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?