The Elements of Evolution – Plant Evolution

Plant Evolution

Plants spread and speciated throughout the world from the time of their terrestrial debut. Their proliferation was both of number and variety. Even the earliest forests had widely divergent plant species.

Polyploidy was the key to plant innovations. Polyploidal cells have more than 2 paired (homologous) sets of chromosomes. Whole-genome duplication afforded simultaneous gene conservation and experimentation.

Gene duplications allow evolution of genes with new functions. ~ Swedish microbiologist Joakim Näsvall et al

Gene duplication events were intensely concentrated 319 and 192 MYA. The 1st begat seed plants. The 2nd brought the proliferation of flowering plants. 90% of herbaceous plants duplicated their genomes.

At least 14,000 protein-coding genes existed in the last common ancestor of all flowering plants. Many of these genes are unique to flowering plants, and many are known to be important for producing the flower as well as other structures and other processes specific to flowering plants. ~ American botanist Joshua Der

The innovative advances in angiosperms prompted one of Earth’s greatest terrestrial radiations, famously characterized by Charles Darwin as “an abominable mystery.”

Trees

The first tree-sized land plants evolved during the Devonian. Stalks in the Silurian were no more than a meter in height. The Carboniferous brought towering trees: woody and tough-leafed. These measures evolved as defensive gestures and included the production of discouraging toxins. Tree sap, resins, and gums help isolate and block infections, as well as quickly healing over wounds.

To animals, trees represented another opportunity to rob the copious cradle which plant life created: begetting beetles that could chew leaves and bore into wood. Another predator-prey evolutionary cycle had begun, one that has been unceasing. Insects and plants have been at war over 300 million years.

When plants could manage it, the interplay between plants and insects eventuated into a truce. Like the symbiotic relationship with nitrogen-fixing bacteria and fungi at their roots, plants managed to cajole insects to do their bidding: trading sweets for pollination services.

Cooperation between divergent species is not readily had. Among macroscopic life, plants have been the most successful in coaxing cooperative relationships with beings in other kingdoms.

Early in the Paleozoic era, most rivers in the world were shallow and wide. The coming of trees, with their tough root systems and woody debris, diverted water in ways beneficial to life. Trees and rivers coevolved. The rise of the forests created river channels and islands, deepening rivers, altering the landscape, and creating opportunities for animal speciation, as new niche habitats opened by both the change in flora and river flow. Trees carving rivers was an extension of work by smaller plants, which created and then employed mud to make their mark on water flows, fashioning favorable wetlands.

Mudrocks were rare before the appearance of plants and common thereafter. In addition to inhibiting erosion, plants also interact with river flows and promote the deposition of fine-grained sediment. This can help armor riverbanks and slow their lateral migration, aiding the preservation of muddy floodplain deposits. ~ American geologist Woodward Fischer

During most of Earth’s history, the decay of dead plants oxidized, producing CO2. The complete process is the carbon cycle. During the Carboniferous, the bulk of carbon was deposited in peat, oxygenating the atmosphere. As carbon dioxide is a greenhouse gas, its reduction meant more solar heat reflected into space. Consequently, with land covering the polar regions, the Carboniferous was a period of global cooling.

 Fire Ecology

An atmospheric oxygen concentration above 13% allows lightning to create wildfires. The rise in O2 was the handicraft of plants: their exhaust and increasing pile of biomass created the conditions for wildfire to become a significant feature of terrestrial ecology.

Fire and the vegetation that feeds it, in concert with the atmospheric CO2 and O2 cycles, developed into a feedback mechanism regulating the oxygen level. Besides destroying vegetation, fire alters soil conditions, which negatively affects plant growth in the short term.

Depending upon aridity, an atmospheric oxygen level beyond 25% can inspire widespread wildfires. Moist air suppresses combustive tendencies, albeit only to a degree.

Combustion has much the same role as respiration in carbon cycling, though, of course, fire is faster and the energy produced is almost entirely heat. Fire creates charcoal, which results in a much higher rate of carbon burial than otherwise.

The Late Palaeozoic era – the Carboniferous and Permian periods – was a time of high humidity, with lush vegetation. Oxygen levels reached over 35%. Wildfires raged.

The Carboniferous coincided with the evolution of conifers: plants adapted to a fire ecology. Fire resistance included thicker bark, deeper embedding of vascular tissue, and sheathes of fibrous roots surrounding the stem.

Numerous seed-bearing plants, such as species of pine, only germinate after a forest fire cracks their seed coats. This is a strategic withholding, so that new plants germinate only after an open habitat becomes available.

Fires did not ravage forests worldwide owing to the evolution of fire-resistant traits and the swampy biome covering vast tracts of land. The humid coolness of the Carboniferous also helped keep fires in check.

Geology seemed to be in cahoots with other changes favorable to vegetation. Continental drift altered global weather patterns and climate, rendering them more conducive to plant growth.