Evolving Darwin’s big theory

The Galapagos Islands – and their contribution to Charles Darwin’s theory of evolution through natural selection – have played such a central role in the development of our modern identity it seems there ought to be little left about them to discover. But research led by University of Hong Kong geologist Jason Ali and published on Thursday may have solved a puzzle left half-answered by the great naturalist himself.

When Darwin, a geologist by training and inclination, reached the cluster of islands 900 kilometres off the west coast of Ecuador in 1835, deep fissures had already opened up in Victorian society’s understanding of the relationship between their creator and the physical world. It was during the 19th century that the Age of Reason’s curiosity and desire to question the unknown turned into an obsession with counting, collecting and qualifying.

When faced with thorny, existential problems, we humans have always woven stories to hang over the gaps in our knowledge. Tradition and ignorance often preserved these tapestries for centuries.

One such problem had been to explain the sheer abundance and diversity of the natural world.

“Each little flower that opens, each little bird that sings, He made their glowing colours, He made their tiny wings,” Cecil Alexander resolved in her 1848 hit hymn All Things Bright and Beautiful.

If not God, then how?

Enter the world’s islands. Although, by definition, they’re isolated by often deep channels of water, all are populated – however poorly – by life forms of some kind.

Islands such as those belonging to Hong Kong don’t present much of a barrier to some – wild boar make it to Kau Sai Chau golf course, off Sai Kung, although Lantau’s water buffalo might find Kowloon a tougher task.

Other islands emerge “sterile as a newborn baby”, as Steve Jones, professor of genetics at University College London, puts it in Almost like a Whale, his modern-day rewrite of Darwin’s The Origin of Species (1859). The Hawaiian and French Polynesian groups are among this type.

These volcanic islands are too far from the mainland and too young to have been populated by anything but ocean-going mammals, such as Polynesian settlers and the tropical Hawaiian monk seal, as well as birds – and bats – capable of long-distance flight, wind-born spiders and other small creatures. Some seeds may have floated, or been deposited from the gut of a bird. The native set of flora and fauna – or biota – on such oceanic islands is typically a pretty impoverished crew, even if they mask their lack of diversity with tropical abundance. All mouth and no trousers, as it were.

The Galapagos, though, are more of a Goldilocks archipelago.

Close enough to land to allow sufficient strandings to create a viable breeding stock, but far enough from the continental land mass to enable the process of natural selection to usurp that divine gift of the creator – separation of the species. After gaining a toehold on a new island, any inhabitants enjoy first-mover advantage.

While a mainland species invading territory or a niche already occupied must enjoy a clear superiority in some crucial aspect to displace the incumbent lodgers (aggression, perhaps, or fecundity, or durability), new arrivals on unoccupied islands can fan out to take advantage of all unexploited niches.

This explains the much-studied variety in the archipelago’s finches – some have heavy bills to crack tough seeds while others fell into a jack-of-all-trades role.

“It’s a bit like when immigrants arrive in a country,” Ali says, in strident northern English tones that belie his part-Indian ancestry and illustrate his point. “The next generation tends to be culturally distinct from their parents. Furthermore, they are invariably disconnected from their parents’ families and friends back in their homeland.”

As Darwin digested the evidence collected during his voyage, it became clear that the land animals and small birds on the Galapagos were related to similar forms on the South American mainland. This is a crucial step to understanding the concept of origin from a common ancestor.

But why were, he wondered, some species found on lots of islands while others were unique to just one or two, even though the climate and topography were almost identical? If they had evolved from common ancestors to fit the various niches offered by the islands, then shouldn’t they be universally distributed, or isolated to discrete land masses?

“The islands, though in sight of each other, are separated by deep arms of the sea,” he wrote in Origin.

Strong ocean currents would have swept unfortunate animals that fell in the water past the shores; the relative lack of storms meant finches and other small birds were unlikely to get blown to neighbouring islands … at least, not in sufficient numbers to establish a viable breeding stock. The geologist wrote: “There is no reason to suppose that they [the islands] have at any period been continuously united.”

But according to the research paper by Ali and University of Sydney geologist colleague Professor Jonathan Aitchison, published in the Journal of Biogeography, Darwin got that part wrong.

Of course, in many ways he was groping in the gloom. Just as Jones’ reworking has the benefit of the 1953 discovery of the genetic code embedded in DNA to buttress Darwin’s theories, so Ali can draw on geological knowledge that was unavailable a century and a half ago.

There is a deep irony at work here.

It seems incredible today that the theory of tectonics – the almost imperceptible drift over eons of huge plates of the Earth’s crust – should only have found universal acceptance in the past half-century. For Darwin and many of his contemporaries, explaining the obvious ancestral ties between continental and island life forms was left to educated guesswork: land bridges that provided precarious stepping stones, only to crumble into the sea, for example.

In fact, Darwin had been closer to thinking his way through the problem than he had realised. It was geology that fired the intellectual furnace of the young Darwin, who took his commission on the legendary voyage of the HMS Beagle primarily as an oceanographer. It was Darwin’s acceptance and understanding of geological time, where tiny, incremental changes have the power to transform a seemingly immutable landscape, which underpinned his natural selection theory.

