto a detailed family tree of mammalian evolution, the researchers could trace changes over approximately 172 million years.

The team’s analysis revealed a complex pattern of evolutionary change throughout mammalian history. They identified 417 instances of directional change – cases where body size consistently increased or decreased along particular branches of the evolutionary tree. They also found 119 cases of altered evolvability, where certain groups of related species became more or less able to explore different body sizes.

Breaking Down Dramatic Changes

One of the most significant findings from the study challenges the idea that dramatic evolutionary changes require special explanations beyond normal Darwinian processes. Professor Pagel’s team showed that even the most extreme changes they observed could be explained by ordinary genetic variation accumulating through natural selection.

The most dramatic example they found was in the evolution of baleen whales. These massive creatures evolved from primitive dolphin-like ancestors, becoming nearly 100 times larger over just 7.6 million years. While this might seem like an impossibly rapid change, the researchers demonstrated that it could be achieved through the same processes that drive smaller evolutionary changes.

The team calculated that the amount of genetic variation needed to produce these changes would have been well within the range that an evolving population could produce over that amount of time. This suggests that no special mechanisms are needed to explain even the most dramatic observable evolutionary changes.

Understanding Evolvability

The team’s analysis of evolvability revealed several surprising patterns. Throughout mammalian evolution, there were many more instances of increased evolvability than decreased evolvability – by a ratio of about eight to one. These ‘watershed moments’ of increased evolutionary potential often occurred in small groups of closely related species.

Professor Pagel explains that increased evolvability might arise for several reasons. It could result from genetic changes that allow for greater variation in certain characteristics or reflect ecological opportunities that enable species to explore a wider range of forms. For example, when mammals first began to evolve after the extinction of the dinosaurs, many new ecological niches became available, potentially increasing the evolvability of various mammal groups.

The team also found that changes in evolvability were rarely linked to directional changes. This suggests that these two types of evolutionary change operate independently, with each playing its own role in shaping the diversity of life we see today. A species might experience a directional change without any change in its evolvability, or its evolvability might change without any immediate directional shift in its characteristics.

Tracking the Pace of Evolution

Another important finding concerned the timing of evolutionary changes. The researchers found that both directional changes and changes in evolvability occurred at a relatively steady pace throughout mammalian evolution when considered in relation to the number of species present at any given time. This finding challenges the idea that evolution proceeds through distinct epochs or that certain periods in Earth’s history were particularly conducive to evolutionary change. Instead, it suggests that the opportunity for evolution is more closely tied to the number of species present than to any particular period.

The team also found that the magnitude of directional changes was only weakly related to the length of time over which they occurred. This suggests that evolutionary changes might often happen in relatively short bursts rather than at a steady pace, even though the overall process follows Darwinian principles.

Implications for Understanding Evolution

The research has several important implications for our understanding of evolution. First, it shows that we do not need to invoke special mechanisms to explain even the most dramatic evolutionary changes we observe. The ordinary processes of genetic variation and natural selection, operating over sufficient time, can produce remarkable transformations.

Second, the prevalence of increased evolvability suggests that evolution tends to maintain or enhance the potential for future change rather than gradually becoming more constrained. This contradicts the idea that species inevitably become more specialised and less able to evolve over time.

Finally, the independence of directional changes and changes in evolvability suggests that evolution operates through multiple semi-independent processes. Understanding how these processes interact is crucial for predicting how species might respond to environmental changes in the future.

 Applying the Model to Other Organisms and Traits

Professor Pagel and his colleagues are now working to apply their model to other groups of organisms and other characteristics beyond body size. They want to know whether the patterns they found in mammals are common across all life, or whether different groups of organisms follow different evolutionary rules.

The team is also interested in understanding the genetic and developmental mechanisms that underlie changes in evolvability. This could help explain why some groups of organisms seem more evolvable than others and might have practical applications in fields like agricultural breeding and conservation biology.

Understanding these evolutionary processes becomes increasingly important as we face unprecedented environmental changes in the modern world. The team’s work provides new tools for predicting how species might respond to environmental challenges and what factors might limit or enhance their ability to adapt.

Bridging the Gap Between Theory and the Natural World

Perhaps most importantly, this work helps bridge the gap between Darwin’s original theory of gradual change and the apparently abrupt changes we often observe in nature. It shows that seemingly dramatic evolutionary transitions can arise through the accumulation of many smaller changes, each within the range of normal variation.

Professor Pagel’s team has demonstrated that Darwin’s fundamental insights about the gradual nature of evolution were correct, even though the process might sometimes appear to proceed in jumps and starts when viewed over long periods. Their work provides a new framework for understanding how the remarkable diversity of life on Earth has evolved and continues to evolve today.

As we face global challenges like climate change and biodiversity loss, this deeper understanding of evolutionary processes becomes increasingly vital. It may help us predict how species will respond to environmental changes and guide our efforts to preserve the extraordinary diversity of life on our planet.