According to evolutionary theory, a new species arises whenever a particular lineage splits off into several. How this happens and what criteria to use in defining a species, however, has been a topic of discussion ever since Darwin’s seminal tome – currently, there are more than two dozen competing accounts of what constitutes a distinct taxonomic entity. This is called the “species problem”.
As if to muddy the waters even further, a new study in the leading scientific journal PLoS Biology suggests that the image of speciation as a “tree of life” – where every individual branch represents a distinct species – is far too simplistic. A bush, authors of the new paper claim, would be a more accurate portrayal.
In the study, which expands upon the work done a bit less than a year ago, where a large consortium of scientists had sequenced the genome of around 50 bird species, Alexander Suh and Hans Ellegren – both researchers at the Uppsala University and members of the consortium – analysed it through a new method that focuses on the so-called retrotransposed elements, or “jumping genes”.
“We can see that the very rapid rate at which various bird species started evolving once the dinosaurs went extinct, i.e. around 65 million years ago, meant that the genome failed to split into separate lineages during the process of speciation”, said Hans Ellegren.
The phenomenon has previously been explained theoretically and is a result of the genetic variation passing from one species to another. If new species then continue to evolve quickly, random chance can end up determining which original genetic variants end up in each lineage – a phenomenon called “incomplete lineage sorting”.
Prior to this study, biologists were unable to find instances of this occurring far back in time, which prevented them from determining whether it played an important role in the process of speciation, or whether it was just a relatively minor fluke.
Armed with the new method, Suh and Ellegren had shown that, say, a cuckoo may be more closely related to a hummingbird than to a pigeon in one part of its genome, but not in others. This is the first time researchers had succeeded in documenting and quantifying incomplete lineage sorting far back in time, proving its importance in evolution.
“The more complex kinship patterns that result from this phenomenon mean that the Tree of Life should often be understood as a Bush of Life,” concluded Ellegren and Suh.