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A new twist on rapid evolution in the Anthropocene

Many people view evolution as an extremely slow, long-term process by which organisms gradually adapt and diversify over millennia. But researchers also have found that rapid evolutionary change can occur over mere centuries, or even decades. Such ongoing rapid evolution is the focus of a fast-moving field of empirical work, made easier by new techniques for detailed DNA analyses in wild populations. Rapid evolution by natural selection can allow wild populations to adapt to new environments or keep pace with stressors in their home habitats.

Superimposed on this baseline of ongoing evolution is the increasingly prolific influence of human activities as a force of natural selection. The current epoch of earth’s history known as the Anthropocene – a period shaped by humans – is marked by massively altered ecosystems and alarming rates of species’ declines. Hundreds of scientific studies show that subtle but important evolutionary changes have allowed wild species to persist longer than they would be able to otherwise in the face of human-induced pressures, including climate change and introductions of invasive species.

Eastern hemlock mortality in the Great Smoky Mountains, Tennessee, caused by the hemlock woolly adelgid. Photo credit: Peter Curtis

A famous example of rapid evolution was caused by coal burning in mid-19th century Britain.  Air pollution killed lichens and coated tree bark with dark soot, resulting in selection for the dark, cryptic form of the British peppered moth. Another textbook example of rapid evolution is selection for resistance to pesticides in many weeds and insects in a matter of years or decades. Likewise, pollution in rivers and lakes has selected for super-hardy fish in some cases, and selective harvesting of large-sized fish has led to heritable shifts towards smaller-sized individuals. Wild species that can’t acclimate, avoid, or adapt to human-caused selection pressures may simply disappear.

Until now, the evolutionary effects of humans on wild, unmanaged species have mainly been inadvertent by-products of anthropogenic activities rather than intentionally planned interventions. It’s true that we have radically altered the genomes of domesticated plants and animals, but these species typically do not affect the gene pools of wild populations unless they interbreed with wild relatives or escape to become self-sustaining on their own (think of feral pigs).

This may be about to change.

Already, incentives are mounting to genetically engineer non-domesticated species of commercial importance, such as vast tracts of forest trees that have been decimated by insect damage and diseases. A recent report by the US National Academies of Sciences and Medicine argues for the urgent need to engineer resistance genes into the germlines of harvested tree species to protect them from such damage. Meanwhile, the challenge of tinkering with the genes of non-model species has been greatly simplified by a new gene-editing tool known as CRISPR. Thus, we may be on the brink of engineering wild, free-living species very intentionally for the first time in their evolutionary history.

Another genetic engineering feat plays into this paradigm-shifting scenario. In the past few years, biochemical engineers have developed a powerful technique in which CRISPR is harnessed to a gene drive system. A gene drive can be inserted into the DNA of a wild species in tandem with various types of desired “cargo” genes. A cut-and-paste feature of the gene drive causes the inserted cargo genes to be inherited by all future offspring in a non-Mendelian fashion. In some cases, the cargo could be a resistance gene that protects a wild species from a deadly disease, or one that prevents it from serving as a vector for human pathogens. In other cases, the trait could lead to single-sex offspring (e.g., all male) or could cause sterility, yet it would still be driven into all interbreeding individuals, eventually causing the population to self-destruct. Some people have suggested that gene drives should be used to eradicate pest species, like invasive rodents on islands and feral cats that plague wildlife in Australia.

Feral cat in an Australian national park. Photo credit: Isabelle Sheppard
Australian feral cat. Photo credit: Tim Doherty

Both types of gene drive applications – for introducing a desired trait, or for eradicating unwanted populations – are actively being pursued by researchers who work with mosquito-borne pathogens. Likewise, academic researchers at the Massachusetts Institute of Technology and Tufts University plan to genetically engineer wild, white-footed mice to make them resistant to the spirochete bacterium that causes Lyme disease. If small-scale experiments on islands are successful, these researchers may expand their efforts to include a gene drive designed to alter much larger populations of wild mice in the Lyme transmission cycle.

White-footed mouse, the focus of a genetic engineering project to combat Lyme disease.  Photo credit: Michael Richardson

However, before wild organisms with novel genes and gene drive systems are released into the environment, regulatory agencies, other scientists, and the public will need to weigh in on the wisdom, likelihood of success, and potential risks of each proposed application. Because a gene drive could quickly sweep through all interbreeding individuals, extreme caution is needed to avoid unintentional and unwanted consequences of this futuristic mechanism for rapid evolution. Welcome to the evolutionary Anthropocene.

Featured image credit: “DNA String Biology 3D” by qimono. Free use via Pixabay

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