Not surprisingly, thousands of evolutionists and biogeographers have jumped in to fill the void. In fact whole schools of thought, dominated by vicariance and phylogeography, have arisen, each giving birth to dozens of theories all based on units that can divide by Darwinian evolution, but not fuse.30
To highlight the point, there are about 3 metres of shelf space in my study holding just a fraction of the books on this subject. I have read most of them because I once set about writing yet another book on the subject myself. I was going to call it, surprise surprise, Reticulate Evolution, but it was never finished because climate change and the mass bleaching of corals raised their ugly heads, and that, for me, was a game-changer. However, when reading all those books and the hundreds of research papers that underpinned them, I saw that reticulate evolution overwhelmingly refutes one hypothesis of the mechanism of evolution after another. It also gives direct answers for most of the biogeographic, evolutionary and taxonomic problems that so many authors have pondered and complained about. In short, these authors would not have written what they did if they had embraced the significance of the differences between the two diagrams earlier in this chapter.
I will end this discourse by explaining my reference to syngameons. A syngameon is a reproductively isolated unit, the old definition of a species. Let’s not split hairs over this: for most organisms, syngameons and species are not remotely the same things. For laughing kookaburras they probably are; for eucalypts and most other major taxa they’re not. Depending on your reference book, eucalypts are now divided into two or more genera, with Eucalyptus divided into subgenera, which in turn are divided into species. Those species are divided into subspecies, and subspecies into varieties.
Are the poplar gums around Rivendell, you might therefore ask, a variety? No, they’re not; they’re part of a continuum that keeps on keeping on – forming finer branches long after all books throw in the towel. So where, in all this is, are syngameons, the slices of the pie that are indeed reproductively isolated? Perhaps roughly at the level of subgenera, for most species of eucalypt can readily be crossed with many other species, but certainly not all other species.
If this is so, why not draw the line, for eucalypts, at the level of the syngameon (perhaps a dozen big units and many laughing-kookaburra-like small ones) and leave it at that? Many reasons. For starters, the taxonomic information loss would be enormous; syngameons are very likely to have no defining morphological characters; syngameons cannot be identified from DNA using cladistics; syngameons can be identified by thousands of cross-breeding trials, although the results will vary geographically.
All this raises the question of how, when and where species originate. Looking at those two diagrams again, note that Darwinian evolution has species forming at specific times (and places), while reticulate evolution has species forming without a time or a place of origin. Only syngameons have that time and place, vague though these might be. Note also that extinctions may be similar in both cases; however, with reticulate evolution what appears to be an extinction may be a genetic merger created by an environmental change. The latter can be seen in the fossil record (as ‘punctuated equilibria’) and in virtually any aspect of present-day biology.
So what, in the natural world, can one make of all this? Not much, except to be aware of the fuzziness and not assume that a population in one part of the world has the same properties as a population of the same species in another part. When it comes to conservation, that’s a lot of food for thought.
Isn’t reticulate evolution also a good excuse to let taxonomists off the hook when they’re not too sure about their species? Certainly, but only for those who don’t delve into details or work over big geographic ranges.
For me, as for so many others, the mechanisms that change the life of our planet is an all-engrossing subject, one I would dearly love to write about, but the need to do my bit to help keep that life alive has become much more important.
Bleached coral on the Great Barrier Reef.
A healthy upper reef slope.
A reef slope two years after a mass bleaching event. The corals still alive are being smothered with seaweed because of an ecological collapse.
Goniopora pandoraensis with some branches bleached. This is the first photograph of bleaching on the Great Barrier Reef.
Origins of The Reef
Darwin’s theory of evolution was not that great man’s only profound and everlasting contribution to science. His hypothesis of how atolls form is infinitely less deep and certainly less well known, but for me it was the theory that most mattered when I first started working on coral reefs. My interest was triggered during the Stoddart expedition, when we first dived down the outer face of Tijou Reef and found that it is also both the outer face of the Great Barrier Reef and the western edge of the Queensland Trough.
My journey with this subject has the longest time span of any scientific concept in my life. I first read, and wondered, about the origin of Australia itself when I was a dream-laden kid at Barker in 1960. Then I brought the Great Barrier Reef into that scenario when writing A Reef in Time in France nearly fifty years later.31 To put all this into context I now need to retrace Australia’s journey from its motherland, the supercontinent of Gondwana, where Antarctica still resides.
Continental positions during the Late Cretaceous, about 80 million years ago. Surviving reef limestone is shown in dark grey. X marks the spot where a giant asteroid hit the earth 65 million years ago.
By the Early Cretaceous (around 130 million years ago), Africa, Madagascar and India had rifted free of Gondwana, and India was on its dash to the equator. At that time Australia was dawdling, so that by the Late Cretaceous (about 80 million years ago) it was only just setting out. However, the new continent carried with it all manner of bizarre plants and animals, including Glossopteris and other temperate rainforest trees that we still have, and a suite of giant reptiles, including many dinosaurs, all tolerating the seemingly lethal seasonal daylight regime that Antarctica has today. When it finally broke free, Australia was isolated in all directions, allowing its marsupials to evolve free of competition from the more advanced mammals of other continents. At least that was the stage-set for the evolution of Australia’s unique terrestrial life that we have long been familiar with. The story of its marine life is another matter, one that only corals can tell.
