Dean Nicolle struggles to name his favourite species of eucalyptus.
"If I had to put a name down, my favourite species would change every day," he says.
"It depends upon whether I am in the field, or if I'm out in the wild, or what's in flower or the colours of the bark at the time."
But there is one species, he admits, that has a particular place in his heart at any time of the year because it is so rare.
The Seppelt Range gum (Eucalyptus ceracea) is only found in a couple of patches in the remote Kimberley region of Western Australia.
The small, scrubby tree has big balls of orange flowers set against greyish-coloured leaves and orange fibrous bark.
"I've only seen it in the field once," Dr Nicolle says.
And it is one of just a handful of species he doesn't have growing on his arboretum in South Australia.
Dr Nicolle began collecting eucalypt seedlings nearly 30 years ago when he was 16.
"It's difficult to put my finger on [what drew me to] eucalypts … I guess it's something that you see everywhere in the landscape," he says.
"One of the things that fascinated me was the sheer number of species."
Around 900 species of eucalypts of all different shapes and sizes dominate Australia's landscape, from the alps to the arid interior, and from Tasmania to the tropics.
There's also a handful of species that only live in South-East Asia.
Dr Nicolle has now planted about 95 per cent of all eucalypt species — even the cool-climate snow gum (E. pauciflora) — on his property at Currency Creek in the rain shadow of the Mt Lofty Ranges.
His top pick at the moment is the lobe-fruited mallee (E. preissiana) from Western Australia.
"It's one of those eucalypts that people don't think of when they hear the name gum tree or eucalypt," he says.
His arboretum was proposed as a location for the ill-fated movie Eucalyptus, based on the novel by Murray Bail.
But more importantly, Dr Nicolle's collection sheds light on the genetic roots and branches of the eucalypt family tree, and how it became an evolutionary success story that continues to grow.
By gum! Where did they come from?
Eucalypts are part of the myrtaceae family, a sprawling group of nearly 6,000 species of plants that include bottlebrushes, paperbarks, lilli pilli, and guava.
Scientists have been piecing together the eucalypt family tree since French magistrate and keen botanist Charles Louis L'Héritiert de Brutelle coined the name in 1788.
He used the term "eucalyptus" to describe the hard-covered flower buds of a plant collected from Bruny Island on James Cook's third voyage, and called the new species Eucalyptus obliqua.
The genus Eucalyptus, which includes the bulk of species we see today, was the first of the three main groups botanists used to describe eucalypts based on the shape of their flowers and fruits.
The second group is Angophora, which includes about 12 species from eastern Australia, such as the stunning Sydney red gum (A. costata).
The third (and hotly contested) group is Corymbia, or bloodwoods.
This group, which was split off from Eucalyptus in 1995, includes species found across the north such as lemon-scented gum (C. citriodora) and several species of ghost gums in central Australia.
There are also a few species in south-west Western Australia, such as the red flowering gum (C. ficifolia).
Commonly known as "gum trees", the species in these three eucalypt groups grew to dominate our continent like no other plant group in the world.
But where did they come from?
There are very few clues in the fossil record, says David Cantrill, executive director of science at the Royal Botanic Gardens in Victoria.
"The fossil record is pretty poor really, given how important they are in Australian vegetation," Professor Cantrill says.
The oldest-known fossils are impressions of fruits, flowers and leaves that were sandwiched between layers of volcanic sediment 52 million years ago.
"It's got such exceptional detail … that allows you to say 'this is definitely eucalyptus'," Professor Cantrill says.
The trees lived around the edge of a lake in the balmy Eocene period in what is now Patagonia, which was once connected to Antarctica and Australia in the supercontinent Gondwana.
"Just because the South American ones are the oldest occurrence [in the fossil record], it doesn't mean that's where they came from," Professor Cantrill says.
So could these trees, or ancestors of these trees, have lived in Antarctica before the continents split?
