You are going to read a newspaper article about trees and leaves. Choose the answer (A, B, C or D) which you think fits best according to the text.
As trees across the northern areas of the globe turn gold and crimson, scientists are debating exactly what these colours are for. The scientists do agree on one thing: the colours are for something. That represents a major shift in thinking. For decades, textbooks claimed that autumn colours were just a by-product of dying leaves. ‘I had always assumed that autumn leaves were waste baskets’ said Dr. David Wilkinson, an evolutionary ecologist at Liverpool John Moores University in England. ‘That’s what I was told as a student.’
During spring and summer, leaves get their green cast from chlorophyll, the pigment that plays a major role in capturing sunlight. But the leaves also contain other pigments whose colours are masked during the growing season. In autumn, trees break down their chlorophyll and draw some of the components back into their tissues. Conventional wisdom regards autumn colours as the product of the remaining pigments, which are finally unmasked.
Evolutionary biologists and plant physiologists offer two different explanations for why natural selection has made autumn colours so widespread. Dr. William Hamilton, an evolutionary biologist at Oxford University, proposed that bright autumn leaves contain a message: they warn insects to leave them alone. Dr. Hamilton’s ‘leaf signal’ hypothesis grew out of earlier work he had done on the extravagant plumage of birds. He proposed it served as an advertisement from males to females, indicating they had desirable genes. As females evolved a preference for those displays, males evolved more extravagant feathers as they competed for mates. In the case of trees, Dr. Hamilton proposed that the visual message was sent to insects. In the autumn, aphids and other insects choose trees where they will lay their eggs. When the eggs hatch the next spring, the larvae feed on the tree, often with devastating results. A tree can ward off these pests with poisons. Dr. Hamilton speculated that trees with strong defences might be able to protect themselves even further by letting egg-laying insects know what was in store for their eggs. By producing brilliant autumn colours, the trees advertised their lethality. As insects evolved to avoid the brightest leaves, natural selection favoured trees that could become even brighter.
‘It was a beautiful idea’ said Marco Archetti, a former student of Dr. Hamilton who is now at the University of Fribourg in Switzerland. Dr. Hamilton had Mr. Archetti turn the hypothesis into a mathematical model. The model showed that warning signals could indeed drive the evolution of bright leaves – at least in theory. Another student, Sam Brown, tested the leaf-signal hypothesis against real data about trees and insects. ‘It was a first stab to see what was out there,’ said Dr. Brown, now an evolutionary biologist at the University of Texas.
The leaf-signal hypothesis has also drawn criticism, most recently from Dr. Wilkinson and Dr. H. Martin Schaefer, an evolutionary biologist at the University of Freiburg in Germany. Dr. Wilkinson and other critics point to a number of details about aphids and trees that do not fit Dr. Hamilton’s hypothesis. Dr. William Hoch, a plant physiologist at the University of Wisconsin, argues that bright leaves appear on trees that have no insects to ward off. ‘If you are up here in the north of Wisconsin, by the time the leaves change, all the insects that feed on foliage are gone’ Dr. Hoch said. In their article, Dr. Schaefer and Dr. Wilkinson argue that a much more plausible explanation for autumn colours can be found in the research of Dr. Hoch and other plant physiologists. Their recent work suggests that autumn colours serve mainly as a sunscreen.
Dr. Hamilton’s former students argue that the leaf-signal hypothesis is still worth investigating. Dr. Brown believes that leaves might be able to protect themselves both from sunlight and from insects. Dr. Brown and Dr. Archetti also argue that supporters of the sunscreen hypothesis have yet to explain why some trees have bright colours and some do not. ‘This is a basic question in evolution that they seem to ignore’ Dr. Archetti said. ‘Idon’t think it’s a huge concern,’ Dr. Hoch replied. ‘There’s natural variation for every characteristic.’
Dr. Hamilton’s students and their critics agree that the debate has been useful, because it has given them a deeper reverence for this time of year. ‘People sometimes say that science makes the world less interesting and awesome by just explaining things away’ Dr. Wilkinson said. ‘But with autumn leaves, the more you know about them, the more amazed you are.’