The Galápagos Islands are something of a mecca for wildlife biologists and enthusiasts alike. The journey there is perhaps the closest thing those in the sciences have to a holy pilgrimage.

While Charles Darwin's theories about evolution and natural selection would take decades to fully form, it was on these islands that some of the initial ideas were planted deep within his mind. As a result, these are some of the most intensively studied, best understood ecosystems on the planet. They are also remarkably well protected.

Thanks to the fame of the Galápagos, just about everybody with even a passing interest in wildlife knows about their giant tortoises, flightless seabirds, Darwin's famous finches, and seagoing iguanas.

But even here, evolution continues to surprise. There is an extremely uncommon kind of iguana here hiding in plain sight. Perhaps fewer than two dozen have ever been seen.

In July 2016, I was in an open-top boat called a zodiac with biologist Aaron Pomerantz, speeding towards a tiny uninhabited island called Plaza Sur. We knew that the odds were against us. We were after the rarest of Galápagos stories: one that has been left largely untold.

The Galápagos is home to two groups of iguanas: the terrestrial, or land-based ones, and the marine ones. They are separated by 4.5 million years of evolution, and live very different lives.

There is an extremely uncommon kind of iguana here hiding in plain sight

Galápagos marine iguanas are the only modern lizards that can forage in the sea. They scrape algae off rocks in the intertidal zone. Their flat faces help them gobble up more algae with each bite.

The marine iguanas have large, menacingly sharp claws. These help them grip the rocky seafloor, so that the waves do not thrash them about. They are mostly black, although on some islands they also sport a bit of red or blue.

The yellow land iguanas, as their name suggests, lead a drier lifestyle. Their pointier faces allow them to bite off chunks of cactus without getting needles in their eyes.

But on Plaza Sur, the land iguanas and the marine iguanas do something they cannot do anywhere else: they make babies together.

Plaza Sur is hardly an island at all. It is a tiny speck of dry land spanning barely one-tenth of a square kilometre, its highest point rising just 75ft (23m) above sea level.

It is the only place in all the Galápagos where you can find hybrid iguanas. Our goal was to film and photograph these rarest of rare animals.

In 2013, Alizon Llerena of the Charles Darwin Foundation in Puerto Ayora in the Galápagos conducted a census of Plaza Sur's iguanas. She only found four hybrids.

Only a small part of the already tiny island is even accessible to tourists

By the time we visited in 2016, the entire archipelago was in the midst of a long, multi-year drought. Iguanas everywhere were suffering. Nobody knew if those four hybrids were still alive, or if any new ones had been born.

Our guide, Mario, laughed at us. Since nobody is allowed inside the protected areas of Galápagos National Park without a guide, he has been leading tourists through the archipelago for years. Despite hundreds of day trips to Plaza Sur, he had never seen a hybrid.

What's more, only a small part of the already tiny island is even accessible to tourists. Not only would there have to be hybrids in the first place, they would have to be visible on the part of the island we were permitted to explore.

Howard Snell was the first to notice the hybrid iguanas. Now a professor of biology at the University of New Mexico, he first came to the Galápagos in 1977 with his wife Heidi.

They were both volunteers with the US Peace Corps, there to work on a land iguana conservation project. But they were not having much luck coaxing baby iguanas out of their artificially-incubated eggs.

Eventually he realised he had caught the same strange iguana twice

"We realised there was a lot of natural history that we just didn't know," Snell recalls. "So we decided to use Plaza Sur as a basic ecology site to try to learn a lot of things about iguanas, so that we'd be more effective at the captive breeding."

They soon began to suspect that Plaza Sur was particularly special.

"In one of the years, I think it was probably '79, or maybe it was '80, when we were capturing marine iguanas, we captured this individual that we thought was just an aberrant land iguana," he says. "It looked kind of odd."

At the time, Snell and his team did not pay that much attention to the curious critter. During a land iguana capture, Snell caught another peculiar animal, but this time he dismissed it as an odd-looking marine iguana.

Eventually he realised he had caught the same strange iguana twice, and that he could not quite decide how to classify it. It did not fit in with the marine iguanas, nor did it seem to be at home among the terrestrial ones.

"We realized that this individual was very intermediate in a lot of the characteristics between land and marine iguanas, and we hypothesised then that it was a hybrid," he says. "But of course we didn't know that for sure."

The hybrids are not just genetic mosaics; they are physical mash-ups as well

Over the next few years, the Snells would go on to find another handful of hybrids. Eventually their surveys also turned up some hybrid hatchlings.

Finally, in a study published in 1997 scientists used genetics to confirm what seemed obvious to Snell. "The hybrid status of this morphologically unusual iguana from this island is confirmed," the researchers wrote.

