Scientists unravel the evolutionary history of "helicopter" seeds.
If you spent your childhood in any way similar to my own, you probably remember searching the ground underneath mighty conifers or maples and picking up “helicopters” to toss into the air. Their dizzying descent makes for an curious sight and occasionally you could get lucky enough to find one with a minuscule shoot to examine, popping right out of the seed attached to the wing.
In addition to being a lazy way to kill time on a spring day, there is another interesting aspect of these helicoptering seeds: their evolutionary story.
Last week, a team of scientists at the University of California, Berkeley published a study in Paleobiology detailing the evolution of wings as a mechanism of seed dispersal in conifers. They found that, although all winged seeds of modern tree species have settled on a one-wing morphology, this design was not the original.
Upon analyzing fossilized seed specimens from the extinct tree Manifera talaris, which lived in Texas during the middle Permian era (~270 million years ago), the research team found a spectrum of seed wing shapes. There were seeds with two equally sized wings, some with one small and one large wing, and others with a single wing. They postulated that these differences probably arose from underdevelopment or misdevelopment of seeds (which assumes an original two-wing design). However, this developmental error turned out to be evolutionarily beneficial to the tree because these differences were acted upon by natural selection to produce the single-wing design now ubiquitously found today.
To understand the probable mechanism behind this single-wing selection, the team created synthetic seeds using the fossils as a template. They used high-speed photography to determine speed of descent and rotation of the seeds with varying wing structures. From this data, they were able to conclude that single wing designs create a slower (more gradual) downward trajectory because of faster rotation speed. This slower descent would have allowed wind and air currents to disperse these singletons farther from their source compared to double-winged seeds, thereby increasing the geographic distribution of the tree species.
Interestingly, there were several unrelated tree species during the Permian that were evolving the ability to use winged seeds for dispersal – seed dispersal by animals was rare during that time. Since trees (or any other organism, for that matter) don’t look around at other trees to decide what kind of seed organization they should try out, it is fascinating that every living tree has independently settled on the same core design principle of one wing.
The mechanism described in this paper provides an explanation for this phenomenon, which is called convergent evolution (not a term coined in this paper). I would highly recommend watching the videos of seed trajectory from this publication – or, better yet, make your own prototypes and try them out!