Plant evolves to escape being harvested, says S.Ananthanarayanan.

Human activities have affected animal behaviour and animal and plant populations the world over. They have affected the climate and the earth itself. Could human actions have visibly affected the course of evolution?

Yang Niu, Martin Stevens and Hang Sun, from the School of Botany, Chinese Academy of Sciences, at Kunming, the capital of the Yunnan province, and the Centre for Ecology and Conservation, University of Exeter, describe, in the journal, Current Biology, an instance of a plant, which is sought after by humans, that has developed a variety with leaves that cannot be easily seen, and is hence less harvested. The researchers find that this variety appears more often in regions where there is heavy harvesting, which indicates that this is a case of evolution in action – a genetic adaptation to evade a predator – in this case, the human harvester of the plant.

Animals have evolved coats that merge with the background, either to avoid being spotted and hunted, or for concealment while hunting. Birds have evolved plumage to show the readiness to mate. Plants have adapted, along with conditions of soil and humidity, to supply the needs of pollinators, even of species that could feed on pests that affect the plants, apart from the colours and appearance, to signal their identity and presence to pollinators.

These adaptations, however, have come about over geological time scales, in the time it takes for the conflicting pressures of natural conditions or populations of species of predator or prey to settle into an ecological balance. A result of this gradual evolution of the biodiversity in the natural world is that features of species are largely fixed and even specialised. The web of inter-relatedness makes changes that affect one population percolate to other species and ecological systems have evolved, over long-time spans, to come to the aid of dwindling species and maintain the relative numbers.

While the pressures of geological changes and the pressures of predators have shaped the ecology over millennia and millions of years, a question is whether natural systems have undergone genetic changes in response to human pressures. Humans, in the course of domestication of plants and grasses, and animals, as well as forming population concentrations, even urban centres, have clearly placed a heavy load on the environment. This has led to disturbance of habitats, the decline, even extinction of species and damage to the environment. But, has human activity, in the short time since human activity began, had evolutionary impact on species?

To answer this question, the group writing in Current Biology take the case of Fritillaria delavayi, a small flowering plant found on high, rocky slopes in southwest China and Tibet. The bulb of this plant is valued in traditional Chinese medicine and has been actively harvested by humans over the last 2,000 years. The question to answer is: has this intense attack by humans led to any defensive changes in the appearance of Fritillaria delavayi?

“Leaf colour of F. delavayi varies among populations from grey to brown, to green. Grey or brown types appear well camouflaged, while green individuals are conspicuous,” the paper says. Recent studies have revealed a number of instances of plants using camouflage, where the appearance of the plant matches the background of where the plant is found, for protections from being eaten by animals, the paper says. In the case of Fritillaria delavayi, however, over the last five years, no herbivore that is a natural enemy of the plant could be found in all accessible populations in the region of Yunnan, the paper says.

Even if there are no animal predators, the plant has been the target of harvesting by humans ever since medicinal properties of its bulb were discovered, some 2,000 years ago, the paper says. If there was correlation between the distribution of the grey to brown variety, which is the less visible variety, and the conspicuous, green variety, and the intensity of harvesting, this could be an instance of human activity impacting the course of evolution.

The team hence carried out a survey of the variations in the colour, and luminosity, of leaves, and the colours of rock and soil in eight locations in southwest China. It was found that there was significant divergence of colours and more in the case of camouflaged, that is, the grey-brown varieties among similarly coloured rock, than the green varieties. And the match of colours was best in the native background of the plants. This indicated that the variation of colours was not random, but “a result of population-specific selection,” the paper says.

Next was the association between how closely the colour of the leaves matched the background and the intensity of the harvesting pressure. The harvest pressure was both how intensely the bulbs were collected and how easy or difficult it was to get the bulbs. It was found that the more intense the collection, the closer the match between the leaves and the background, which is to say, the quality of the camouflage. As for difficulty of harvest, which was reflected by the time it took to reach the bulb, it was found that plants that were more difficult to harvest (when the bulbs were deeper, for instance) were less efficiently camouflaged.

The notion was that a closer colour match resulted in longer detection time, and hence in less harvest. To make sure that this was a fact, there was an on-line citizen science experiment, where people were asked to “to locate a fritillary target as quickly as possible in each of 14 randomly allocated photo slides, simulating the herb collection process by collectors.” As expected, better camouflaged plants took longer to be detected.

The images used were both based on 3 colours – red, green, blue – or on 2 colours - blue and yellow. This allowed for a comparison between the sensitivity of human eyes and possible natural herbivores, which see in only two colours. It was seen that the plants in 3 colour images were spotted faster. As the plants are intensely harvested by humans, having the right colour would hence greatly improve a plant’s chances of avoiding detection, the paper says.

In principle, camouflage could have arisen as a result of selection against detection by animal herbivores, if any, even before human harvesters became significant. This possibility, however, is remote, the paper says, as no animals that targeted the plants or the bulbs were seen in any of the eight populations studied. In any case, the paper says, the bulbs were rich in alkaloids, chemicals that keep animals, like rodents away. While it is the alkaloid content that makes for the use of the bulbs in traditional medicine, it is clear that the colour-specific distribution of the plants is an evolutionary response to human predation.

There have not been many studies of the how plants have evolved under harvesting pressure, the paper says. Given the complexity of the visual environment, the subject is one with severala aspects to examine. As a reverse of camouflage, there is the incidence of mimicry, by weeds, of cultivated plants, an unintentional result of selection by humans, the paper says. “Given that humans have long collected animals and plants for a variety of traits, we expect there to be many other analogous examples of humans driving changes in coloration in the wild,” the paper concludes.

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