Just how new species are established is still one of the most central questions in biology. In an article in the leading scientific journal Nature, researchers at Uppsala University in Sweden describe how they mapped the genomes of the European pied flycatcher and the collared flycatcher and found that it is disparate chromosome structures rather than separate adaptations in individual genes that underlies the separation of the species.
We have generally defined species by their ability to produce fertile offspring in the wild. Sure, we can force donkeys and horses, or lions and tigers, to mate and produce offspring. But that does not normally happen nor are fertile offspring the normal result.
The main thing that really defines species, at least in my opinion, is how much gene flow – let’s call it sex – there are between populations. If there is ample, than various genomic changes that occur in one population will inevitably occur in the other.
So there is no net accumulation of unique mutations in one population. They stay one species.
But when there is no gene flow for whatever reason – mountain ranges separating populations or just appearance – then each population is able to accumulate DNA changes not present in the other. Eventually the changes are so great that the biological processes needed to produce fertile offspring can no longer mesh.
Here, the researchers found that the genomic sequences of these two bird species are very similar. The reason they are separate species and why gene flow is nil between them are due to changes in specific regions of their chromosomes called centromeres. Not in the mass accumulation of many genetic changes.
Small, localized changes in specific structures of the chromosome may be all that is really needed to create a new species.
Meiosis is the process by which eggs and sperm are produced. Centromeres are very important for the process.
When an egg and a sperm combine, the chromosomes from each parent pair up, with the centromeres on each pair playing a critical part.
If the centromeres do not match, then the whole process stops. So even if the two species wanted to have sex, there would not be any productive offspring.
When the genomic sequences of these two bird species were lined up, there were changes in the centromeres, even though most of the genes were very similar.
This is not unexpected. Centromeric changes have long been seen in different species. This work shows that purely changes in chromosome structures such as centromeres can pretty much create species by themselves.
So, perhaps one of the most important steps in the creation of a new species is not the slow accumulation of lots of genetic changes between two separated populations. Perhaps it is relatively rapid changes in centromeric structures between the populations.
Once this has happened, preventing any gene flow at all between the two groups, then there can be increasing accumulation of changes, eventually producing two very different groups.
Again, not unexpected but nice to see more data demonstrating the principle.