Plant Evolution: It’s Not About Temperature

Plant evolution has always been assumed to happen faster in the tropics. Find out how a massive new study overturns that assumption.

When singer-songwriter Bruce Cockburn made a big comeback in the late 80s, I especially enjoyed one of his then-new songs. It was called If a Tree Falls. Part of the first verse goes like this:

 “Rain forest

Mist and mystery

Teeming green

Green brain facing lobotomy

Climate control center for the world

Ancient cord of coexistence”

 Bruce Cockburn was expressing our culture’s growing awareness of the importance of rainforests to the ecosphere. Of course, he was also expressing our concern about the impact of rainforest destruction on the global environment.

When Cockburn talks about the “ancient cord of coexistence,” he’s referring to the rainforest’s reputation as a hotbed of biodiversity. Since the days of Charles Darwin, scientists have believed that tropical rainforests had unique properties that promoted the origin of new species.

 Recent Discoveries Seemed to Challenge Assumption

We covered how recent discoveries about life in the ocean seemed to challenge that perception in an earlier story. There was actually more biodiversity in many fish species in the Earth’s temperate zones.

 This week, the journal Nature Communications published a study that questions that understanding for plant evolution as well.

 About a quarter of all flowering plants share a common ancestor and are called rosids. Readers have probably guessed that roses fall into this plant group, but other rosid plants include mangroves and oak trees and about 100,000 other plant species. It’s an extensive category.

 Rosids Evolving Twice as Fast in Temperate Zones

Researchers from the Florida Museum of Natural History have found that rosids are evolving twice as fast in temperate zones as in the tropics. This contradicts the conventional wisdom that the enormous diversity of life in the tropics is because rates of evolution are more rapid there.

 Nobody questions that the tropics have many more species than other regions of the Earth. Nor do they question that most rosids originated in tropical areas.

Yet, when the study team looked at 20,000 rosid species, they found that the pace of plant evolution in the temperate zones far outstripped what they saw in the tropics. Professor Ryan Folk of Mississippi State University is one of the lead authors of the study.

 “Temperate Regions – Where the Action is Taking Place”

He explained, “Everyone knows about the diversity of tropical rainforests. You would assume all the action in evolution is happening in them. But we found out that it is really the temperate regions of the Earth – really our own backyards – where a lot of the recent action is taking place.”

 This is another study that was made possible by access to today’s massive and sophisticated databases. The researchers looked at the genetic databases GenBank, IDigBio and Global Biodiversity Information Facility to pull together DNA data from more than a million plant occurrence records.

 Professor Folk explained, ““The paper showcases the power of harnessing massive datasets to provide totally different perspectives and upset what we thought we knew about classic hypotheses. This work excited me because it was not simply a new finding, but exactly opposite of my gut instinct. No one was really predicting this before huge data resources became available to perform these types of studies.”

Charles Darwin Called It “An Abominable Mystery”

Charles Darwin once said that the evolutionary history of how so many plant species could spread everywhere so quickly was an “abominable mystery.” Even today, scientists don’t have a firm grasp of what exactly caused that.

Intuitively, most people think that the reason so many species thrive in the tropics is the temperature. Yet, when biologists have looked at the temperature’s role in plant evolution, their findings have been mixed.

 The same holds true for latitude. Researchers haven’t found much correlation between distance from the equator and rates of evolution.

Correlation Between Latitude and Evolution Rates

The study team consists of evolutionary biologists. They wanted to work together to gain a better understanding of the correlation between temperature and how plants evolved..

 They chose rosids as their subject because they’re a large group of land plants that thrives in just about every terrestrial environment t in the world. A lot of rosids are trees, and they play an essential role in forest health.

 Professor Douglas Soltis is a curator at the Florida Museum. He shared that, “To me, that was one of the biggest terrestrial evolutionary events – the rise of the rosid-dominated forests.”

 Forests Provide the Habitat for Many Species

As we know, forests provide the habitat for many species. As Professor Soltis went on to say, “Other lineages, such as amphibians, insects and ferns, diversified in the shadow of rosids.”

Surprisingly, the study’s findings show that plant evolution, at least among rosids, took off when global temperatures gradually started to fall. Lower humidity also supported the rapid development of this broad category of plants.

 The team has concluded that the critical factor in plant evolution is neither temperature nor latitude. The origin of new plant species results from a habitat’s stability.

 Origin of New Plant Species From Habitat’s Stability

As Professor Folk explained, many exotic plant species in the tropics, “simply failed to go extinct, so to speak.” About 95% of species end in extinction, but natural selection seems to be less strict in places where the climate is stable.

 Miao Sun from Aarhus University in Denmark is another lead author of the study. As he put it, “There seems to be a trend forming that, together with our study, shows a lower diversification rate in tropical regions compared with temperate zones. But it’s still hard to tell to what extent this pattern is true across the tree of life.”

 This raises an interesting question about climate change. If plants diversity more quickly in stable habitats with moderate temperatures, how will global warming affect plant evolution?

 How Will Global Warming Affect Plant Evolution

Professor Pamela Soltis is another curator at the Florida Museum. Her answer to that question was, “Warming temperatures will likely slow the rate of diversification, but even worse, we don’t expect species currently living in arctic or alpine areas to be able to respond to quickly warming temperatures.”

 In more bad news, climate change forces species to migrate out of their traditional ranges and leads to extinction. Professor Soltis added, “The change is happening too rapidly, and we are already seeing species moving northward in the Northern Hemisphere or up mountains, with many more species facing extinction or already lost.”

 There’s a growing body of evidence that the tropics’ biodiversity isn’t about temperature or humidity. It’s about stability.

 Rapid Increases in Temperature Discourage Plant Evolution

Rapid increases in temperature won’t encourage plant evolution. They will drive plant species to extinction.

 We’ve known for a while that climate change is a crucial driver of the rapid and unprecedented mass extinction happening worldwide. This study is part of a growing body of literature suggesting that disrupting the temperature of stable ecosystems also contributes to mass extinction.

 Another lead author on the study, Florida Museum Coordinator Robert Guralnick, wrapped up the discussion saying, “Rosids are an enormously successful group of flowering plants. Look out your window, and you will see rosids. Those plants are there because of processes occurring over millions of years, and now we know something essential about why.”

 We always have more to learn if we dare to know.
Learn more:
Plant study challenges tropics’ reputation as site of modern evolutionary innovation
Groundbreaking, evolutionary plant study by MSU biological sciences faculty reverses previous hypotheses
Recent accelerated diversification in rosids occurred outside the tropics
Origin of Life Before the Origin of Species – Four Theories
What Causes Tropical Biodiversity?
Biodiversity Always Wins


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