A bee crawls into the flower of morning glory. Sasha Bishop, a recent graduate of University of MIchigan researcher Regina Baucom, studied the declining rates of adaptation in morning glories, finding that morning glories may be adapting to attract pollinators at the expense of adapting to a warming climate. This trade-off may be leading to an overall decline in rate of adaptation.
Image Credit: Grace Zhang, the Baucom Lab, University of Michigan
Scientific Frontline: Extended "At a Glance" Summary: Evolutionary Trade-Offs in Plant Adaptation
The Core Concept: Plants confronting the dual crises of climate change and dwindling pollinator populations are evolving to prioritize pollinator attraction over climate adaptation, leading to a steep decline in their overall rate of adaptation.
Key Distinction/Mechanism: Instead of adapting to environmental stressors independently, traits such as flower size and flowering time have become genetically linked covariants. The intense selective pressure to attract scarce pollinators favors larger flowers, which overrides the evolutionary advantage of an earlier flowering time necessary to survive a warming climate. This linkage locks the plant into a specific evolutionary trajectory, limiting its ability to respond efficiently to other selective pressures even when sufficient genetic variation exists.
Major Frameworks/Components:
- Genetic Covariance and Constraint: The biological mechanism where the genetic linkage between two distinct traits restricts a population's capacity to adapt to multiple stressors simultaneously.
- Pollinator-Driven Selection: The strong evolutionary pressure exerted on plant morphology (e.g., flower size) caused by the widespread decline of insect pollinators due to human development and agricultural pesticide use.
- Phenological Adaptation: The alteration of biological timing, such as advancing flowering dates, which serves as a primary adaptive pathway for plants responding to shifts in global temperature and precipitation.
- Adaptive Lag: The observed discrepancy between the theoretical capacity of an organism to evolve rapidly and the actual, constrained rate of adaptation documented in wild populations.
Branch of Science: Evolutionary Biology, Ecology, and Botany.
Future Application: This research will improve predictive models regarding how agricultural weeds adapt, inform crop management strategies, and guide conservation efforts aimed at mitigating the compounding effects of climate shifts and pollinator collapse on global ecosystems.
Why It Matters: The study demonstrates that possessing ample genetic variation does not guarantee a species can survive rapid environmental shifts. By illustrating how the immediate need for pollination can stall critical climate adaptations, it exposes a profound, hidden vulnerability in wild plant populations facing human-caused global change.
Facing both climate change and a plummeting pollinator population, plants may be evolving to attract pollinators rather than adapting to a warming climate, and this trade-off has resulted in a steep decline in the plants’ rate of adaptation, according to a University of Michigan study.
Researchers studying morning glories observed a 96% decrease in the population’s rate of adaptation over a nine-year period. This declining adaptation rate could impact farmers, who deal with the morning glory as an agricultural nuisance.
“Because pollinator pressure strongly favors larger flowers, that linkage may limit how efficiently the population can respond to other selective pressures. Whether that ultimately makes the weed more or less of a problem for farmers is hard to predict—and that unpredictability is itself part of the story,” said Regina Baucom, a professor in the U-M Department of Ecology and Evolutionary Biology.
The reason for the decline is compelling, Baucom said. Currently, the need to attract pollinators with larger flowers is superseding the evolutionary advantage of an earlier flowering time, and these traits have become linked in a way that limits the plants' ability to adapt. This decline is occurring even though the newer plant populations possess ample genetic variation, suggesting they should be able to adapt normally.
“The plant isn’t running out of evolutionary fuel—it’s increasingly locked into a trajectory that favors pollinator attraction, potentially at the expense of climate adaptation,” Baucom said.
Human-Caused Global Change
The researchers, led by recent doctoral graduate Sasha Bishop in collaboration with University of Toronto researcher John Stinchcombe, examined the morning glory in the context of human-caused global change. In addition to a warming climate, they note, humans have altered the environment in ways that negatively impact both plants and animals. People build over previously untouched habitats, and widespread agricultural practices, such as pesticide and herbicide use, have led to a steep decline in pollinators.
Bishop stated that their study, published in Evolution Letters, examines the gap between theoretical expectations—that organisms should be able to adapt quickly to rapidly changing environments—and what scientists observe in the wild populations of many plants.
“Instead of evolving, there are all these wild populations that are dying off, declining, or going through genetic bottlenecks,” Bishop said. “So we’re looking at a situation in which there’s a lag in what we’re seeing in the adaptive rate in wild populations compared to what we think might be theoretically possible in terms of rapid evolution.”
An Adaptation Trade-Off?
For this study, the researchers grew morning glory seeds collected from wild populations at two different points in time, nine years apart. Focusing on plant traits that could be impacted by climate change and pollinators, they recorded the date of each plant’s first flower, flower size, nectar sugar, flowering time, and the distance between the flowers’ anthers (the pollen-producing parts of the flower) and the stigma (the structure that receives pollen).
The researchers then examined the adaptation rate using a statistic called R, which calculates how a population is predicted to adapt when considering how traits within it are linked, versus focusing on a single trait at a time. This approach allowed them to study how one trait might “constrain” another—meaning that one trait could influence how another trait changes or does not change.
When two traits become linked, they are considered “covariants.” Bishop and Baucom’s study also revealed that flower size and flowering time became linked in morning glories—all over the brief, nine-year period of the study.
They found that in their original populations of morning glories, the rate of adaptation was about 76% of what was expected without covariance. Nine years later, however, the rate of adaptation dropped to about 9% of what was expected without covariance.
“There are quite literally thousands of studies showing that flowering phenology is a really important adaptive path when it comes to climate change, particularly temperature changes and precipitation changes, both of which happened in these wild populations in the locations where they were collected from,” Bishop said. “The implication in my mind is that pollinator decline, or the lack of pollination and selective drive to attract pollinators, is making these plants potentially less able to adapt to climatic shifts.”
Published in journal: Evolution Letters
Title: A resurrection experiment reveals reduced adaptive potential in a common agricultural weed
Authors: Sasha G D Bishop, John R Stinchcombe, and Regina S Baucom
Source/Credit: University of Michigan
Edited by: Scientific Frontline
Reference Number: ebio071226_01