Sixty-five million years ago, a massive asteroid hit what is now the Gulf of Mexico, shaking the whole of Earth and triggering the last great mass extinction, which wiped out the remaining dinosaurs and much else besides. About that time, Papua New Guinea started rotating counterclockwise away from north-east Australia, creating the Queensland Trough and, I have to suppose, the shallow platform on which the Great Barrier Reef now resides. Reefs did not proliferate anywhere at this time because major carbon dioxide peaks snuffed most of them out, worldwide.
The Antarctic Circumpolar Current was established about 40 million years ago when South America rifted free of Antarctica, the last continent to do so. This formed a marine thermal barrier around Antarctica, helping the polar ice cap to form and giving most of Australia its all-time lowest water temperatures, while the continent was less than halfway along its journey to where it is today. At that time, reefs would not have formed anywhere around Australia as we now know it. However, they did form along the north coast of New Guinea, for Greater Australia (meaning Australia and New Guinea combined, as they have been for most of their existence) had come within seeding range of the coral reefs of Asia.
By the Early Miocene (around 20 million years ago) terrestrial Australian life (seabirds presumably excepted) was still isolated from the rest of the world, but Australia’s marine life wasn’t. Minimum temperatures were often higher than now, and we know that coral reefs proliferated down the coast of Western Australia as far south as Perth, so there’s no reason whatsoever to believe that the same did not happen down the east coast.
All this points to the
Great Barrier Reef, roughly the size and shape it is now but probably bigger in the south, being formed well over 20 million years ago.
On 12 April 1836, Darwin knew nothing about continental drift – not even he imagined that – but after climbing a mountain slope in Tahiti, he gazed out on the beautiful island of Moorea, often called the Bali Hai of the South Pacific, and that night he made an extraordinary entry in his journal: ‘. . . if we imagine such an island arc after successive intervals, to subside a few feet in a manner similar to but with a movement opposite to the continent of South America; the coral would be continued upwards, rising from the foundation of the encircling reef. In time, the central land would sink beneath the level of the sea and disappear, but the coral would have completed its circular wall. Should we not then have a lagoon island [atoll]? Under this view, we must look at a lagoon island as a monument raised by myriads of tiny architects to mark the spot where a former land lies buried in the depths of the lagoon.’32
He then turned this wonderful piece of deductive logic into his unlikely theory of the origin of atolls. As the greatest geologist of his time, Sir Charles Lyell, put it: ‘I am very full of Darwin’s theory of Coral Islands [atolls] . . . Let any mountain be submerged gradually . . . there will be a ring of coral and finally only a lagoon in the center. Why? For the same reason that a barrier reef of coral grows along certain coasts: Australia etc. Coral islands are the last efforts of drowning continents to lift their heads above water.’33
Darwin developed this theory just by looking and thinking, nothing more. Another of his eureka moments? Certainly looking at atolls can prompt pondering – they encourage any amount of that – but seen from the sea, atolls keep their secrets. It’s only when seen from above, from an aircraft – in short supply in Darwin’s time – or on a chart, that they are seen for what they are, which is usually a string of narrow reefs that form a circle or an irregular loop, some dotted with little cays. The centre of the loop, the atoll lagoon, is almost always shallow and full of sand, while the outer wall plunges to a great depth.
Darwin’s theory conjures up a spectre of mountains conveniently sinking at just the right pace for corals to keep growing, yet this must rarely be the case. Some mountains go in the opposite direction – up – and these often have dead reefs on their slopes. Others sink too quickly, and if these have reef remnants at all they are dead for want of energy-giving sunlight. Atolls only form when the rate of sinking is tolerable for coral growth, and this must happen in the context of ever-changing sea levels. So the chances of an atoll forming are really quite slim.
Was Darwin right? About twenty of his contemporaries got stuck into the arguments. If he was right, reefs would be thick, in places very thick. If he was wrong, they would just be veneers atop the submerged remains of mountains. Most of the conjecture was about how reefs would survive changes in sea level resulting from polar ice sheets forming and melting during ice age cycles, creating sea level changes of 100 metres or more. Reefs would be shaved off when the sea level went down and then would need to be rebuilt as it rose. There were many views published about these theories and variations of them, and they clearly nagged at Darwin. Just before he died, although rather more preoccupied with human evolution than with reefs, he wrote to one of his antagonists, Alexander Agassiz of the Museum of Comparative Zoology, Harvard University: ‘If I am wrong, the sooner I am knocked on the head the better . . . I wish that some doubly rich millionaire would take it into his head to have borings made in some of the Pacific and Indian atolls.’
