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DNA may unravel an old story of species dispersal

Posted on 04-12-2017 by Toine Baken

Crested newts show enclaves and potential species replacement, with possible implications for humans.

One of the most lively debates on the history of the human species is on how Homo sapiens replaced other human species and conquered the Earth. We know that H. sapiens dispersed across the globe replacing other human species such as the Neanderthal over time. By studying how species disperse and replace one another today, we may learn what happened in the past. And what may happen in the near future!

Today, researchers (with Ben Wielstra from Naturalis as leading author) published a new article in Proceedings Of The Royal Society. They studied whether species enclaves could be a sign of past species replacement. Firstly, they established what an enclave is in this context and how it might be formed. When two closely related species live adjacent in an area and one of the two starts dispersing into the other species’ territory, their geographical ranges will overlap. When the dispersing species has a competitive edge over the residing species, it may cause the latter species to recede. However, a small local population may endure, which is subsequently enclosed by the invading species: an enclave is formed. Since the two species are closely related, they may hybridize and exchange genes. In the area around the enclave, a hybrid zone may gradually establish. The invading species may then over time incorporate part of the DNA of the enclaved species in their genetic code, even when the enclave or the whole enclaved species eventually disappears. Like this, the genes of an organism may unravel an old story about the species dispersal.         

This theory has been tested on crested newts of the genus Triturus (T. ivanbureschiT. macedonicusT. cristatus and T. dobrogicus) in central Serbia. In this area, part of the distribution of T. ivabureschi is detached from the main population and forms an enclave. This enclave is likely formed by dispersal of another species (T. macedonicus) northward into the area of T. ivanbureschi. The possibility of T. ivanbureschi having crossed the area of T. macedonicus to form the enclave is ruled out by the authors, due to the large distance (more than 80 km) between the main distribution and the enclave. The researchers have studied the DNA of the vast amount of 664 Triturus individuals from 251 (!) localities and determined nuclear DNA markers for all four species. For each locality, they determined the percentage of ancestry of the other three species in the specimens with these markers and plotted these data on a map.

Triturus dispersal map

A figure from the original publication, showing the dispersal of the four Triturus species, the enclave of Triturus ivanbureschi and the hybrid zone of mixed DNA.

The results were quite as they had expected: the DNA of the enclaved species (T. ivanbureschi) has been taken up by the invading species (T. macedonicus) in the hybrid zone and this zone also reaches into the initial territory of the invading species. There is much less genetic exchange between T. ivanbureschi and the other two neighbouring species.  

With the theory confirmed by their results on extant crested newts, researchers may have also found more support for the theory of species replacement of – for example – human species such as Neanderthals by Homo sapiens. Would this have taken place, DNA of the superseeded human species would be expected in sapiens DNA today. Finding DNA markers of these human species in our DNA would support the theory of species replacement in humans and very potentially show the possible location of ancient human species enclaves! Last author Pim Arntzen adds: “We know that about 4% of our DNA is of Neanderthal origin. We also know that our species superseded the Neanderthals at some point. But it is hard to determine if there have been Neanderthal pockets of resistance and where they would have been based on our DNA, because recent humans disperse across the globe rapidly.” This makes it hard to locate any possible hybrid zones, if they would have been there.

In addition, these finds may also shed light on how and whether neighbouring species will persist, as man-made habitat alteration and climate change causes species to disperse and territories to overlap. Therefore, researchers from Naturalis continue researching the concept of “moving hybrid zones”.