Geneticists also verified another of Snell's predictions. The hybrids were all produced by matings between male marine iguanas and female land iguanas.

There was reason to think that the hybrids were sterile. Nobody ever found any marine iguanas that had a few land iguana genes, or vice versa. If the hybrids had been able to breed, other mixed iguanas would have turned up, such as ones that were 75% marine iguana and 25% land iguana.

The hybrids are not just genetic mosaics; they are physical mash-ups as well.

Like the marine iguanas, the hybrids are mostly black. However, their necks and torsos are adorned with long yellow stripes; evidence of their terrestrial heritage.

The Galápagos continue to tell us how evolution works

They inherited long claws from their seagoing fathers, but they use them for climbing trees, not for clinging to rocks. Land iguanas cannot do either.

Marine iguanas have flat tails that they can use as rudders while swimming. The hybrids have them too, though they have never been seen in the water. It is as if somebody took half a land iguana and half a marine iguana, chopped them up, and stitched them back together.

This description was at the forefront of our minds when our boat reached the wooden dock and we scrambled up the lava rocks onto the island. We had only walked a few metres when Mario stopped us.

There was an iguana, the first we had seen, lazily sunning itself under a cactus. "I don't believe it," said Mario. It was a hybrid.

If the unique colouration pattern was not evidence enough, the iguana's body was also covered with cactus spines, suggesting that it was a cactus climber.

Once they get to be five or six years old, they'll live a long time

When I tell him our story, Snell is not surprised. "There was one that lived near the dock for years and years," he says.

Out of respect for the animal – and because it would have been illegal – we did not flip our hybrid onto its back to see whether it had been marked. But if Snell is right, then this could have been one of the animals tracked by his 24-year study. He caught one hybrid, already several years old, every year for a decade.

"Once they get to be five or six years old, they'll live a long time," says Snell. In all, he can recall finding 16 hybrids between arriving at Plaza Sur in 1977 and the end of his project there in 2000.

How do these strange animals even exist?

Plaza Sur offers something of a Goldilocks situation for iguanas, says evolutionary biologist Gabriele Gentile of the University of Rome in Italy. Circumstances there are just right.

The two species are forced into promiscuity

First, Galápagos iguanas only breed for a short time each year. On Plaza Sur, those seasons overlap. When male marine iguanas arrive at the island ready to mate, they catch the tail end of the land iguanas' reproductive season.

"So female land iguanas that have not yet mated may be forced to copulate by larger, aggressive male marine iguanas," Gentile says. By the time female marine iguanas are ready to mate, the male land iguanas have already done the deed.

Second, the island is truly tiny. Other islands have enough space for the two groups of iguanas to find disparate habitats, but on Plaza Sur their territories overlap. "It is quite easy to see marine iguanas in the more interior part of the island," Gentile says. "The two species are forced into promiscuity."

Even more surprising than our once-in-a-lifetime sighting is that, nearly two centuries after Darwin visited the islands, the Galápagos continue to tell us how evolution works.

"Hybridisation is actually a powerful factor in evolution," says Gentile.

In the past, hybridisation was considered a bad thing

Because hybridisation allows two groups of animals to combine their gene pools, the hybrid animals represent entirely new arrangements of genes. Compared with the usual tricks evolution employs to create new sets of genes, like mutation, hybridisation between species can make changes a lot faster.

This raises an important question for conservationists: what does it mean to protect a species? What should we do with animals that cannot be classified as a unique species at all, like the hybrid iguanas?

Gentile says that, as well as preserving biodiversity, we also need to protect the evolutionary processes that generate biodiversity.

"In the past, hybridisation was considered a bad thing," says Gentile.

There is a good reason for this. When two species begin mating, they end up as one species and their distinct features are lost. The International Union for Conservation of Nature includes hybridisation as a potential risk factor for extinction, alongside more obvious threats like habitat loss, diseases and pollution. As a result, Gentile says, "hybrid animals would not be considered for conservation action."

The hybrid iguanas of Plaza Sur appear to be sterile

This is a reasonable attitude when hybridisation is the result of human meddling. But if hybridisation is part of a natural process, as it seems to be in the case of the hybrid iguanas, then it may be worth preserving. That means preserving the unique habitat on Plaza Sur that allows these unlikely creatures to come into being.

For now, the hybrid iguanas of Plaza Sur appear to be sterile and make up less than 2% of the island's total iguana population. That means they do not pose any threat to the existence of marine and land iguanas.

Big changes would need to occur for hybridisation to impact the evolution of Galápagos iguanas in a way that posed a philosophical problem for conservationists. But that does not mean it cannot happen, says Snell. "Anybody that says they can't reproduce is overstating their knowledge," he says. "But it is unlikely."

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