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Geology and Genes

Roger Cooper

Has New Zealand's geological history influenced the genetic composition of its flora and fauna? How do we go about studying evolutionary history of New Zealand land animals through the Cenozoic (65 million years ago to present day) in the absence of any fossils prior to the last 5 million years?

These questions have been addressed in a multidisciplinary study that involves tectonic history, changing shore lines and land area, climatic and sea level history, and DNA studies of fossil and living birds.

DNA contains historical information that can be used to compensate, at least in part, for the lack of a fossil record.

Geologist Roger Cooper and son Alan, a molecular biologist, have come up with a bottleneck theory that has widespread implications for the present day flora and fauna and their evolutionary history.

Alan developed and exploited a new technique for recovering "ancient DNA" --  DNA extracted from extinct birds such as the moa and bush wren, enabling them to be compared with living relatives. The mitochondrial 12S and ND6 genes have been used in a comparative study of six lineages of moa, three species of kiwi, and three of wrens. The moa and kiwi are flightless, the wrens poorly flighted, and all are endemic to New Zealand.

Unexpectedly, the genetic diversity of the three groups found in mitochondrial DNA was found to be similar, and less than expected for groups thought to have been isolated since New Zealand separated from Gondwana 80 million years ago.

The DNA sequence data indicates that within each group an ancestral genotype started radiating about 18-25 million years ago. Consequently, if the groups had been here for 80 million years they had a remarkably low genetic diversity at the beginning of Miocene time.

The common pattern is highly interesting, and since it is found in three ecologically diverse taxa and at two genetic loci, it is unlikely to be an artifact or coincidence.

New Zealand's geological and climatic history may provide the answer. About 25-30 million years ago New Zealand was reduced to an archipelago of low lying islands, with a land area only about 20% of present. Climatic fluctuations resulting from the onset of Antarctic glaciation, combined with reduced niche diversity and reduced population size, is thought to have stressed the biota and significantly increased the background extinction rate, leading to a genetic bottleneck effect.

The rapidly expanding land area, warming climate and diversification of topography that followed (from 24 million years ago to present day) provided ideal conditions for rapid speciation and diversification, to produce the present day diversity in our biota.

The hypothesis predicts that many radiations of endemic land animal and plant groups will be found to have commenced about 20-25 million years ago, and has implications for the rate of morphological evolution, and for our concept of the biota of Cenozoic New Zealand. It is testable through further DNA studies of a variety of endemic animal and plant groups.

Roger Cooper works for the Institute of Geological and Nuclear Sciences