THE sexual habits of wild sunflowers have resolved a century-old debate in evolutionary biology. The flowers have revealed that new species can indeed form when two species mate to produce hybrid offspring.
In fact, hybridisation causes such an explosion of genetic variation that sunflower hybrids become new species capable of invading novel ecological niches. That suggests hybridisation may be more important than genetic mutations in causing rapid, widespread evolutionary transitions.
Biologists agree the formation of new species, or speciation, is driven by two processes. In ecological selection new species form as plants or animals adapt to changing environments. And in sexual selection, preference for certain traits drives speciation, for instance, when females choose only to mate with males of a certain colour.
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Both processes select new variations of genes. These can appear either through mutations, or by hybridisation. Many biologists argue that because hybrids are mostly infertile, hybridisation introduces only transient genetic variation. But Loren Rieseberg of Indiana University in Bloomington, Indiana, and his colleagues have now conclusively shown that the tiny proportion of fertile hybrids can produce enough variation to lead to new species.
The researchers studied five species of annual sunflowers of the genus Helianthus. Two species, H. annuus and H. petiolaris, grow across central and western US. The other three, H. anomalus, H. deserticola, and H. paradoxus, grow in extreme environments such as the dry, sandy soils of Nevada and Utah, or the salt marshes in west Texas. Genetic studies have shown that these three hardy species, which originated between 60,000 and 200,000 years ago, are hybrids of the first two parental species. But Rieseberg’s team wanted to know whether they could cross the parent species and recreate the extreme traits present in the hybrids – such as decreased mineral ion uptake and increased leaf succulence – that allow them to survive in salty soil.
The team crossed H. annuus and H. petiolaris plants, and backcrossed the resulting hybrids with the parent species twice. While no single plant displayed all the extreme traits present in any of the ancient hybrid species, all the 11 extreme traits of H. deserticola, 11 of the 13 extreme traits of H. anomalus and 10 of the 15 extreme traits of H. paradoxus were present in a population of laboratory hybrids. In short, the artificial breeding programme showed that hybridisation could account for a wide range of extreme traits seen in the ancient hardy species.
To see if the lab hybrids could adapt to extreme habitats, Rieseberg’s team transplanted them into the salt marshes where H. paradoxus grows. Preliminary results show that increased leaf succulence and the ability to exclude mineral ions allowed some of the hybrids to survive, whereas all the individuals of the much less hardy parent species died.
Computer simulations suggest these hybrids can evolve into new species within 50 to 60 generations, says Rieseberg. “A blink of an eye in terms of evolutionary time.” And this ability to bring about dramatic changes may allow hybridisation to drive speciation more quickly than random mutations. “What if an adaptation requires changes in 5 genes? How long do you have to wait before changes occurred in all 5 genes?” says Rieseberg. “Hybridisation can create variations in many genes simultaneously.” Even though the parent species each have some of the genes for a given trait, it’s the gene combinations that turn up in hybrids that make extreme variations possible (see Graphic).
Rieseberg believes this is really the first unambiguous example of how hybridisation has contributed to some major adaptive changes. Michael Arnold, an expert on hybridisation and evolution at the University of Georgia, agrees. “It’s a very significant finding,” he says. “It tells us that hybridisation can lead to new ecological forms.”