25 February 2010

Invasive species runs out of evolutionary "steam" as it invades

ResearchBlogging.orgFor invasive plants, flowering time is a trait that may often be under selection during colonization—when a plant flowers determines its climatic tolerances, its vulnerability to herbivores, and its compatibility with the local pollinator community. In a study just released online at Proceedings of the Royal Society B, Colautti and coauthors examined the evolution of this trait in a plant that has swept across eastern North America since its introduction from Europe: purple loosestrife, and found that it may be reaching the evolutionary limits of its invasive-ness.

Purple loosestrife, Lythrum salicaria, may be running out of evolutionary steam as it invades more northerly climes. Photo by Steve_C.
Loosestrife, like many organisms, faces a trade-off in establishing a time to reproduce, between early flowering and accumulating resources for seed production. Early flowering means producing fewer seeds, or provisioning them less thoroughly—but as loosestrife colonizes more and more northerly climes, it will be under selection to flower earlier in compensation for shorter and shorter growing seasons.

But natural selection can only do so much. In order for natural selection to operate on a population, individuals in the population must vary in some trait that affects how many offspring they produce—if all individuals have the same trait value, or the same number of offspring, they'll all have equal chances to contribute to the next generation, which will probably look pretty much like the parental generation. Furthermore, species colonizing new territory may actually lose variation, either because new popualtions may be founded by just a few individuals, or because of the action of natural selection itself. Finally, there may be a point at which plants simply cannot flower any earlier, because they must reach a certain developmental point before reproducing.

Add these up for an invasive species moving north, and you might expect that the most recently-arrived (and most northerly) populations would flower earlier, and have less variation in flowering time, than more southerly populations. Using theoretical and experimental approaches, Colautti et al. show that exactly this process is occurring in purple loosestrife. They first built a mathematical model of natural selection acting on flowering time, which behaved as I've outlined above. They followed this by raising loosestrife seeds from northern and southern populations together in experimental sites located at different latitudes, and found that, even raised in southern conditions, seeds from northern sites grew into smaller, less productive plants. Raised in greenhouse conditions, seeds from southern populations produced plants with a much wider range of flowering times than seeds from northern populations. Together, these suggest that loosestrife has evolved earlier flowering times at northern sites—and may be running out of variation, the raw material for natural selection, as it moves north.

Invasive species often evolve in response to their new habitats, and force native species to evolve in response to their arrival. As they colonize Australia, for instance, cane toads (soon to be a major motion picture) have evolved longer legs [PDF] so as to win the race for unoccupied breeding ponds; and exerted selection on native black snakes to tolerate the toads' defensive toxins and to attack the toads less frequently. Better models of how newly introduced species respond to and exert natural selection may help conservation biologists anticipate the results of biological invasions.


Colautti, R., Eckert, C., & Barrett, S. (2010). Evolutionary constraints on adaptive evolution during range expansion in an invasive plant. Proc. R. Soc. B DOI: 10.1098/rspb.2009.2231

Phillips, B., Brown, G., Webb, J., & Shine, R. (2006). Invasion and the evolution of speed in toads. Nature, 439 (7078) DOI: 10.1038/439803a

Phillips, B., & Shine, R. (2006). An invasive species induces rapid adaptive change in a native predator: cane toads and black snakes in Australia Proc. R. Soc. B, 273 (1593), 1545-50 DOI: 10.1098/rspb.2006.3479

Vellend, M., Harmon, L., Lockwood, J., Mayfield, M., Hughes, A., Wares, J., & Sax, D. (2007). Effects of exotic species on evolutionary diversification Trends in Ecology & Evolution, 22 (9), 481-8 DOI: 10.1016/j.tree.2007.02.017


  1. Great post I enjoyed that thank you. I do question whether producing an equal number of offspring = equal chances of contributing to the next generation. Because of course if some trait is found in some individuals that makes them have better survivability then the proportion of species of surviving would differ. But I may have misunderstood what you were saying?

  2. Daniel -- thanks! So, yes, there could also be some trait that determines offspring survival rather than offspring number, and which varies among families. There are really any number of points in the developmental cycle when natural selection may be operating. But the critical point is that, for natural selection to operate, there must be variation in the traits that determine the ultimate number of surviving offspring, whatever point in the cycle you choose to take a count.