Imagine a photo of your great-grandparents, grandparents, and parents side by side. Although you will see similarities, each generation will look different from the previous one.
This is the process of evolution in its simplest form: descent through transformation.
A huge amount of modifications can be made over several generations. This is how the variety of life forms on Earth arose.
But this idea has long been misunderstood as a one-directional path toward a “higher” or “better” organism. For example, Rudolph Zallinger’s famous 1965 Time-Life illustration “way to go sapient” shows the step-by-step evolution of humans from ape-like ancestors to modern humans.
Extending this view beyond humans, early paleontological theories about ancient life supported the following ideas: Orthogenesis or “progressive evolution”,” each generation’s genealogy developed into a more elaborate or optimized form.
But evolution has no finish line. There is no end goal, no end state. Organisms evolve as follows: natural selection They act at specific geological moments, or simply by drift, without strong selection in any direction.
In a recently published study I conducted, Makale SmithAs undergraduate research interns at Harvard University, then funded by the National Science Foundation, we sought to study whether the one-sided model of reproductive evolution always holds true in plants.
On the contrary, we discovered the evolution of reproductive strategies in several species of ferns, one of the oldest plant groups on Earth. It was a two-way streetPlants sometimes evolve “backwards” into less specialized forms.
The path of evolution is not linear
Selection pressures can change from moment to moment and lead evolution in unexpected directions.
Consider dinosaurs and mammals, for example. For over 150 million years, dinosaurs exerted strong selective pressure on Jurassic mammals. Jurassic mammals had to live small and underground to avoid being hunted to extinction.
Then, about 66 million years ago, Chicxulub Asteroid It wiped out most non-avian dinosaurs. Suddenly, small mammals were able to live on land, free from strong predatory selection pressures, and eventually evolves into a larger formIncluding humans.
Dolo’s Law have been criticizedAnd although its original ideas have largely disappeared from public discourse, this perspective still influences aspects of biology today.
The March of Plants and Progress
Museums often depict animal evolution as follows: Linear progression to higher levelsBut they are not the only source for this story. This also appears in education about the reproductive evolution of plants.
The first vascular plants (vascular plants with tissue that could move water and minerals throughout the plant) were Telome, a leafless, stem-like structureThey have a capsule at the end called a sporangium that produces spores.
Telomes performed both of the plant’s two main roles: 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 reproductive and photosynthetic functions.
moving through the plant lineage Lycopods containing spores From ferns to flowering plants, reproduction becomes increasingly specialized. In fact, flowers are often depicted as the ultimate goal of plant evolution.
Throughout the plant kingdom, once species evolved reproductive structures such as seeds, cones, and flowers, they did not revert to simpler, undifferentiated forms. This pattern supports a gradual increase in reproductive complexity. However, ferns are an important exception.
Evolving, but not always evolving
Ferns have a variety of reproductive strategies. Most species combine spore development and photosynthesis in a single leaf type, a strategy called monomorphism. Others separate these functions, having one leaf type for photosynthesis and another for reproduction. This strategy is called dimorphism.
If broad patterns of specialization are universal across plants, one would expect that once a lineage of ferns has evolved dimorphism, it will be unable to change course or revert to monomorphism. But using natural history collections and algorithms to estimate the evolution of ferns, Smith and I found exceptions to this pattern.
within a known family Chain fern (Blechnaceae)We found several cases where plants evolved highly specialized dimorphisms but then reverted to a more general form of monomorphism.
Lack of seeds makes ferns flexible
Why do ferns have such flexible reproductive strategies? The answer is a lack of seeds, flowers and fruits. This distinguishes them from the more than 350,000 species of seed plants living on Earth today.
Imagine taking a fertile fern leaf, shrinking it, and wrapping it tightly into tiny pellets. Basically, the unfertilized seeds are highly modified dimorphic fern leaves contained in capsules.
A seed is just one of a series of highly specialized structures, each of which is formed last, creating a form so specific that reversal is almost impossible. However, because living ferns do not have seeds, they can change where they place the spore-producing structures on their leaves.
Our findings suggest that not all reproductive specializations in plants are irreversible. Instead, it may depend on how many specialization factories you have acquired over time.
In today’s rapidly changing world, knowing which organisms or traits are “fixed” can be important for predicting how species will respond to new environmental challenges and human-imposed habitat changes.
Organisms that have evolved along “one-way” pathways may lack the flexibility to respond to new selection pressures in specific ways and must find new strategies for change. In lineages such as ferns, species may retain the ability to “reverse evolve” even after specialization.
Ultimately, our study highlights fundamental lessons from evolutionary biology. There is no ‘right’ direction in evolution.There is no march towards the final goal.
The path of evolution resembles a tangled spider’s web, with some branches diverging, others converging, and some looping around on their own.
Jacob S. SuisaAssistant Professor of Plant Evolutionary Biology, University of Tennessee
This article is republished from: conversation Under Creative Commons License. read original article.