It is a possibility that Professor Cantrill and his team are exploring, but so far there have been no eucalypts discovered among the fossil plant species from Antarctica.
Fossil evidence of eucalypts in Australia is also very patchy.
"There are quite a few localities where fossil leaves have been called Eucalyptus, but as there are no flowers or fruits, these records are considered doubtful," he says.
They are also much younger than the Patagonian fossils.
"Most of the [Australian] records are from sediments that are younger than 30 million years," he says.
Family tree connections
While we don't know where eucalypts originated, DNA studies are starting to fill in the gaps about how different species are related.
Andrew Thornhill sequenced DNA of more than 700 eucalypt species — including samples from trees in Dr Nicolle's arboretum — and their close rainforest relatives, which are divided into two groups: mesicalypts and newcalypts.
These ancient rainforest relics include anbinik (Allosyncarpia ternata) in the Northern Territory, Vic Stockwell's puzzle (Stockwellia quadrifida) in Queensland, as well as species in Papua New Guinea and New Caledonia.
"These are like the leftovers or the last living relatives of what probably the original eucalypts looked like," says Dr Thornhill of the University of Adelaide and State Herbarium of South Australia.
The trees we know as gumtrees in the Eucalyptus, Corymbia and Angophora lineages broke away from their shared common ancestor with the mesicalypts and newcalypts around 58 million years ago.
Some of the deepest Eucalyptus lineages go back between 40–44 million years.
They include closely related species such as yellow tingle (E. guilfoylei) from Western Australia, and tallowwood (E. microcorys) from northern New South Wales.
The fact these two trees live in isolated pockets on different sides of the country suggests there were other species across Australia back then that are now extinct, Dr Thornhill says.
"To actually figure out where they started is almost impossible without having a fossil record."
Many West Australian species, such as jarrah (E. marginata) appear in the DNA tree around 30 million years ago.
But the bulk of the species we see today evolved in eastern Australia between 10 and 2 million years ago.
Some of the youngest species include the tall forest trees in south-east Australia and Tasmania, such as the mighty mountain ash (E. regnans) and the Sydney blue gum (E. saligna).
"It is likely that once the eucalypts hit new territory they exploded, plus the continent kept becoming warmer and drier, giving them the chance to outcompete their rival trees," Dr Thornhill says.
The east and west coast species also became separated by the gentle uplift of the Nullarbor Plain from the sea floor, which led to pockets of localised diversification.
"Esperance is still one of the most diverse areas for eucalyptus," Dr Thornhill says.
The Blue Mountains and the border of New South Wales and Queensland are other diversity hotspots.
There are also some species of eucalyptus, such as the rainbow gum (E. deglupta), that are native to South-East Asia.
The majority of these species have only evolved in the last 1-2 million years.
"They are recent escapees from Australia," he says.
Meanwhile, genetic mapping of the other two genera — Corymbia and Angophora — shows species in each genera diverged from each other between 30 and 6 million years ago.
The older lineages of Corymbia, like Eucalyptus, include Western Australian species such as the red flowering gum (C. ficifolia) and marri (C. calophylla) — the two species made famous by May Gibbs' gumnut babies.
Younger species include the lemon-scented gum (C. citriodora) from north-eastern Australia.
But there are anomalies in how some species are related to each other, fuelling debate as to whether Corymbia needs to be split up into two groups, merge with Angophora, or vice versa.
"The sticking point is that the genetic data does not throw out a conclusive answer," Dr Thornhill says.
"Sometimes Angophora is inside Corymbia and sometimes it is next to Angophora.
"If it was up to me … [they] should be merged."
Masters of adaptation
The success of eucalypts comes down to their ability to rapidly adapt to, and even change, the nature of environments.
They've developed strategies to attract local pollinators.
The general rule of thumb (although there are always exceptions) is that species that have flashy red, pink, orange, or yellow flowers attract birds and small mammals.
"Generally species that are pollinated by birds or small mammals, such as honey possums, have larger, more colourful flowers, but fewer of them," Dr Nicolle says.
