whatWhat is the speed 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 that it was so slow that it could only be observed on geological time scales. However, over the past century, several cases of adaptive evolution have been documented that occurred over only a handful of generations. Thus, the birch moth, a butterfly, changed color in a few decades when air pollution blackened the walls and bark of trees.
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This butterfly, which was mostly white, quickly turned black due to predatory selection. In fact, black moths camouflaged themselves better on dirty surfaces, and the genes that produced black moths became more and more common.
In another example, the frequency of tusked elephants increased in response to poaching, with poachers prioritizing killing tusked animals.
However, it remains difficult to say how fast adaptive evolution is currently occurring. Could it be fast enough to influence the response of populations to current environmental changes? Until now it was assumed that the answer was no, however there is no precise 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 evolved two to four times faster than previous work suggested. This shows that adaptive evolution may 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” enunciated by biologist Ronald Aylmer Fisher (1890-1962) in 1930, the genetic variance (a measure of differences) in the ability to survive and reproduce among individuals in 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 use the theorem in wild populations have been few and present statistical problems.
We work with twenty-seven research institutes to collect data on nineteen wild populations that have been tracked for long periods, some dating back to the 1950s. The birds and mammals studied include blue tits in Corsica, sheep in Canada, hyenas in Tanzania, or even baboons in Kenya. . Generations of researchers have collected information about 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 field work. The investment may seem exorbitant, but the data has already been used in thousands of scientific studies and will be used again.
New statistical method
We then use 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 two key properties of the unequal distribution of survival and reproduction among wild populations.
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First, most individuals die before reproducing, which means that there are many individuals without reproductive success. Second, while most adults sparsely reproduce, a few give birth to large numbers of offspring, leading to a skewed distribution.
Among our nineteen populations, we found that, on average, genetic change in response to selection was responsible for an 18.5% per generation increase in the ability of individuals to survive and reproduce.
This means that children are, on average, 18.5% “better” than their parents. In other words, an average population could survive an environmental change that reduces survival and reproduction by 18.5% each generation.
Given this speed, we found that adaptive evolution could explain more recent changes in characteristics of wild animals (such as size or timing of reproduction). Other mechanisms are also important, but this result indicates that evolution must be considered in conjunction with other explanations.
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, will evolution help animals adapt?
Unfortunately, this is where things get complicated. Our research only estimated genetic changes due to natural selection, but, in the context of climate change, there are other forces at play.
First, there are other evolutionary forces (such as mutations, chance, and migration). Second, environmental change itself is probably 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 be able to fully compensate.
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Finally, adaptive evolution can, by itself, modify the environment experienced by future generations. Particularly when individuals compete for a resource (such as food, territory, or mates), any genetic improvement will lead to increased competition within the population.
Our work alone is insufficient 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 extremely slow and operating observably throughout our lifetimes.
* Timothée Bonnet is a researcher in evolutionary biology at the Research School of Biology at the Australian National University in Canberra.