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- Here, we'll delve into plant evolution.
- First, we'll define what is the evolution of plants.
- Then, we'll study the origin and evolution of plants.
- We'll continue with the enormous importance of plant evolution that changes the conditions on Earth forever.
- We'll also analyze the timeline of plant evolution.
- We'll finish with some examples of adaptions that plants developed through their evolution.
The definition of plant evolution
Plant evolution refers to how plants originated and all the changes they have gone through since then while dispersing through Earth and adapting to different environments.
Plants have evolved through the same mechanisms affecting all life on Earth. Much like animals, bacteria, and fungi, the different conditions plants faced influenced their evolution.
- Diversity within populations and between individuals occurs naturally through genetic variation. A phenomenon where, thanks to differences in the DNA sequences from one individual to another (i.e. different allele frequencies), different morphological traits are present within a species. Genetic variation can be caused by mutations, sexual reproduction or genetic drift, but no matter the cause the outcome is always the same: slight differences between individuals.
- Some differences caused by genetic variation can be beneficial, or harmful, to an individual's chance of survival. What is beneficial varies across different periods, environments, and in the presence or absence of predators and resources. The environmental factors affecting survival are known as selection pressures.
- Left to time, populations may change to the point where they are no longer recognizable as descendants of their ancestors. These morphological changes are the result of a gradual change, over many generations, in the genetic makeup of a population.
This process is known as evolution. Natural selection is one of the mechanism by which evolution occurs.
Evolution: a gradual and cumulative change in the heritable genetic traits of a population of organisms over the course of many generations.
Natural selection: a process where individuals with traits that help them survive in their environment are more likely to survive and reproduce because of those traits. These beneficial traits become more and more common within the population with each passing generation.
Tracking these changes in individuals and conditions through time paints a picture to better understand how the diversity of plant life across the terrestrial world came to be. This knowledge can help us to predict responses to climate change, droughts, and other challenges our society will face, and may even influence how we respond to these threats.
The origin and evolution of plants
It's widely believed that life started underwater. The origin and evolution of plants started roughly 430 million years ago when the ancestor of plants migrated to terrestrial land and gave rise to today's land plants. The ancestral streptophyte algae is thought to have been the only plant ancestor to survive the move onto land4.
The origins of life itself are hotly contested, but it is mostly agreed that all life stems from a single common ancestor.
- This Last Universal Common Ancestor (LUCA) formed roughly 3.5 billion years ago.1 LUCA gave rise to all living organisms we see today, plants, animals, fungi and bacteria alike.
- Early life forms were simple unicellular organisms, reliant on diffusion to gather all the energy and nutrients needed from their surrounding environment. With time, life evolved complex processes to make its own energy. This early cellular evolution underpinned the processes of glycolysis, respiration, and photosynthesis.2
- Photosynthesis is thought to have originated in bacteria and allowed organisms to harness sunlight for energy.3 Early plant ancestors, in the form of simple eukaryotic cells, are believed to have absorbed photosynthesizing cyanobacteria. These previously free-roaming cyanobacteria gave rise to chloroplasts, the photosynthetic organelles found in plants.
Modern-day plants have complex stress signaling pathways with many similarities to the ancestral streptophyte algae4. This indicates it was not an easy transition from water to land and strong selection pressures would have been at work.
This symbiotic relationship may have occurred due to food scarcity. In an environment lacking prey, capitalizing on freely available sunshine for energy would be very beneficial. By absorbing rather than consuming photosynthesizing cyanobacteria plant ancestors would have also gained this beneficial trait.3
Plant evolution: the move to land
In the eyes of evolution, you either adapt or face extinction. Initial terrestrial environments were rife with available sunlight, and space to grow whilst lacking predators and competitors. However, the move to land was still a particularly stressful time for early land plants.
Land plants that couldn’t adapt to their new environment were outcompeted for resources, and simply couldn't survive the harsh conditions. The threat of Extinction was constant for early land plants. Some of the deadly threats and consequences of terrestrial life included:
Desiccation - Early land plants couldn’t transport water, so relied heavily on damp conditions.
