Day: May 7, 2009
WHY do honeycombs have hexagonal cells? Why are the florets in a sunflower arranged in a double spiral? In medieval times, these questions would have been met with a simple answer. God established the heavens, setting “a compass on the face of the deep”, leaving evidence of His presence in all creation. Most scientists today would rather invoke Charles Darwin to explain these patterns as products of evolution. They emerged from myriad possible shapes through natural selection. In other words, they are particularly suited to the task at hand.
Philip Ball, a British science writer, sets out to document the current understanding of what caused nature’s multifarious shapes. Is it just evolution or are there physical and chemical forces at work? One of Mr Ball’s heroes is Sir D’Arcy Wentworth Thompson, a Scottish biologist and polymath, who in 1917 wrote “On Growth and Form”. This mammoth and erudite tome, the first formal attempt to analyse patterns and shapes in nature, was described by a colleague, Sir Peter Medawar, as “the finest work of literature in all the annals of science that have been recorded in the English tongue.” Yet it was a difficult task, and Thompson never quite achieved the results he hoped for.
Take the beehive, and its hexagonal honeycomb. Why does a hive not have triangular or square cells instead? In the 18th century a French scientist, René de Réaumur, worked out that the hexagonal structure ensures that bees fill the volume efficiently yet limit the total wall area of the cells. Put slightly differently, hexagonal cells allow bees to focus on producing honey and expend the least amount of energy making wax. Darwin used the beehive as an example of evolution, saying it “was absolutely perfect in economising labour and wax”.
Thompson argued for a simpler physical explanation, saying that natural selection need not enter the picture at all. If one treated each individual cell as a bubble of wax created by a bee, and every bee was trying to create as large a cell as possible, the physics of surface tension would ensure that all the bubbles took on a hexagonal shape within the hive.
Mr Ball finds the real answer to be more complex than either explanation. To Darwin’s supporters, Mr Ball points out that decoding the honeybee genome still does not explain how the bee makes the honeycomb. On the other hand, Thompson’s explanation of surface tension ignores the bees, which painstakingly build the honeycomb, and possess capabilities that are not yet fully understood. For example, the honeycomb is somehow aligned to the earth’s magnetic field, though no one quite knows why. From the curl of a ram’s horn to patterns of spider webs and the development of an embryo, Mr Ball examines the possible causes of the shapes and forms we observe. His book contains a lot of fascinating detail about the different physical, chemical and evolutionary processes at work. In the end, he concludes, nature is an opportunist.
in The Economist