Using DNA to put them in place

Published: Saturday 15 August 1992

A VULTURE is really a stork and albatrosses belong to the same super-family as the flightless penguins. This is what US ornithologists, Charles Sibley, Jon Ahlquist and Burt Monroe have concluded in their recent rearrangement of traditional classification, based on a controversial new technique developed by them known as DNA-DNA hybridisation.

Since the days of the pioneering Swedish systematist, Linnaeus, the search has been on for a foolproof way of identifying the actual, historical sequence of events by which evolution occurred in order to solve what scientists like to call "the classification puzzle".

Until the discovery of the structure of DNA, biologists relied on comparative form, structure and function of different organisms as a basis for classifying species and families. However, as this concentrates mainly on external features, this technique is open to misinterpretation. For example, Darwin's finches evolved different-shaped beaks to fit their various habitats. Conversely, two unrelated species sharing the same environment can develop similar features by a process known as "convergent evolution". Thus, a systematist can never really be sure whether two similar species share a common ancestor or are an example of convergent evolution.

DNA-DNA hybridisation, or comparison of the DNA of two species, claims to over-ride this problem. The more matching the DNA, the closer the relationship of species to one another.

However, Sibley, Ahlquist and Monroe have opened a new chapter of controversy and academic-infighting, directed at how the test results have been interpreted and analysed, in particular the statistical methods used. There is also a growing body of evidence that evolutionary rates of different avian species fluctuate, which would seriously affect the interpretation placed on their results.

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