If you look closely at pictures of great-grandparents, grandparents and parents, you will notice similarities, but each generation will be different from its predecessors. This is the
process of evolution in its simplest form: descent with modification.
Over many generations, an incredible amount of modification is possible, which is how the diversity of life on Earth came about.
However, this idea has long been misunderstood as a one-way path to "higher" or "better" organisms. For example, the famous Rudolf Zallinger illustration "The Road to Homo sapiens," published in Time-Life magazine in 1965, depicts the step-by-step evolution of humans from ape-like ancestors to modern humans.
Extending this perspective beyond human development, early paleontological theories about ancient life supported the idea of orthogenesis, or "progressive evolution," in which each generation of a genus progressed toward more complex or optimized forms.
But evolution has no finish line. There is no final goal, no end state. Organisms evolve as a result of natural selection acting at a particular geological moment, or simply as a result of drift without strong selection in any direction.
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In a recently published study conducted by Jacob S. Suissa, an assistant professor of plant evolutionary biology at the University of Tennessee, along with Makaleh Smith, a National Science Foundation-funded research intern at Harvard University, the scientists sought to determine whether a one-sided model of reproductive evolution is always valid for plants.
The opposite phenomenon was found; in many species of
ferns - one of the oldest groups of plants on Earth - the evolution of reproductive strategies was two-way, with plants sometimes evolving "backwards" to less specialized forms.
The path of evolution is not linear
Selection pressures can change in the blink of an eye and steer evolution in unexpected directions.
For example,
dinosaurs and mammals. For more than 150 million years, dinosaurs exerted strong selection pressure on
Jurassic mammals, which were forced to stay small and live underground to avoid predator attacks until extinction.
Then, about 66 million years ago, the asteroid Chicxulub wiped out most non-Asiatic dinosaurs. Suddenly, small mammals were freed from the intense pressure of predatory selection and were able to live above ground, eventually evolving into larger forms, including humans.
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In 1893, the Belgian paleontologist Louis Dollot proposed the idea that once an organism has reached a certain level of development, it does not return to its previous state in exactly the same way it evolved, even if it encounters conditions identical to those in which it once existed.
Dollo's Law, as it has come to be called, implies that specialization is a one-way street, and organisms accumulate layers of complexity that make reverse evolution impossible.
Although Dollo's law has been criticized and its original idea has largely disappeared from popular discourse, this view still influences some aspects of biology.
Plants and the march of progress
In museums, animal evolution is often portrayed as a straightforward progression to higher stages, but these are not the only sources of this narrative. It is also present in the teaching of the evolution of reproduction in plants.
The earliest vascular plants - those with tissues that can move water and minerals throughout the plant - had leafless, stem-like structures called telomes, with capsules at their tips called sporangia that produced spores.
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Telomeres performed both important functions for plants: converting sunlight into energy through photosynthesis and releasing spores to produce new plants. The fossil record suggests that, over time, plants developed more specialized structures that shared these reproductive and photosynthetic functions.
Advancing along plant lineages, from spore-bearing lycophytes to ferns and flowering plants, reproduction becomes more and more specialized. Indeed, the flower is often portrayed as the ultimate goal of botanical evolution.
Throughout the plant kingdom, species that have evolved reproductive structures such as seeds, cones, and flowers have not reverted to simpler, undifferentiated forms. This pattern confirms the progressive complexity of the reproductive system. Ferns, however, are an important exception.
Evolving, but not always forward.
Ferns have a variety of reproductive strategies. Most species combine spore development and photosynthesis on a single leaf type, a strategy called monomorphism. Others separate these functions and have one type of leaf for photosynthesis and another for reproduction - a strategy called dimorphism.
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If the patterns of specialization that are widespread among plants were universal, it would be expected that, having evolved dimorphism, the fern genus would be unable to change course and revert to monomorphism. However, using natural history collections and algorithms to estimate fern evolution, scientists Suissa and Smith found exceptions to this pattern.
In the family known as the derbyan ferns (Blechnaceae), researchers have found many instances where plants develop highly specialized dimorphism but then revert to a more general form of monomorphism.
The lack of seeds gives ferns their flexibility
Why do ferns have such flexible reproductive strategies? The answer lies in what they don't have: seeds, flowers, and fruits. This distinguishes them from the more than 350,000 species of seed plants living on Earth today.
If you take a fertile fern leaf, reduce it in size, and tightly roll it into a tiny pellet, you essentially have an unfertilized seed - a highly modified dimorphic fern leaf in a capsule.
Seeds are just one highly specialized structure in a set of reproductive traits, each building on the previous one, creating a shape so specific that it becomes almost impossible to change. But because living ferns have no seeds, they can change the arrangement of their spore-forming structures on the leaves.
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Suisse's results suggest that not all reproductive specialization in plants is irreversible. On the contrary, it may depend on how many levels of specialization plants have acquired over time.
In today's rapidly changing world, knowing which organisms or traits are "locked in" may be important for predicting how species respond to new environmental challenges and human-imposed habitat changes.
Organisms that have evolved along a 'one-way' path may lack the flexibility to respond to new selection factors in a particular way, and may have to find new strategies for change. Species such as ferns may retain the ability to "evolve backwards" even after specialization.
Ultimately, the new study underscores a fundamental lesson of evolutionary biology: there is no "right" direction in evolution, no march toward an ultimate goal.
Evolutionary paths are more like a tangled web, with some branches diverging, others converging, and still others looping.