Complex Evolutionary History With Extensive Ancestral Gene Flow in an African Primate Radiation | Molecular Biology and Evolution | Oxford Academic
12 years ago, I wrote a blog post to explain why, because speciation is a process, not an event, we often don't even know it's happened until well after the event when we can see we have a new population with distinct characteristics. I illustrated this with a hypothetical example of monkeys in a forest being split into isolated populations by climate change.
In it I said:
But, gradually, due to climate change or continental drift, or maybe a change in ocean currents, the forest begins to get drier and turn into grasslands, with trees surviving only close to rivers. In other words, the monkey population is broken up into isolated groups which can no longer interbreed because they simply don't come into contact any more. Each group will be free to evolve according to the local conditions in its woodland. Eventually, maybe after a few hundred thousand years, maybe a million or two, these groups may evolve to the point where they not only look different to each other but may not be able to interbreed even if they do meet up.And now, as though to confirm my hypothetical example was close to the real thing, a team of archaeologists and geneticists, led by Axel Jens and Katerina Guschanski of the Department of Ecology and Genetics, Animal Ecology, Uppsala University, Uppsala, Sweden, have carried out a whole genome analysis of 22 species of West African guenons (monkey of the Cercopithecini tribe - one of the world's largest primate radiations) and shown how the different species diverged with frequent gene flow across species boundaries and hybridization events playing a part in the process of radiation and diversification.
So where and what was the 'speciation event'? At what point in the process could an observer say, "Hey! I've just seen speciation occur! It happened when...". In fact, we only know that speciation has occurred retrospectively because, according to our rules of taxonomy, failure to interbreed means they are now different species. Maybe if we had been able to examine them a hundred thousand years ago we might have found that they could still interbreed. Maybe we would have found an incompletely speciated 'ring species'.
There was no sudden emergence of a new species; no sudden branching of the 'tree of life'; no mutation which brought a new species into being and no 'macro-evolution' event. There was no event which creation pseudo-scientists proclaim to be impossible and which they claim has never been seen. All there was was a slow accumulation of difference, directed by natural selection with each group doing nothing but struggling to survive and reproduce with the ones which left the most descendant contributing the most genes to the gene-pool.
Now, take the same scenario, only this time the climate changed again after a few tens of thousands of years and the isolated scattered groups could once again mix freely. But this time maybe they had not diverged sufficiently to prevent interbreeding, or maybe one group now had a significant advantage over the others. In these cases, the group with the genes which gave them greater success would come to dominate and possibly replace the others.
Is this speciation? Is this the point at which we can say a new species arose and the 'archaic' form went extinct? Or is this merely evolution of the entire species? Were those groups isolated for a few thousand years new twigs on the monkey branch of the tree of life, or were they merely groups of individuals with the potential to become new species, but which never quite made it? Certainly, the day they came back into contact, nothing happened to their genes. It was not a change on their part which caused them to re-establish contact. It was the environment which changed.
Rosa Rubicondior: Evolution - Making a Monkey (4 July 2012)
The team have published their findings, open access, in the journal Molecular Biology and Evolution. In it they say:



















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