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Humans Are Now Causing Species to Artificially Evolve

Extinction isn't the only end game.
Two brown bear and polar bear hybrids at the Osnabrueck Zoo, Germany. Image: EPA/Friso Gentsch

Species are disappearing at speeds unseen since previous global extinction events, and there's little doubt that humans are to blame. Biologists estimate that dozens of them vanish every single day, which is nearly 1,000 times the rate of extinction levels predating modern Homo sapiens. Our heavy-handed impact on Earth's ecosystems is now so noticeable, it's even been labeled as its very own epoch: the Anthropocene.

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But man-made ecological damage doesn't always come in the form of extinctions. Instead of succumbing to the pressures of human activity, some species are rapidly evolving. This relatively new—and deceptively catastrophic—adaptation, called "human-mediated speciation," was explored in a study published this week in the Proceedings of the Royal Society B.

When a plant, animal, or bacteria undergoes "speciation," its evolutionary lineage will split off into two or more unique gene pools. Sometimes, this results in a noticeable difference between the two lineages, but other times, discrepancies are harder to find. As Scientific American once pointed out, orcas (Orcinus orca), which biologists long-believed to be a singular type of whale, have actually been branching off for hundreds of thousands of years into distinct species with their own traits and traditions.

Antarctic orca species. Image: NOAA/ORCA/J.P. Sylvestre

Since evolution is such a difficult process to quantify, especially over shorter periods of time, biologists at the University of Copenhagen decided to investigate whether recent known species adaptations could have occurred as a result of anthropogenic stressors. What they discovered was resounding evidence for human-mediated speciation because of habitat disruption, invasive species, domestication, hunting, and hybridization (think: "pizzlies" and "grolar bears").

One of the most obvious drivers of speciation is the destruction of ecosystems by the combined forces of climate change, natural resource extraction, development, and the introduction of harmful invasive species. Along these lines, the study found that 38 species—for example, the brown anole (Anolis sagrei) in Hawaiʻi and Taiwan, and cane toads (Rhinella marina) in Australia—showed signs of rapid evolution following their introduction to foreign environments. Nearly 70 percent of invasive plant species in Australia, many of them unfit for the region's arid climate like the trampweed (Facelis retusa), changed at least one morphological trait, or structural feature, over a period of 150 years.

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More interestingly, when a deadly South American pathogen called the "myxoma virus" was transplanted to Australia during the 1950s to control wild rabbit populations, within a year, the rabbits had begun to co-evolve alongside the virus, developing a resistance to it. According to the study's authors, evolutionary changes like this aren't uncommon but often go unnoticed among species that aren't considered "economically important."

Domestication is another likely cause of speciation, and considering that humans have "tamed" 474 animal and 269 plant species over the last 11,000 years, its effects are well-documented in modern species like the domestic dog (Canis lupus familiaris). A side effect of this practice, however, is that humans now also interact with species considered "pests," such as agricultural weeds, which has weakened the selective pressures that previously kept them under control.

Number of recorded animal and plant species extinctions; number of recorded established invasive species; number of domesticated species. Light grey, since AD 1500; dark grey, during the Holocene. Chart: Proceedings of the Royal Society B/J. W. Bull et al.

"We also see examples of domestication resulting in new species. According to a recent study, at least six of the world's 40 most important agricultural crops are considered entirely new," said lead author Joseph Bull, a postdoc at the Center for Macroecology, Evolution and Climate at the University of Copenhagen, in a statement.

"In this context, 'number of species' becomes a deeply unsatisfactory measure of conservation trends, because it does not reflect many important aspects of biodiversity. Achieving a neutral net outcome for species numbers cannot be considered acceptable if weighing wild fauna against relatively homogenous domesticated species."

Even humans are falling victim to their own ecological manipulation. Right now, bacterial resistance to over-prescribed antibiotics is one of the most serious threats to public health, according to the World Health Organization. The microbe responsible for gonorrhea and Staphylococcus aureus (MRSA) are just a few of today's "superbugs" that have evolved a resistance to most medications. Treatments can now require the use of multiple antibiotics to compensate for the likelihood that one or more might fail.

While the addition of novel species might seem exciting or beneficial during a time of mass extinction, Bull and his colleagues encourage the scientific community to consider the net worth of anthropogenic change, rather than just its artificial contributions. Wild species can't simply be replaced with new ones, and the biological consequences these organisms pose to complex, and often fragile, ecosystems is dangerously unpredictable.

"The prospect of 'artificially' gaining novel species through human activities is unlikely to elicit the feeling that it can offset losses of 'natural' species," Bull added.

"Indeed, many people might find the prospect of an artificially biodiverse world just as daunting as an artificially impoverished one."