IT MAY be possible to predict the winners and losers ahead of a mass extinction 鈥� those species with the greatest variability in their appearance should be most likely to survive.
Mark Webster at the University of Chicago has shown that , an extinct group of marine arthropods, evolved most rapidly when individuals within a species had bodies that varied in size, shape or number of body segments. When this variation fell away so did the rate of evolution, which may have left the animals more susceptible to extinction by climate change.
鈥淭rilobites evolved rapidly when individuals within a species had bodies that varied in size, shape or number of body segments鈥�
Advertisement
Webster reviewed a large number of earlier trilobite studies. These showed that 70 per cent of the earliest trilobite species, which emerged around 520 million years ago during the Cambrian period, had highly variable bodies. After the Cambrian, that number dropped to 30 per cent and remained so for most of the next 250 million years, until the trilobites died out in the Permian (Science, ).
The decline in variation within a species coincided with a fall in the rate at which new trilobite species appeared. 鈥淚t鈥檚 been known for a long time that Cambrian trilobites evolved quicker than at other times,鈥� says Webster, but his study is the first to show a clear link between the rate of evolution and variation within species. 鈥淐ambrian species were throwing more variation out there for natural selection to operate on. So potentially you can see how a species could evolve quicker,鈥� Webster says.
He suggests that within-species variation declined after the Cambrian because most species of trilobites had evolved to exploit particular niches or lifestyles. This would have made any variation from the optimal body form less competitive, constraining patterns of growth and development.
But there is an evolutionary trade-off: a species that is too specialised may no longer be able adapt to new conditions, becoming more likely to perish during periods of rapid environmental change. 鈥淚t鈥檚 possible that lineages could become so channelled in their development that they鈥檙e walking down a one-way street that they can鈥檛 back out of,鈥� Webster says.
