Evolutionary theory holds that one of the primary drivers of speciation – the divergence of a single species into two or more – is physical isolation.
If a single population is divided by an insurmountable barrier – a sea, for instance, or a mountain range – then the separate sections will likely take different evolutionary paths.
Now, however, researchers have found that speciation can be propelled by simple differences in environmental temperature. Physical obstacles are not necessary.
A team of scientists from South Africa and Australia looked at a common species of saltwater fish, the Knysna sandgoby (Psammogobius knysnaensis), which is found in coastal waters around South Africa.
Externally, any one Knysna sandgoby looks much like any other, but when the researchers – headed by Peter Teske from the University of Johannesburg and Jonathan Sandoval-Castillo from South Australia’s Flinders University – delved into the genetics of the fish they found a different picture.
Looking at a total of 8532 short genetic sequences generated from 109 individuals, the scientists noted key differences in protein-coding genes that were strongly associated with temperature tolerance.
These differences tended to correlate with the average water temperature of the locations from which the fish were sourced. Water temperatures in South African form distinct bioregions, and run along a gradient ranging from quite cool to quite warm.
The differences in the genetic sequences of the fish indicated that subpopulations were undergoing changes that influenced the fitness of the animals in terms of the water conditions in which they lived.
In classic evolutionary style, the fish with better genetic adaptation to particular temperatures were more likely to reproduce in those bioregions, resulting in each adaptation becoming more common. Fish with different genetic temperature tolerances were less likely to thrive.
In other words, the researchers concluded, the Knysna sandgoby population was undergoing the early stages of speciation – driven by the exclusionary effects of heat gradients.
“Each regional goby population is already adapted to its preferred thermal habitat, and migrants that disperse into nearby regions that are too warm or too cold will not do as well as the locals,” explains Teske.
At this stage, the changes affect only temperature-adaptive genes, with the rest of the goby genome identical across all populations.
“Over time, the remainder of the genome will ‘catch up’ with the temperature-selected genes, and even later, the new species will also change morphologically,” says Teske.
“Only then will they be recognisable without the help of genetic methods.”
The research is published in the journal Proceedings of the Royal Society B.