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Imagine a picture of your great-grandparents, grandparents and parents side by side. You would see a similarity, but each generation would be different from its predecessors. This is the evolutionary process in its simplest form: descent with change.
Over many generations, dramatic change is possible. This is how the diversity of life on Earth came about.
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This idea, however, has long been misunderstood as a path in the direction of “higher” or “better” organisms. For example, Rudolph Zallinger’s famous 1965 Time-Life illustration “The way of Homo Sapiens” shows that humans are evolving in a stepwise fashion from ape ancestors to modern humans.
Extending this perspective beyond humans, early paleontological theories of ancient life supported this idea orthogenesis, or “progressive evolution”, in which each generation of a lineage progressed towards a more sophisticated or optimized form.
But evolution has no destination. There is no final goal, no final state. The organism evolves natural selectionActing at a specific geological moment, or simply drifting without strong selection in any direction.
Which I did in a research I published recently Makaleh Smiththen a research grant from Harvard University, funded by the National Science Foundation, we tried to examine whether the unidirectional model of reproductive evolution was always true in plants. On the contrary, we have seen the evolution of reproductive strategies in many types of ferns – one of the oldest groups of plants on Earth. it has been two-wayplants sometimes evolve “backwards” to less specialized forms.
The path of evolution is not linear
Selection pressures can change in heartbeats and steer evolution in unexpected directions.
Take dinosaurs and mammals, for example. For more than 150 million years, dinosaurs exerted great selection pressure on Jurassic mammals, forcing them to stay small and live underground to avoid being hunted to extinction.
Then, about 66 million years ago, Asteroid Chicxulub It wiped out most of the dinosaurs. Suddenly, the small mammals were relieved of a great deal of selection pressure and could finally live off the land. evolving into larger formsincluding humans.
In 1893, a Belgian paleontologist Louis Dollo He presented the idea that when an organism progresses to a certain point, it does not return to a previous state in the exact way it evolved, even if it encounters the same conditions it once experienced. Dollo’s LawAs it became known, this means that specialization is largely unidirectional, accumulating layers of complexity that make backwards evolution impossible for organisms.
Dollo’s law again has been criticizedand his original idea has largely disappeared from popular discourse, a view that still influences aspects of biology today.
Plants and the march of progress
Museums often depict the evolution of animals direct progression to higher levelsbut they are not the only sources of this narrative. It also appears in the teaching about the evolution of plant reproduction.
The earliest vascular plants – tissues that could move water and minerals throughout the plant – had them structures without leaves, called telomesCapsules called sporangia produced spores at the tip. Telomes performed the two major jobs of plants: converting sunlight into energy through photosynthesis and releasing spores to produce new plants.
The fossil record shows that over time, plants developed more specialized structures that divided these functions of reproduction and photosynthesis. Moving through the plant lineages, from one lycophytes with spores from ferns to flowering plants, reproduction is increasingly specialized. In fact, the flower is usually outlined as the ultimate goal of botanical evolution.
Throughout the plant kingdom, after species evolved reproductive structures such as seeds, cones, and flowers, they did not revert to simpler forms. This model supports a progressive increase in reproductive complexity. But ferns are an important exception.
Evolving, but not always forward
Ferns have multiple reproductive strategies. Most species combine spore development and photosynthesis in a single type of leaf, a strategy known as monomorphism. Others separate these functions into one type of leaf for photosynthesis and another for reproduction, a strategy known as dimorphism.
If the patterns of specialization generally seen in plants were universal, we would expect that once a fern lineage had evolved dimorphism, it would not be able to reverse course and return to monomorphism. However, using natural history collections and algorithms to estimate fern evolution, Smith and I found exceptions to this pattern.
within a family known as chain ferns (Blechnaceae)we found multiple cases where plants evolved highly specialized dimorphism, but then reverted to the more general form of monomorphism.
The absence of seeds gives ferns flexibility
Why might ferns have flexible reproductive strategies? The answer lies in what they lack: seeds, flowers and fruits. This sets them apart from the more than 350,000 species of seed plants that live on Earth today.
Imagine taking a fertile fern leaf, chopping it up and rolling it tightly into a small pellet. That’s basically what an unfertilized seed is: a highly modified dimorphic fern leaf in a capsule.
Seeds are just one highly specialized structure in a set of reproductive traits, each building upon the last, creating a shape so precise that it becomes almost impossible to reverse. But since living ferns have no seeds, they can change where they place their spore-producing structures on their leaves.
Our findings suggest that not all plant reproductive specialization is irreversible. Instead, specialization may depend on how many layers plants have acquired over time.
In today’s rapidly changing world, knowing which organisms or traits are “locked in” could be important for predicting how species respond to new environmental challenges and human-imposed habitat changes.
Organisms that have evolved along “one-way” paths may not have the flexibility to respond in a particular way to new selection pressures and may have to devise new strategies to change. In fern-like lineages, species can retain the ability to “evolve back” even after specialization.
Ultimately, our research underscores a fundamental lesson in evolutionary biology: There is no “right” direction in evolution.no march to an end goal. Evolutionary paths are like intricate webs, with some branches diverging, others converging, and some even turning in on themselves.
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