The intellectual barriers to evolution weren’t just those of creationists.

The paradigmatic frame of reference for our modern world is measured in billions of years; in Darwin’s lifetime, you wouldn’t have been considered medieval for viewing the Earth as the work of a few millennia.

Darwin was very familiar with the power of unnatural selection – his own work on pigeons made him a master in the principles of selective breeding for desired traits. Selection by nature, though, depends on mutations.

By their laws, they are rare and appear random – more prone to grotesqueness and failure than to delivering the best fit for a given (often dynamic) habitat.

Darwin’s peregrinations through South America exposed him to a fossil record that hinted heavily at an ancestral evolutionary chain of great antiquity but also of marked difference to the more familiar European one.

Moreover, Darwin’s observations of geological formations led him to suggest the sea levels had not always been constant. Indeed, had Darwin never devised his theories on evolution, his geological endeavours alone would have ensured a place among the giants of Victorian science: his work on the formation of oceanic islands set the benchmark. A few more leaps of logic may even have led him to the theory of plate tectonics that helps explain how the Galapagos is populated today.

The islands are part of a plate that is slowly moving east towards South America, where it grinds beneath a westerly-moving cousin. Their interactions explain the soaring spine of the continent, the Andes. As the plate passes over a zone of hot spots, volcanic islands are thrust through the Earth’s skin, erupting like boils on a teenager’s face.

As these islands move east towards the continent, they are eroded and battered by the sea and wind, disappearing beneath the waves. But for their brief – in geological terms – existence, they provided stepping stones for the distribution of today’s Galapagos castaways. (And helped explain how the evolution of species was possible on islands that seemed too young to have harboured the process of adaptation through mutation.) Ali is no stranger to the impact of geology on the distribution of animals. He made headlines with a 2010 study co-authored by Matthew Huber – a palaeoclimate modeller then at Purdue University in Indiana, in the United States – that explained how the ancestors of modern-day Madagascar could have drifted on floating vegetation from Africa.

Due to its isolation, most of the mammal species and half the birds on the world’s fourth-largest island exist nowhere else on Earth. The first mammals are thought to have appeared there about 60 million years after Madagascar split from the African continent.

It’s always tricky to go against the flow, but Ali’s research, published in Nature, showed that the currents would have been going in the opposite direction at the crucial (geological) moment in time when life made the leap to the north-drifting island.

“It was while I was hunting for something to follow the Madagascar research that I was looking at the map and noticed the ocean bed in and around Galapagos. I was amazed to see that about half the islands were connected by shallow sea floor. Drop the ocean surface and many of the islands would sit on a single mega-platform,” Ali says.

Ali and Aitchison set about creating a model to account for changes in sea levels around the archipelago caused by the advance and retreat of ice over the past half-million years.

The continental ice sheets suck huge amounts of water from the oceans, leading to significant fluctuations of global sea levels. On top of that, islands get an extra boost when the weight of that water is lifted from them – a bit like how losing a few kilos puts a spring in your step. Based on what Ali describes as a conservative gauge, the research showed land bridges would periodically open up between islands as the sea fell by 150 metres or more, allowing populations that may have been well on the path to becoming different species, thus incapable of interbreeding, to reconsolidate their gene pools.

“Over the past half-million years, major changes in sea levels, caused by climatic and geological processes, have regularly reconfigured the Galapagos’ geography,” Ali says.

For those of us who get their pleasures in life from the (glacial) accumulation of knowledge, Ali’s research brings its own rewards – not least of which is to fill in another of the few remaining cracks in the Darwinian evolutionary narrative.

If there’s a bigger, wider takeaway, however, it’s this: when faced with an intractable problem, take a breath. Then try looking at it through a different lens.

Tort a lesson

The Galapagos' giant tortoises do not fit the new model. As the archipelago's most iconic animals, these lumbering legends have been under scientific scrutiny more than most - right down to their genetic make-up. Molecular biological data indicates that their gene pools are hugely complex. The evidence suggests that water barriers created by high sea levels were not insuperable; members of the various sub-populations were able to float or swim between islands, sharing genetic material with their not-so-distant neighbours. Moreover, in recent centuries, the tortoises were moved between islands by sailors and pirates who valued them as living stores of much-prized meat, and so their story has an extra level of complexity.

Gaga iguana?

Spare a thought for the pink land iguana (Conolophus marthae). The study by Jason Ali and Jonathan Aitchison highlights an interesting moral dilemma for conservation biologists working on the Galapagos archipelago. Discovered just a few years ago, these supersized lizards are restricted to a small area on the northern Isabela Island. Ali and Aitchison's explanation for this distribution is that they are being hounded off the island (and potentially into extinction) by a less-flashy land iguana cousin by the name of Conolophus subcristatus.

Ali says, "If they are slowly being driven to extinction, do you let nature take its course or do you instead take a sub-group and put them into a zoo from which they can never, ever return?"

To Ali, it's a delicious conundrum.

"Personally, I would opt for the latter," he says. "Perhaps Lady Gaga would be up for sponsoring the effort, as the reptiles would make perfect props in one of her lavish music videos.''