The flower size also relates to the size of the fruit (or gum nut as many people mistakenly call them).
The lower growing a species is, the more woody and larger the fruits are.
This may be a strategy to protect the plant against fire, Dr Nicolle says.
"Larger forest tree species where the capsules are higher up, or where they shed their seed annually, have smaller flowers."
Many species in eastern Australia have small white or cream-coloured flowers that attract insects.
Eucalypts also evolved to have essential oils in their leaves.
"Essential oils probably evolved as a defence mechanism," says Carsten Kulheim of the Michigan Technological University.
But we're only just starting to understand the genetics behind these oils, Dr Kulheim says.
"There's really not enough information on what type of terpenes (the main component of essential oils) are present in what species."
River red gums (E. camaldulensis), for instance, differ greatly in their production of essential oils depending upon where they live.
"That makes sense because Western Australia has different insects and herbivores than NSW or southern Victoria," he says.
Along with understanding how these chemicals might play a role in helping to protect the plant against diseases, Dr Kulheim believes they could also be important sources of biofuels in the future.
But the adaptation that probably gave eucalypts the upper hand over their rainforest cousins was the ability to harness fire.
On one hand, the chemicals in their leaves make them very flammable. But on the other, most species bounce back from fire from buds under their bark or at the base of their trunk.
"It's a bit of a chicken-and-egg question, which came first: The high concentration of essential oils or the epicormic sprouting?" Dr Kulheim says.
Dr Thornhill agrees.
"It's possible they did the old divide and conquer of the rainforest.
"Slowly, they just outcompeted all the other rainforest trees, dumped their leaves and set themselves on fire, then spread a bit further.
"The more they take over, the drier it gets too."
They also breed with each other, which helped them spread like wildfire.
Evolution happening before your eyes
Unlike many plants, different species of eucalypts can cross-breed to produce progeny that eventually become new species or subspecies.
"Other groups of plants tend to not hybridise because they've got incompatibility receptors on the plants or in the style and stigma that prevent hybridisation," Professor Cantrill explains.
The flowering seasons of different eucalypts often overlap, creating an evolutionary boom time with the help of the birds and the bees.
One of the best places to see this happening in front of your eyes is on Dean Nicolle's arboretum, says Justin Borevitz of the Australian National University.
Professor Borevitz and his team is analysing the genetics of all the species growing at Currency Creek to understand how adaptable eucalypts will be in the face of changing environmental conditions.
"It's kind of a good news story that within and between closely related species, there's a tremendous amount of adaptive potential and genetic diversity."
For example, yellow box (E. melliodora) and white box (E. albens), which are some of the youngest lineages in the family tree, have a history of hybridisation and share a lot of genetic diversity.
"When you plant seed, even off one tree, there are so many different things that come out. They're not pure.
"I always joke with people that our next generation Currency Creek is going to have more diversity than ever existed in the evolutionary history of eucalypts, because Dean planted all these trees next to each other and they've been hybridising."
So just when does something become a new species to add to the family tree?
This can be tricky to work out without genetics because eucalypts are the ultimate shape-shifters.
For example, the Tasmanian blue gum (E. globulus) is the second-tallest species of eucalypt. It can grow up to 100 metres tall in sheltered valleys, but on the coast it is a scrubby tree about 2 -3m tall.
While it has developed traits to survive in different environments through natural selection, it is still the same species.
But over time, different populations of the same species can develop separate genetic lineages if they become isolated and no longer breed with each other.
By growing different species of trees under the same environmental conditions at Currency Creek, scientists are able to tease out how much of a plant's appearance is due to genetics and how much is due to the environment.
"Occasionally there's a new species named by a botanist on the basis of it looking very different in the wild," Dr Nicolle says.
"But when you grow it with closely related species in a common garden such as an arboretum and it turns out to be exactly the same, then you certainly disappoint the botanist."