UV radiation - Water may filter sunlight, and reduce the amount of energy absorbed by chlorophyll pigments, but it also acts as a barrier against harmful UV radiation. A barrier absent on land.
Lack of structural support - Water offers aquatic plants support and buoyancy, but in terrestrial environments, plants must devote energy and nutrients to rigid features like cell walls.
The importance of plant evolution
The harsh selection pressures of early terrestrial environments shaped land plant's evolutionary journey. Yet, since their emergence in the late Ordovician Period, land plants have reworked our planet to suit their own needs. Paving the way for some species to blossom, whilst ensuring the extinction of others.
Importance | Description |
The development of roots changed the earth's physical environment | As plants spread across land, previously bare riverbeds became flourishing plant habitats. Plant roots held the earth together and reduced erosion on river banks. This resulted in an increase of meandering rivers, rather than the wide braided channels common before the emergence of land plants.
|
Land plants drove early mass extinctions | As plant roots burrowed down into the earth, the rocks beneath were worn down. Releasing minerals that found their way into earth's river systems and oceans. This sudden increase in nutrients caused the eutrophication and anoxia of past oceans, killing half of marine life in the Devonian Mass Extinction. |
Plants and algae changed the earth's atmosphere | Plants and algae are autotrophs. They absorb carbon dioxide and energy from the sun, whilst releasing oxygen. Plants and algae dramatically increased the ratio of oxygen in the atmosphere during the carboniferous period, allowing a boom in animal evolution. With oxygen no longer a limiting factor, huge arthropods emerged. |
Plants influence the global climate | Photosynthesis directly increases atmospheric oxygen concentrations, but plant roots also played a role by breaking up the earth and releasing minerals which react with carbon dioxide. These reactions draw down atmospheric carbon dioxide and lock it away in the earth and oceans. This dramatically increased the concentration of oxygen in the atmosphere, leading to global cooling periods and ice ages. The mass extinctions which occurred during ice ages, opened up niches for surviving species to adapt and colonize. |
Table 1: The main changes that plants caused in Earth’s environment reflect the importance of plant evolution.
Evolution of plants: Timeline
The timeline of plant evolution spans, according to the existing evidence, from about 430 million years ago in the late Ordovician Period until modern times. Land plants' ability to flourish is largely attributed to adaptations gained through four key evolutionary steps, which no doubt evolved under harsh selection pressures.
Fig. 1: The origina and evolution of plants. Key steps in plant evolution throughout different geological periods.
Angiosperms, which underwent each key stage of plant evolution, are now the most abundant of all land plant types.
Examples of plant evolution
Billions of years of plant evolution have allowed land plants to conquer every corner of the globe. So much so that land plants now make up 82% of global biomass. Let's see some examples of plant evolution that show the different ways they have adapted to land.
Adaptation | Example of the Benefits Bestowed on Plants. |
Waxy cuticle | Prevent water loss, reducing the risk of desiccation. |
Increased gas exchange needed for respiration and photosynthesis. Guard cells control how open or closed the stomata are, reducing water lost by transpiration and desiccation risk. | |
Rhizoids | Provide structure and some uptake of water in bryophytes. |
Vascular system | Transport nutrients, water and energy in the form of ATP from where they are absorbed, or produced, to tissues where they are needed. Vascular plants are able to grow much taller, outcompeting other plants for sunshine, because of their vascular systems. Vascular plants also have increased rigidity and support. |
True roots | Anchor and support plants, and aid in the absorption of water in vascular plants. |
Protective flavonoids and pigments | Protect plants from UV radiation by filtering harmful UV light whilst still allowing for some energy absorption for photosynthesis. |
Nectar & variations in color, scent and size of flowers | Sweet nectar encourages insects and other pollinators to travel deep into the flowers, where sticky pollen attaches to their skin or fur. As pollinators visit multiple flowers in a day, some of this pollen will rub off on future plants, fertilizing and spreading the initial plant's genes. Plants make their flowers more inviting for pollinators through the use of bright colors, appealing scents, and different sized and shaped petals. Angiosperms, or flowering plants, have coevolved with pollinators. |
Seeds and pollen | Allow the genetic material, and eventually fertilized embryos of plants to travel far away from their parents and reduce competition for resources. Seeds and pollen both also have protective coats which protect their contents from mechanical damage and desiccation. Seed plants are able to survive and spread through much harsher environments thanks to this adaptation. |
Fruit surrounding seeds | Angiosperms' seeds are surrounded by fruits or ovaries. Sweet fleshy fruits invite animals to eat them, dispersing the seeds contained within through their feces. This increased dispersal reduces competition from parent plants in angiosperms. Other fruits are dry and hard. These fruits confer additional protection to the seed. Some of which may have hooks that attach to the fur of pollinators aiding dispersal. Not all seed plants benefit from the evolution of protective fruit, as gymnosperms lack ovaries. |
Table 2: The key adaptations that plants developed to adapt to terrestrial conditions.
