Feature

Geology and Genes

The ups and downs of New Zealand's land masses over millions of years have not only shaped the land, but also the plants and animals living on it.

Robert Hickson

A recent gathering of geologists and biologists saw the last 65 million years of New Zealands geological and biological history drawn together as the interdisciplinary meeting looked at how geological changes have affected the evolutionary paths of the country's plants and animals.

The Geology and Genes conference, sponsored by the Geology Society of NZ and the Systematics Association of New Zealand, followed on from a similar meeting in 1994, enabling geologists and evolutionary biologists to gain a better understanding of each other's fields and where they interact.

Systematics, or the study of relationships between species, has had a renaissance with the widespread use of DNA sequencing. Consequently, there is a need for detailed geological and geographical information to attempt to relate the formation or extinction of new species with physical events.

This is particularly relevant in New Zealand, which has a long history of extensive geological change and habitat alteration. The focus of the conference was, therefore, on how life -- especially in terrestrial habitats -- has evolved in New Zealand under such conditions.

Separating Out Species

One of the major problems in evolutionary studies is determining the times of separation of populations and species.

Ewan Fordyce, from Otago University, has found fossils to be useful and valuable in this regard. Some of the New Zealand fossil vertebrates represent some of the earliest examples of, for example, penguins and whales, and so are internationally important. Good fossils, and dates for them, are equally highly prized by molecular systematists.

Unfortunately, for many studies, well-dated relevant fossils are not available and so the estimation of times of separation frequently rely upon assumptions about rates of DNA change to infer a molecular clock. Such an approach can be fraught with difficulties, but there are several strategies which can be used to investigate the timing of events.

One method involves calculating confidence limits from molecular data. This technique has been used by Lindell Bromham, from the University of Queensland, to investigate whether there was a "Cambrian Explosion" or a sudden burst of diversity in animal forms around 500 million years ago (Mya). Matt Phillips, from Massey University, has used the same technique to see whether birds and mammals began to diversify before or after the extinction of dinosaurs, about 65 Mya.

Both these investigations use new analytical techniques which, rather than producing specific dates, instead generate confidence intervals for time estimates. These can then be used to more rigorously evaluate hypotheses about timing of events or, even more importantly, to say that the data is inadequate to be able to answer the question.

Both researchers have concluded that the formation of diverse new groups occurred well before the appearance of representative fossils. Such results are to be anticipated from the generally slow nature of the evolutionary processes and the low probability of finding suitable fossils.

For more recent events (that is, from a few to tens of million years), confidence limits can be too broad to exclude several explanations, but obtaining more sequence data can reduce the limits. As molecular techniques improve, the couple of hundred base pairs of sequenced DNA that were the standard just a few years ago are now seen as pitiful. Thousands of base pairs of sequence can often now be quickly determined.

Alan Cooper, now based at Oxford University, has used examples from New Zealand birds, working on completely sequencing the 16,000 base pairs of mitochondrial DNA from two extinct moa species to more accurately infer times of separation for ratite birds. Thanks to his industrious efforts, there is now more sequence data available from these extinct species than there are for living relatives, such as kiwi and ostrich, making it difficult to fully analyze the moa data.

Gina Lento of Auckland University has found increasing amounts of sequence data helpful in clarifying the volutionary relationships of the worlds pinnipeds (walruses, seal and sea lions), although some issues, such as when and by which route sea lions moved into the southern hemisphere, are still unresolved. [Oct 1995, Oct 1996]

Ticking of Genetic Clocks

A third way for a biologist to try and make temporal sense of some of their molecular data is to assume a particular rate of "ticking" for a molecular clock, based upon calibrations of a group of species for which fossil dates are known. They can then see how these derived dates fit with known geological events. This is where New Zealand, with its volatile geological past, has a great deal yet to contribute.

Studies of the evolution of New Zealand cicadas provide an excellent example of congruence between DNA sequence data and geological dates. Thomas Buckley of Victoria University has used an insect mitochondrial DNA molecular clock and extensive evaluation of the most appropriate model of DNA sequence evolution to determine the best estimate for times of origin of these cicada. The results indicated this origin to be in the range of 2-3 Mya, conforming very closely with the rising of the Southern Alps.

Plant molecular systematists often have more trouble attempting to estimate rates of change from their data because few reliable calibrations have been determined. Studies of the evolution of New Zealand alpine plants from lowland relatives (and between sibling species on the mainland and some offshore islands) will help refine our understanding of rates of change in plant DNA sequences.

Peter Lockhart, from Massey University, has been examining new methods to find phylogenetically useful sequences to study some of these groups, both for species and population comparisons. Finding such markers for plants is a very important objective because, in contrast to zoologists, botanists have a more limited range of genetic loci available for evolutionary studies.

Comparative studies of plant communities, as well as DNA sequencing of related species in New Zealand and the Southern Andes, provides interesting contrasts, with both superficial and real ecological and genetic similarities between the two regions. Some of the genetic data has revealed that, despite the width of the Pacific, some species have been able to cross it relatively recently, probably aided by the southern winds.

Delving into geological journals can be daunting for the uninitiated, and many local biologists have relied on very useful but now dated texts, such as the two-volume Geological History of New Zealand, for geological information. For the biologists the conference was well worthwhile because they were informed about the latest predictions of New Zealands geography over the past 65 million years.

Orogeny Replicates Phylogeny

A geological overview of major events between 10 and 65 Mya shows extensive changes in the shape and extent of land in the area of present-day New Zealand. Geographic changes and climatic ones can be tracked in the comings and goings of pollen belonging to different species and groups.

