Fear of flying -- in plants
"Every one is familiar with the difference between the ray and central florets of, for instance, the daisy... But with respect to the [two types of] seeds, it seems impossible that their differences in shape...can be in any way beneficial"--Charles Darwin
The theory of evolution is famously linked to the Galapagos Islands, but this week's paper "Rapid evolution of seed dispersal in an urban environment in the weed Crepis sancta," published in Proceedings of the National Academy of Science, studied much smaller "islands." In an urban environment dominated by concrete, patches of soil around sidewalk trees (below left) are among the few places where plants can grow.
Photo credits: Gilles Przetak and Eric Imbert.
Members of the daisy or sunflower family (Asteraceae) often produce two types of seeds (above right) on the same disk-shaped composite flower head. Seeds from the center of the disk are light in weight and plumed, so they are easily dispersed by wind. Those from the outer edge of the disk are heavier and not plumed, so they tend to fall near the mother plant. Although Darwin apparently failed to see the benefit of having two types of seeds, this kind of diversity acts as a form of bet-hedging. Wind dispersal of seeds over a wide area decreases the chances that all of a plant's offspring will be killed.
Then why not disperse all of the seeds? Because, given that the mother plant managed to reproduce -- many plants don't -- conditions near the mother plant may be better than where most wind-blown seeds might land. This was particularly true in the study discussed here. Earlier, Jonathan Silvertown pointed out, in an essay titled "When plants play the field," that the ratio of the two seed types changes in beneficial ways with changes in flower head diameter. The area of a disk increases four-fold as the circumference doubles, giving proportionally more of the wind-dispersed central seeds. So the plant will always drop some seeds in the same place that it managed to reproduce. But if favorable conditions lead to larger flower heads, more seeds will be dispersed by wind over a larger area, where they can compete with other plant's seedlings rather than with each other.
So, without any genetic change, this disk-size dependence adjusts the ratio of dispersing to nondispersing seeds to match current conditions. But what if conditions consistently favor more or less seed dispersal? Can this ratio also evolve, with a genetic change over generations?
Pierre-Olivier Cheptou and colleagues in Montpelier, France, hypothesized that urban environments would select for fewer dispersing seeds. They showed that most wind-blown, i.e., dispersing seeds fall far from the mother plant. In an urban environment, where the mother plant may occupy the only patch of soil for many meters, most dispersing seeds will land on concrete pavement and die. Therefore, if there are versions of genes that decrease the ratio of dispersing to nondispersing seeds, relative to alternative versions of the same genes, plants with those versions should have more of their seeds survive, making those versions (alleles) more common in each subsequent generation. (Making smaller diameter flower heads, which would generally allow making more of them, would be one way to achieve this.)
To test their hypothesis, the authors collected seeds from the soil "islands" in the urban environment and from outside of town, where dispersing seeds would be more likely to land on soil than concrete. They grew all the seeds in a greenhouse and compared the ratios of dispersing to nondispersing seeds produced there. (By growing seeds from both sources in the same environment, they mainly saw genetic rather than environmental differences, although there is a small possiblity of "maternal effects." For example, if seeds from urban environments had higher lead content, could that effect the development of flower heads on plants grown from those seeds, even apart from any genetic differences? Probably not.) As predicted, seeds from urban islands grew into plants that made relatively few dispersing seeds. Interestingly, they did not find a correlation between flower head diameter and the ratio of seed types. With additional measurements and modeling, they calculated that this genetic difference between city and country plants would only have taken about 12 years to evolve.
Evolution is widely misunderstood. First, some species may have changed little in millions of years, at least in overall body shape, but significant changes can occur in only a few generations (hours for bacteria, years for annual plants), especially in a new environment. Second, just as a river flowing downhill may not always reach an ocean, let alone a particular ocean, evolution responds to current conditions, rather than pursuing some long-term goal. In this example, the descendants of seeds that blow from city to country, or vice versa, will start evolving in the opposite direction from their recent ancestors. And, finally, evolution does not necessarily have anything to do with monkeys! On the other hand, the principles that apply to plants often apply to animals as well, as the quotation below shows.
"Madeira, like many oceanic islands... is much exposed to sudden gales of wind... insects which flew much would be very liable to be blown out to sea and lost. Year after year, therefore, those individuals which had shorter wings, or which used them least, were preserved...." - Alfred Russell Wallace