Every spring in the Sierra Nevada foothills, evolutionary history unfolds in the form of a delicate pink flower. The rugged landscape of southern California is home to two emerging species of Clarkia xantiana that coexist in the same foothills despite diverging nearly 50,000 years ago. They’re so closely related they can still technically reproduce. But does that mean they do? And if so, what does it mean for the species’ past, present and future?
According to Shelley Sianta, a postdoctoral researcher advised by faculty Dave Moeller and Yaniv Brandvain, it depends. The two species (or subspecies, depending on who you ask) are what Sianta calls reproductively isolated in that they have staggered flowering times and distinct mating systems – one self-pollinates while the other “outcrosser” relies on pollinators. Hybridization is rare. But in a new publication for PNAS, the team also suggests that climate variability can shift or weaken barriers between the taxa from year to year.
“Precipitation can affect things like when each species blooms,” says Sianta. Drought, temperature and rain all play a role in determining when (and how many of) each species germinates, blooms and distributes their seeds. “They might flower closer together one year and then the next year have flowering times that are further apart. There’s lots of fluctuation over time.” Their findings illuminate new clues pertaining to the complex variables underpinning the species’ origins.
“The evolutionary transition from pollinator-dependent to self-pollinating is one of the most common transitions in flowering plants,” says Moeller, whose lab seeks to identify the ecological and evolutionary mechanisms behind the diversity of flowering plants. “What we don’t know is how this transition influences the evolution of new species.” In the case of Clarkia xantiana, Sianta and Moeller suspect the difference in mating system is ever so slowly driving the two species apart. But the dynamics affect each species unequally.
Their team’s genomic analysis of multiple Clarkia populations allowed them to trace the genetic history of about 200 Clarkia plants. They used shared genetic markers to provide insight on gene flow between multiple populations of the two taxa. “Genetic material flows much more from one species to the other than the reverse,” says Sianta. Their findings revealed more genetic variants transfer from selfer genomes to the outcrosser genomes than vice versa. Using these data they concluded Clarkia xantiana hybrids are more likely to mate with outcrosser flowers. “The outcrossers have retained larger flowers that are attractive to pollinators,” notes Moeller.
Sianta and Moeller are still investigating how the persistent selfer genetic variants affect the outcrossing Clarkia populations. But gene flow from one species to another, called introgression, affects more than just flowers. In humans, for example, neanderthal DNA that introgressed into human populations lingers even today and affects things like autoimmune disorders.
The complicated factors that go into the fate of a species can seem overwhelming. “Part of me feels like one of the biggest implications of my work is appreciating how evolutionary forces vary over time,” says Sianta. Earth is undergoing rapid change brought on by anthropogenic (human-caused) forces, making it possible for scientists like Sianta to observe evolutionary shifts happening over shorter timescales. As extremes in climate fluctuate from year to year, changes could drive emerging species apart or bring them closer together.
“It’s really interesting to think about the fact that environmental changes have the ability to weave into these dynamics,” she says. “Having a connection with nature and understanding how these simple processes add up really makes it feel important.” – Adara Taylor