As a specific example, Peter Heenan, of Landcare Research, has studied the evolution of New Zealand native legumes, such as Carmichaelia or broom. These and other native legumes have undergone convergent morphology, where they have adopted similar anatomical characterisitcs which Heenen suggests is probably due to a response to changes in the New Zealand environment. Conditions in the Miocene, about 10-20 Mya, were cold and dry, because of the development of a major Antarctic ice cap. As a consequence Charmichaelia developed forms which were drought-resistant. The rapid uplift and erosion of the Southern Alps in the late Miocene saw new habitats for the plants and rapid adaptive radiation in the eastern South Island.

A Long Time Ago...

Changes in New Zealand 10-40 Mya are of great interest to New Zealand evolutionary biologists for two reasons.

Firstly, a series of islands to the north west of New Zealand led up toward New Caledonia, and could have provided a convenient set of land connections, enabling some plants and animals to migrate into New Zealand. This avenue seems to have disappeared by the middle Miocene, about 10 Mya.

Secondly, during the Oligocene (25-35 Mya), the New Zealand land area was very much less than before or after, and the reduction in size, followed by extensive increases in land area and topography, may have had a major influence on the survival and diversification of species.

The speciation of skinks in relation to New Zealand's geographical past.
A: Late Eocene, 35-45 million years ago. An ancestral skink is inferred to be present in the land area during the late Eocene. New Zealand then consisted of low rolling plains and extensive coastal swamps, surrounded by a relatively shallow sea.
B: Mid-Oligocene, around 30 million years ago. During the Oligocene, the ancestral species becomes isolated on the islands formed as the land wears away, and evolves into new species.
C: Late Miocene, 5-10 million years ago. Increasing land area results in greater dispersal of some of the skinks and additional speciation.

Information from Roger Cooper, of the Institute of Geological and Nuclear Sciences (GNS), covering the improved assessment of some of the hypothesized islands, has been very valuable for biologists, as is his preliminary modeling of the impact of the changes on species diversity.

The conference demonstrated a satisfying concurrence between the geologists' view that an island archipelago during the Oligocene would have stimulated species diversification for some groups, and my own interpretations for the evolution of New Zealand skinks.

While dating of the timing of diversification of lizard sequences is uncertain, an approximate estimate, along with the number of these lizard species in New Zealand and their apparently rapid diversification from each other, can be explained by the geographical structure of New Zealand in the Oligocene. This presents a very nice island model of speciation for further testing. [The Great Skink Migration, Sept 1990]

In contrast to the finches of the Galapagos islands, there is little morphological differentiation amongst the skinks, which may be a consequence of the skinks' less specialized diets. An absence of land mammals offering potential competition or predation in New Zealand for most of its history also probably contributed to high lizard diversity.

Rod Hitchmough at Victoria University has worked on new species of geckos identified genetically and morphologically, which has dramatically increased the numbers of recognised lizards in New Zealand. Too few DNA sequences are currently available from the geckos to determine whether they show a similar pattern and timing of diversification as the skinks.

Once Were Islands

Several of the biologists at the Geology and Genes conference wanted to know how their phylogenetic hypotheses fitted with former geography or geological processes.

The genetic distinctiveness and anomalous distribution of some populations of Northland mudfish from what was believed to be other populations of their species raised the question of how, or whether, this could be explained by former islands within the last few million years.

Mary Morgan-Richards of Otago University has been studying the chromosomes, allozymes and now DNA sequences from populations of the Auckland tree weta. These have also shown some surprisingly large differences, possibly echoing the occurrence of once-were-islands in the Northland region.

Geologists, too, seemed to benefit from the meeting, appreciating that biology can help refine their geological scenarios.

Hamish Campbell of GNS described his geological studies of the Chatham Islands, where he has been trying to determine how long they have been above water. Genetic studies of the Chatham Island biota and estimation of at least approximate limits for when they may have reached there would, he acknowledged, help in this regard.

David Penny, from Massey University, closed the conference with cautions to be aware of over-confidence in molecular data, but also to avoid being dismissive of conflicts between fossils and molecular data. The challenge is to develop new research projects to understand how and why differences occur.

One very important aspect of the meeting was that geologists and biologists gained further insights into not only what can reasonably be inferred about New Zealand's biological and geological past, but also where uncertainties still remain.

The range of different geological and climatological events which have affected New Zealand, and for which there are reasonable estimates of time, make New Zealand an important evolutionary showcase, perhaps particularly with respect to examining the reliability of molecular clocks.

As illustrated by Buckley's research, assumptions about rates of change can be tested with New Zealand taxa using phylogenetic and geological knowledge.

The genetic side of the conference was dominated by studies of vertebrates, but the abundance of plant and invertebrate groups (as well as coastal biota) for testing evolutionary models will hopefully be reflected at the next meeting, along with longer DNA data sets and confidence limits for date estimates.

There is still plenty of our island experience left to discover.

A book of extended abstracts (Geology & Genes; ISBN 0-908678-69-X) from the meeting is available from The Publications Officer of the Geological Society of New Zealand, c/o Institute of Geological and Nuclear Sciences, PO Box 30-368, Lower Hutt. Price $15 (includes postage and packaging).

Robert Hickson did his PhD in molecular evolution, concentrating on New Zealand skinks. He is currently at the Zoologisches Institut, Universitat der München.