Plant Evolution - Key takeaways
- All plants share the same common ancestor as all other life on earth. Specifically, the group of plants probably originated from an ancestral green algae.
- Photosynthesis evolved in bacteria. Early eukaryotes engulfed cyanobacteria gaining the ability to photosynthesize themselves. Engulfed cyanobacteria gave rise to chloroplasts.
- Plant evolution has shaped our natural world, by changing the composition of the atmosphere and the geology of their environments.
- The move to land brought about many stressors and strong selection pressures. Natural selection ensured plants that didn't adapt when they faced harsh competition and predation, eventually went extinct.
- There are many examples of plant evolution seen in the adaptive features land plants require to survive within their niches.
References
- Christie Wilcox, Evolution: Out Of The Sea, Scientific American, 2012.
- Cooper GM, The Origin and Evolution of Cells, The Cell: A Molecular Approach. 2nd edition, 2000.
- Robert E. Blankenship, Plant Physiology: Early Evolution of Photosynthesis, 2010.
- Jan De Vries et al, Plant evolution: landmarks on the path to terrestrial life, New Phytologist, 2018.
- Lumen, Bryophytes, Boundless Biology
- C. Jill Harrison et al, The origin and early evolution of vascular plant shoots and leaves, Philosophical Transactions of the Royal Society B, 2018.
- Ada Linkies et al, The Evolution of Seeds, New Phytologist, 2010
- Hannah Ritchie and Max Roser, Biodiversity, Our World In Data, 2021.
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Frequently Asked Questions about Plant Evolution
What is an example of evolution in plants?
There are numerous examples of evolution in plants under natural selection. In the late Devonian period vascular plants evolved ‘naked seeds’ as an adaptation to dry conditions.
How is the evolution of gymnosperm plants best explained?
The evolution of gymnosperm plants can be explained as follows: they were the first seeded plants to evolve from vascular plants during the late Devonian or early carboniferous period. Gymnosperm plants’ novel seed and pollen adaptations to wind allowed them to disperse throughout and survive the dry conditions.
What major events highlight the evolution of plants?
The major events that highlight the evolution of plants are: the move to land, the evolution of the vascular system, seeds, and flowers. These adaptations increased the survival of plants within the dry terrestrial conditions as they conquered the land.
What is the proposed sequence in plant evolution?
The proposed sequence in plant evolution starts with the move of the ancestral streptophyte algae onto land. Next plants evolved vascular systems, gymnosperm seeds and eventually the angiosperm seeds and flowers which make up 80% of known plant life.
What are the four major periods of plant evolution?
The four major periods of plant evolution are: the move to land during the Ordovician-late Ordovician period, the development of vascular tissue in the Devonian period, gymnosperm emergence in the carboniferous period, and angiosperm emergence in the Cretaceous period.
What is the timeline for the evolution of plants?
The timeline for the evolution of plants, based on the major events, spans from their emergence about 430 million years ago in the late Ordovician Period to the rise of flowering plants in the Cretaceous Period.
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