At the tail end of the Pleistocene, history's last ice age was violently shaping the earth. Tremendous glaciers upheaved mountains and entombed continents. When the ice finally receded, a radically new world emerged. And all the while, the diminutive kiwi, New Zealand's most symbolic bird, was puttering around the underbrush.
Kiwi are among the most mysterious birds alive, both genetically and behaviorally. Regarding their lineage, conflicting theories have divided biologists and even countries. Now, brand new evidence, published in the Proceedings of the National Academy of Sciences, could topple even the most conventional beliefs about kiwi genetics. What was assumed to be a small pool of species is actually teeming with diversity. According to researchers at the University of Toronto Scarborough, there are three times as many kiwi species as we think, and they "explosively" diversified much later than records seem to suggest.
"We've been gathering kiwi blood samples from New Zealand over the past 20 years, so this data set has taken a long time to develop," Jason Weir, an associate professor of ecology and evolutionary biology at the University of Toronto Scarborough, told me.
"Traditionally, we've sequenced only one or two genes, but now we're able to sequence thousands and thousands, randomly spread across genome."
Scientists have been chasing the kiwi's ancient origins for more than century. On the outside, this little ratite, or flightless bird, looks physically primitive. It's raptor-like claws and elongated beak appear straight out of the Cretaceous. Archaic mutations, such as color blindness and acute smell receptors, make the kiwi a genetic oddball. Yet, scant fossil evidence places them firmly within the Holocene, which arose 12,000 years ago and, depending on who you ask, persists through today.
Previous investigations of kiwi DNA gave rise to the five species theory. Today, we have the brown kiwi, great spotted kiwi (roroa), little spotted kiwi, rowi, and tokoeka. All of these are believed to have diverged from a common ancestor, sometime before the Middle Pleistocene. During this epoch, periods of global cooling are thought to have stifled biodiversity everywhere, but the effect of glaciation on some species remains largely unknown.
Using sophisticated sequencing techniques, the team saw another history unfold for the kiwi. According to thousands of genetic markers, traced from 300 blood samples and fossilized material, it's possible that 17 species or subspecies of kiwi once existed—11 of which are still alive.
"In almost all studies during the 1990s, when genetic sequencing was just getting going, people were lucky to get one or two markers. But with a genome wide perspective, we're able to see that kiwi diverged in the past 800,000 years, which is very recent for evolutionary time scales," Weird added.
Millions of years ago, as glaciers ebbed and flowed across New Zealand, the birds were forced into isolated pockets along the North and South Island. Here, they diversified and evolved new traits. Instead of slowing down, the kiwi's speciation rates increased fivefold. Not even Darwin's finches, which are considered the hallmark of adaptive radiation, saw diversification at this scale, the study notes.
The team hasn't determined if the new lineages are entire species or subspecies. Genetically, they're different, which renders them distinct, according to phylogenetic species concepts. But some scientists define species based on whether they interbreed freely with other populations. If these kiwi remained reproductively isolated, they could qualify as a species; however, that's something researchers just don't know yet.
Another question that still needs to be answered is how, morphologically or behaviorally, these groups varied from one another. Kiwi tend to be cryptic animals, which means biologists can't always tell them apart just by looking at them. Some things that future studies could reveal is whether these birds produced different vocalizations or smells. "We don't know what these genes are coding for," Weir said, "and we're not investigating that yet."
Regardless, these findings could offer major conservation benefits for New Zealand's kiwi. Two kiwi species are currently endangered—the northern brown kiwi and the rowi—while two others are vulnerable to extinction. One of the biggest threats these birds face is predation by feral cats, weasels, rats, and other animals that feed on young kiwi.
Weir told me that experts at New Zealand's Department of Conservation hope to use his data to inform their breeding decisions. If new kiwi species or subspecies do exist, should they be treated as different conservation units? What are the potential risks of mixing populations? Hypothetically, if one group is bred with another, their separate lineages could become genetically extinct.
When I asked if we're going to see the declaration of new kiwi species in the near future, Weir was only speculative. "Taxonomy is always highly controversial because people have different species concepts," he said.
"Our paper's going to add a little more fuel to that controversy."