Evolution has its limits, study finds
A recent scientific study investigating how quickly species evolve when they spread to new geographical areas reveals that some invading species hit evolutionary boundaries, halting their ability to adapt to new environments. Findings from the study could be used to discover the evolutionary limits of invasive species.
“Our results show that evolution can happen quickly when plant and animal species are colonizing new parts of the globe, and humans are causing this to happen at a phenomenal rate,” says Biology professor Chris Eckert. “Every species has geographical range limits, but we don’t know why.”
The researchers studied purple loosestrife, an invasive plant species brought to Canada from Eurasia in the early 1800s on ship ballasts and as garden plants. Since then, the plant has spread up and down the eastern seaboard, as far north as Timmins Ontario, and across North America. It has become one of the most high-profile invaders in North America, where it is reputed to radically change wetlands, push out native plants, and reduce biodiversity.
In the case of purple loosestrife it seems that strong natural selection acting on populations as they spread northward has exhausted genetic variation for important traits. These results provide some hint as to what causes that evolutionary process to stop.
By studying a large number of North American populations from south to north, both in natural populations and in experimental garden environments, the researchers showed that purple loosestrife has undergone striking evolutionary change as it has spread into northern Ontario.
Plants grown from Timmins seeds, even when grown in Virginia near the southern end of the study area, flowered much earlier than their made-in-America counterparts. This is reflective of an evolutionary adaptation in the plant that allows it to survive, flower and produce seeds in colder, less hospitable climates with shorter growing seasons. But the purple loosestrife has paid a price when adapting to a northern climate -- seeds from the northern populations grew smaller, producing less fertile offspring, no matter where they were planted.
Northern populations also exhibited the tell-tale signs of running out of the genetic variation for flowering time that is so essential for natural selection to produce plants adapted to even more northerly climes.
Robert Colautti from the University of Toronto is lead author on the paper, which was recently published in the journal Proceedings of the Royal Society B.
The next step will be to bring these plant populations into the lab and try to make them evolve artificially.