Evolution occurs faster than previously thought in wild animals

QWhat is the rate of evolution? Adaptive evolution occurs when natural selection causes genetic changes that favor the survival and reproduction of individuals.

Charles Darwin, the discoverer of this phenomenon, thought it was so slow that it could only be observed on geological time scales. However, over the past century, several instances of adaptive evolution occurring within just a few generations have been documented. So, the birch moth, a butterfly, changed color over the decades when air pollution blackened the walls and bark of the trees.

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This butterfly, which was mostly white, quickly turned black due to predator selection. In fact, black moths are better camouflaged on dirty surfaces, and the genes that produce black moths have become more common.

In another example, the frequency of tuskless elephants has increased in response to poaching, where poachers prioritize killing animals with tusks.

However, it remains difficult to tell how fast adaptive evolution is currently taking place. Is it fast enough to influence the response of populations facing current environmental changes? Until now, it has been assumed that the answer is no, without specific data on the subject.

A difficult theorem to apply

To measure the rate of adaptive evolution in nature, we studied nineteen populations of birds and mammals over several decades. We found that they changed two to four times faster than previous work suggested. This shows that adaptive evolution can play an important role in how wild animal populations change over relatively short periods of time.

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How to measure the speed of adaptive evolution? According to the “fundamental theorem of natural selection”, stated by biologist Ronald Aylmer Fisher (1890-1962) in 1930, the genetic variance (a measure of differences) in the ability to survive and reproduce between individuals of a population is equal to the rate of adaptive evolution of the population.

This “fundamental theorem” has been known for ninety years, but it is difficult to apply. Attempts to apply the theorem to wild populations are rare and suffer from statistical problems.

We collaborated with twenty-seven research institutes to collate data from nineteen wild populations that have been monitored over a long period of time, some since the 1950s. Among the birds and mammals studied are blue tits in Corsica, sheep in Canada, hyenas in Tanzania or even baboons in Kenya. Generations of researchers have collected information on the birth, mating, reproduction, and death of each individual in these populations.

In total, these data represent approximately 250,000 animals and 2.6 million hours of fieldwork. The investment may seem excessive, but the data has been used in thousands of scientific studies time and time again.

New statistical methods

We then used quantitative genetic models to apply the “fundamental theorem” to each population. Instead of tracking changes in each gene, quantitative genetics uses statistics to capture the total effect resulting from changes in thousands of genes.

We have also developed a new statistical method that fits the data better than previous models. Our method captures the two main characteristics of uneven distribution of survival and reproduction in wild populations.

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First, most individuals die before reproducing, which means many individuals have no reproductive success. Second, while most adults reproduce sparingly, some give birth to large numbers of offspring, leading to a skewed distribution.

In our nineteen populations, we found that on average genetic change in response to selection was responsible for an 18.5% increase per generation in the ability of individuals to survive and reproduce.

This means that the offspring are, on average, 18.5% “better” than their parents. In other words, a typical population can survive an environmental change that reduces survival and reproduction by 18.5% per generation.

Because of this speed, we find that adaptive evolution can explain recent changes in wild animal characteristics (such as size or timing of reproduction). Other mechanisms are also important, but this result suggests that evolution must be considered along with other explanations.

Greater competition

What does this mean for the future? At a time when natural environments are changing dramatically around the world, due to climate change and other forces, can evolution help animals adapt?

Unfortunately, this is where things get complicated. Our research only estimates genetic changes due to natural selection, but, in the context of climate change, other forces are at play.

First, there are other evolutionary forces (such as mutations, chance, and migration). Second, environmental change itself is likely a more important driver of population demography than genetic change. If the environment continues to deteriorate, theory tells us that adaptive evolution will generally not fully recover.

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Finally, adaptive evolution can, itself, change the environment experienced by future generations. Especially when individuals are competing for a resource (such as food, territory or mates), any genetic improvement will lead to greater competition within the population.

Our work alone is not enough to make predictions. However, it shows that evolution cannot be ignored if we want to accurately predict the near future of animal populations.

Despite the practical challenges, we are amazed to witness Darwinian evolution, a process once thought to be very slow, operating in visible ways throughout our lifetimes.

* Timothée Bonnet is a researcher in evolutionary biology at the Research School of Biology at the Australian National University in Canberra.

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