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Unnatural selection: How humanity is shaping evolution

In 1873, the Italian geographer Antonio Stoppani coined the phrase ‘anthropozoic era’ to reflect the increasing power humanity held over nature. Though our terminology has changed slightly, the relentless spread of what we now call the Anthropocene, has continued to propel us into a world where almost every natural system is subject to some degree of human influence. Never before in Earth’s history has the biosphere been so disproportionately impacted by a single species. Not only has the human population increased vastly over the last century, but the ecological impact per person has risen as well. This coupling has enabled the exponential expansion of the sphere of human influence, resulting in tremendous shifts in global conditions as temperatures have shot up by 0.85°C, sea levels have risen 0.2 meters, oceans have become 26% more acidic, and forests shrink by 0.57% annually.

The true extent of the Anthropocene’s impacts, however, are only recently being realised in new research into human-driven changes in natural selection and evolution.

The most apparent change that has accompanied humanity’s growth is the spread of sprawling cities and urban spaces. These environments are dramatic departures from naturally occurring, undesigned habitats, and though they only account for 1% of Earth’s surface, they have the ability to completely redirect evolutionary trajectories. The construction of buildings and roads, for example, have been found to shape bird adaptation as collisions with windows and cars have driven evolution towards sedentary lifestyles and shorter wingspans. In Puerto Rico, the increased prevalence of smooth, artificial surfaces in urban areas has favoured longer limbs and toe pads in local lizards. These sorts of behavioural and anatomical changes are only projected to become more prevalent as urbanisation continues.

Remarkably, it seems that some of these adaptations to urban life may be evolving simultaneously across different cities. Certain mosquito species have been found to have independently adapted to life underground, exploiting the increasing availability of urban subterranea such as subway tunnels that provide abundant human hosts. This urban adaptation is further reflected in the repeated evolution of biting preferences for humans over birds in both Europe and North America, illustrating how cityscapes have become new ecosystems with their own set of selection pressures.

Even naturally-occurring ecological communities are subject to anthropogenic evolution. The rapid movement of people and goods around the globe along with the incidence of industrial revolutions have profoundly disrupted natural systems. Often, human-driven adaptation overrides natural selection, with one study even finding anthropogenic selection to be nearly twice as significant.

Certainly, the most recognizable cases of this are instances of overexploitation. The tendency for humans to hunt and harvest selectively results in organisms evolving away from sought-out traits. This trend has been demonstrated repeatedly in reductions in the occurrence of silver-coated foxes, elephant tusk length, and bighorn sheep horn size. In a similar fashion, overfishing has driven fish populations to adapt to become less desirable, with rapid decreases in size and age of maturity.

Simultaneously, the sprawling network of human travel and international trade have facilitated the introduction of non-native organisms en-masse. From cane toads that have decimated Australian ecosystems to beetles that are driving North American trees to extinction, the havoc caused by invasive species is clear. What is less understood, though, are the adaptive responses that these invasions cause in native wildlife. A study published last month has found that invasive fruit flies in Pennsylvania have driven their local counterparts to evolve larger body sizes and earlier rates of maturation to better compete with the invaders. These sorts of shifts remain understudied but will undoubtedly become more prevalent in the face of increasing globalisation.

To-date it is this array of direct ecological disruption that has generated the most recognizable instances of human-driven evolution. However, in the face of continued greenhouse gas emissions, it seems that within the next century the Anthropocene will unleash a much grander-scale evolutionary pressure onto the global stage. Current predictions suggest that climatic shifts will cause earlier animal migrations at a rate of around 2 days per decade. Already studies have found the impacts of warming temperatures as pink salmon migrations occur two weeks earlier than they did 40 years ago and blackcap warblers seem to be splitting into subspecies in response to shifting migratory patterns.

As we progress further into the Anthropocene it seems increasingly clear that humans will only continue to alter the selective pressures that act on natural systems. Seemingly, the rise of urban ecosystems, anthropogenic disruption of natural habitats, and growing impacts of climate change will be major influences shaping the next stage of evolution in the biosphere. In this sense, the Anthropocene truly represents the beginning of an unprecedented epoch in Earth’s history in which one species now rules over the fates of all others.

Image Credits: The Conversation