This article was originally presented at Conversation.
Human impacts on global ecosystems can be severe, widespread and irreversible. But life on Earth evolved to meet the challenges of the environment over 3.5 billion years: Could those same evolutionary forces help life on Earth survive in a man-made environment?
Our latest research reveals that evolution seems unstoppable during a biological invasion, but then suddenly stops after a century of rapid adaptation. Understanding why this is happening could be key to managing biodiversity over the next century.
Faced with environmental challenges, natural selection can be a powerful force for evolutionary change in modern time frames. Galapagos finches develop different beak sizes to feed on changing seed sources, overfished cod ripen earlier, and purple loose plants bloom earlier in response to shorter growing seasons in northern Ontario. But evolution has limits.
Evolutionary constraints
For almost 20 years, I have studied how some species attack and thrive in new environments. At Queens University, I continue to work with students and associates to study rapid evolution in nature.
A new topic of this paper is the interaction between natural selection and evolutionary constraint.
Adapting to new environments requires new genetic variants. Natural selection can promote genes that improve survival and reproduction. But without new variants, adaptive evolution will stall.
Restrictions are the reason why related species share common traits, and the reason why centaurs, mermaids and dragons exist only in mythology: genes do not produce hooves or fish tails in humans, nor wings in large reptiles. By limiting the options available to natural selection, evolutionary constraints are the ultimate cause of extinction.
As a counterweight to natural selection, it is surprising that evolutionary constraints are not studied so intensively. But there are experimental tools for this.
Joint garden studies
The usual garden experiment was introduced 100 years ago, but it remains the gold standard for studying the genetic basis of rapid evolution.
It involves raising genetically related individuals in a unique environment to spot genetic differences in growth and development. In our laboratory, the usual experiments in the garden with purple lobsters reveal a delicate dance between natural selection and evolutionary limitation.
Purple loosestrife, or Lithrum saliciaria, is known for its attractive purple-pink flowers in infested swamps across Canada and the United States. Over a period of 150 years, the species has spread from Maryland to the north to Labrador and Saskatchewan, and south to the Gulf of Mexico and southern California.
Purple petals, like other plants, have limited resources to invest in growth or reproduction. Some genes produce larger plants, others make plants that bloom earlier. But no gene does both. This is a genetic restriction to bloom or grow earlier in order to gather more resources.
Plants with more resources are more competitive and can produce more flowers. But additional resources are wasted if flowers are produced too late in the season, when temperatures are too low for pollinators and seed development to ensure gene transfer for higher growth. This delicate balance gives the optimal flowering time that accompanies changes in the length of vegetation.
Rapid spread
So how did natural selection and evolutionary constraints shape the flowering time of the purple lobster as it spread across North America? We cannot travel to the past, but collections of natural history provide a tangible connection to the past.
Dried specimens of the purple spatula are stored in the Fowler Herbarium at Queens University and in dozens of other herbarium collections throughout North America. The location and date of collection were recorded with each carefully preserved specimen.
Using historical time records, we reconstructed the local growing conditions of each specimen to computer predict what each plant would look like if grown in uniform growing conditions - a virtual shared garden.
No longer limited to sustainable seed collections, we would use a virtual shared garden to reconstruct 150 years of evolution across North America.
The results are astounding. Early flowering is constantly evolving in response to shorter growing seasons throughout North America. But after about a century, the rate of evolution of the barn seems to be limited by a trade-off between flowering time and size. This type of evolutionary stagnation has also been observed in fossil records over much longer time intervals. This seems to be a common feature of evolution.
Constraints are a good reason to be skeptical that evolution will save species from extinction in a stressful environment. But limitations also make evolution more predictable, at least at shorter intervals that are most relevant to human civilization.
And this is just the beginning - one species among millions. How does the balance between natural selection and restriction take place in other invasive species, or in endangered species? Natural history collections help us to understand the past, to make predictions about our future. It’s time to dump her and move on.
Robert I Colautti is an assistant professor of biology and the Canadian Research Department (Tier II) at Rapid Evolution, Queen’s University, Ontario. Disclosure statement: Robert I Kolauti received funding from the Queen’s University, the Government of Ontario and three federal funding agencies: NSERC, SSHRC and CIHR.