As is well known, this happened some fifty-six years later, when the United States Atomic Energy Commission, in preparation for nuclear testing, sank a series of deep boreholes in the northern Marshall Islands. There was immense public interest in the issue: Darwin had predicted that atolls might be 5000 feet (1525 metres) thick. Two boreholes reached volcanic foundations at 1267 and 1045 metres. Darwin was right, but how could he have been that accurate?34 I have no idea, and nor have the dozen or so reef geologists and historians I have talked to about it.
Before all this happened, and to supposedly prove or disprove Darwin’s hypothesis, some crazies from the University of Queensland decided to take a core from the Great Barrier Reef, apparently not having observed that Australia is just about the oldest continent on Earth and would be exceedingly unlikely to be sinking, rising, or doing any other such thing. In 1937 they drilled a hole on Heron Island, and when that didn’t work out, because their equipment wasn’t the best, they drilled several more holes in and around the far northern ribbon reefs, and when those didn’t work out either they gave up. In hindsight this was fortunate, because there was a lot of interest in these drillings, and newspapers of the time would surely have pronounced Darwin wrong, in line with the narrow-minded, Christian-dominated view most people had about his theory of evolution.
In the 1970s, a group of geologists again began taking deep cores from the Great Barrier Reef, in what developed into the most expensive research project ever undertaken on it. This coring (the word ‘drilling’ being by then taboo because of its association, at least in the minds of the permit peddlers in the Great Barrier Reef Marine Park Authority, with oil exploration) to determine the age of The Reef became famous, especially as its leader, Peter Davies from the Bureau of Mineral Resources, gave engaging talks on the subject at several reef conferences. Peter’s work resulted in two basic notions about the Great Barrier Reef – firstly that it is (horizontally) wedge-shaped, being thicker in the north because it is older in the north, and secondly that it is very young as reefs go. As far as being wedge-shaped goes, the Great Barrier Reef is as thick as the distance between the bedrock and the ocean surface, something that has nothing to do with age. And as for being a baby, this view came to a much dramatised climax in 2001, when an international drilling consortium using a 50-tonne drill to go coring announced that the central Great Barrier Reef had started growing about 600 000 years ago, a conclusion endorsed by a football team of reef geologists.35
I countered this conclusion in A Reef in Time seven years later by pointing out that it’s unreasonable to suppose that just because old reef (at the bottom of their boreholes) isn’t there now it never was there. Older reefs may have grown and been eroded away many times following sea level changes. I’m still waiting for a response to that, and am fairly sure I won’t get one because I don’t believe there is one. If extensive reef development occurred on the west coast, why would reefs not have formed in the east, when environmental conditions and bathymetry were at least as favourable?
The ability of coral larvae to make long-distance journeys, which underpins what I have just described, was only a dawning notion thirty years ago and would have been unappreciated by geologists then, some of whom expressed the view that corals were dispersed by seafloor spreading – by hitching rides on islands and continents as they moved about.
This geocentric view of the origins of reefs is now being clarified by mapping the surface of reefs in detail, using high-resolution, three-dimensional bathymetry.36 Today we have confirmation that the ribbon reefs had a deeper-water forerunner from times of lower sea level, something I’ve seen many times when diving, the second line of reef being clearly visible at depths of 50 metres or more. However, I wonder if we’ll ever know how many times this has happened. Very likely we never will, as neither drilling into reefs nor mapping their bathymetry will give an answer. And given the amount of time involved, the story is certain to be anything but straightforward.
Of course Darwin’s insight into the origin of atolls and my view about the age of the Great Barrier Reef could have been arrived at by getting together information from all relevant fields and just integrating it. However, this bit of integrating occupied me for thirty-five years. One mustn’t rush these things.
Darwin’s theory of atoll formation, where a fringing reef grows around a subsiding mountain (left), which ultimately becomes an atoll after the mountain has become completely submerged (ri
ght).
The Coral Triangle
When John Wells gave me his wall chart of coral distributions, back in the autumn of 1975 on my first trip to America, I wasn’t too keen on copying it all out, so it sat on a shelf in my study gathering dust until fate intervened in the form of a magical machine I bought from Woolworths: a Commodore 64 computer. This was mostly designed for playing games, but with two fingers to the task and a little help from a friend who’d used a computer, I soon had John’s table typed in and, lo and behold, I could easily make changes without reverting to scissors or sticky tape. Better still, I could print out lists of countries where any coral genus had been recorded, as well as lists of genera in each of those countries. My study at Rivendell became littered with piles of printer paper, excellent fodder for a new contour map of the world’s coral genera. I reported on this to the Fifth International Coral Reef Symposium in Tahiti, in 1985.37
In the meantime, I set about putting these records on AIMS’s new computer, an IBM mainframe monster that filled two large rooms. At first I’d regarded computers with suspicion, mostly because the electron microscope I used for my PhD, another monster, had taken a lot of time to master and I’d never used anything like it since. The same sort of thing seemed to apply to computers; better to leave such jobs to a technician.
Using the new IBM was a big mistake on several fronts, the first being that I only narrowly escaped that most terrible of fates – being drafted onto a committee.
A Life Underwater Page 24
