Everyone’s seen Rudolph Zallinger’s “The March of Progress” illustration showcasing the evolution of humans: from early primate ape ancestor, Dryopithecus, and progressing toward modern man, Homo sapiens. Evolution is a fascinating phenomenon, but it doesn’t necessarily always follow a straight path as portrayed by Zallinger.
The idea that evolution marches from simple to complex forms, building irreversibly on each prior form has been around for a long time. Paleontologist Louis Dollo’s law states that once an organism progresses with a specialized structure, it does not revert to the previous state.
But now, a new study published in the journal Evolution is challenging the prevailing belief that life progresses unidirectionally. The findings suggest some plants can evolve backward, i.e., specialized species can revert to their more primitive forms.
The plant in focus is the chain fern (Blechnaceae). Ferns are some of the Earth’s oldest plants, predating even the dinosaurs. Researchers at the University of Tennessee zoomed in on the fern’s reproductive strategies and found that ferns retain two distinct modes of reproduction.
They found some species of chain fern are monomorphic, meaning a single leaf performs both spore development and photosynthesis. Meanwhile, other species of chain fern employed a specialized strategy called dimorphism, producing two different leaf types for photosynthesis and reproduction.
This split in the reproduction mode adds complexity and an opportunity to examine Dollo’s law. If evolution is absolute then those ferns developing dimorphism would not shift their course back to monomorphism.
Using the natural history collections and algorithms for estimating evolution, researchers analyzed over 118 species and revealed that evolution in ferns is “neither stepwise nor irreversible.” Intriguingly, they found multiple instances of chain fern species that developed dimorphism, and then flipped back to monomorphism.
As for why chain ferns pull off this evolutionary U-turn, researchers hypothesize the lack of seeding as a reproductive strategy is the vital factor. Once a specific plant commits to seed, it may be stuck with this complexity and downgrading isn’t a viable option. But since ferns have no seeds, they can modify their leaves into whatever spore-producing structure best suits the environment.
“Evolution has no finish line. There is no end goal, no final state,” says Jacob S. Suissa, Assistant Professor at the University of Tennessee. “Our findings suggest that not all reproductive specialization in plants is irreversible. Instead, it may depend on how many layers of specialization plants have acquired over time.”
The study was published in the journal Evolution.
Source: The Conversation.
