Munch Lab

Elephants and the mesh of life

The evolution and diversification of life are nicely depicted by a tree – with the original first living organism at the root and the main branches splitting into still smaller branches and twigs that eventually lead to the living species at the leaves. But what about all the species that went extinct, you may ask. The dinosaurs found themselves in dire straits 65 million years ago. In fact, species go extinct all the time. Whereas recently emerged species most likely persist, species that arose hundreds of million years ago have most likely gone extinct since then. This continuously removes twigs and branches from the tree of life, which is why it has ended up looking like, well, a tree.

Elephants are an excellent example of a group of species where some have gone extinct and others not. There are three living species: the Asian elephant and the forest and savanna elephants in Africa. However, we would only need to travel back 100.000 years to meet another three species of elephants: The straight-tusked elephant, the woolly mammoth, and the Colombian mammoth. Fortunately, it has been possible to obtain enough DNA from bones of these extinct species, that their genomes could be sequenced. Over the past year or so, I have been part of an effort led by the excellent Elle Palkopoulou, to analyze the full genomes of individuals from these six species.

In the study that we publish in PNAS today, we show that the species tree that relates these six species is crisscrossed by connections between the branches, making it look more like a mesh than a tree. When one species split into two, they diverge only slowly, and it seems to take at least a couple of million years to accumulate enough genetic differences that hybrids are no longer fertile. Until then, hybridization may happen, transferring genes from one species into the other. The most important finding in the paper is that elephant species hybridized extensively even a long time after they initially split apart. The most prominent example is the straight-tusked elephant (the model in picture). Most of its genes can be traced back to the initial split from the ancestor to African elephants, but a large proportion is derived from hybridization with forest elephants and with Mammoths. I bet that as more species groups are studied using full genomes we will see many more examples where hybridizations and even fusions of species make the species tree look more like a mesh than a tree.


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