Species formation typically happens slowly through new mutations over millions of years, but there are exceptions.
The Galapagos Islands is one of the best known; there, for example, 15 bird species are thought to have evolved in just three million years. A series of lakes in eastern Africa appears to hold the record, however.
Research shows that one group of fishes, called cichlids, has evolved hundreds of species in less than a million years – faster than any other group of animals.
In a new study, a team led by Matthew McGee from Australia’s Monash University has used an evolutionary network of all cichlids both to verify the speciation rates in the lakes and to start to find an explanation for how they occur.
With colleagues from Tanzania, Uganda, Switzerland, the US and the UK, McGee sequenced 100 genomes from the fastest evolving location, Lake Victoria, where as many as 500 species formed in less than 10,000 years.
They took a different approach, however, as described in a paper in the journal Nature.
When studying genomes, researchers usually focus on single-letter changes that occur in DNA strands, called single nucleotide polymorphisms or SNPs. McGee and colleagues chose to assemble each individual species’ genome into a large “pan-genome”.
This contains information from all of the genomes, including small changes but also larger ones that occur when mutations insert or remove chunks of DNA.
The researchers say its data revealed that the new species can form very rapidly if they first hybridise with distantly related species and exchange genetic material.
Bacteria are known for their ability to exchange DNA between distantly-related species, allowing them to rapidly acquire traits such as antibiotic resistance, but this process was thought to be uncommon in animals.
The researchers discovered that many cichlids in Lake Victoria contained DNA sequences introduced via hybridisation from an ecologically similar group of cichlid species from southern Africa that are separated by as much as 10 million years of evolution.
“Our results suggest that the combination of ecological opportunity, sexual selection and exceptional genomic potential is the key to understanding explosive adaptive radiation,” the authors write in their paper.
To account for the ability of different species to exchange DNA with each other, they used networks instead of branching trees.
Non-treelike evolution is often neglected for technical reasons, McGee says, but it is important to think about evolutionary relationships as a complex network “or we may miss critically important information”.
Network analysis reveals “fundamentally non-treelike evolution” in the lakes, the researchers write.
They conclude: “Our insights into the factors that underlie cichlid diversification in general, and the special circumstances that permitted the exceptional rates of explosive adaptive radiation in Lake Victoria, make us hopeful that combining genomically informed speciation research with macroevolutionary analyses of diversification will soon enable a comprehensive understanding of why some clades produce spectacular radiations while others do not.”
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