What can these flowers teach us about evolution

This article was originally included on The conversation.

Human impacts on global ecosystems can be severe, widespread and irreversible. But life on Earth evolved to meet environmental challenges for 3.5 billion years: Can these same evolutionary forces help preserve Earth’s life in a man-made environment?

Our latest research finds that evolution seems unstoppable during a biological invasion, but then abruptly stops after a century of rapid adaptation. Understanding why this is happening can be key to managing biodiversity in the next century.

Faced with environmental challenges, natural selection can be a powerful force for evolutionary change on a modern scale. Galapagos finches develop different beak sizes to feed on changing seed sources, overfished cod ripens earlier, and purple vines bloom earlier in response to shorter growing seasons in northern Ontario. But evolution has limits.

Evolutionary constraints

For almost 20 years, I studied how some species invade and thrive in new environments. At Queen’s University, I continue to work with students and associates to study the rapid evolution of nature.

A new topic of this work 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 options, adaptive evolution will stop.

Restrictions are the reason why related species share common features and the reason why centaurs, mermaids and dragons exist only in mythology: no genes produce hooves or fish tails in humans, nor wings in large reptiles. By limiting the options available for 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.

General garden research

The general garden experiment was introduced 100 years ago, but it remains the gold standard for studying the genetic basis of rapid evolution.

This involves raising genetically related individuals in the same environment to observe genetic differences in growth and development. In our lab, the usual garden experiments with the purple vine reveal a delicate dance between natural selection and evolutionary constraint.

Purple vine, or Lythrum saliciaria, is known for its attractive purple-pink flowers in invading wetlands in Canada and the United States. For 150 years, the species has spread from Maryland north to Labrador and Saskatchewan and south to the Gulf of Mexico and Southern California.

The purple vine, like other plants, has limited resources to invest in growth or reproduction. Some genes produce larger plants, others make plants that bloom earlier. But no genes do either. This is a genetic restriction to bloom earlier or grow larger to gather more resources.

Plants with more resources are more competitive and can produce more flowers. But additional resources are lost if flowers are produced too late in the season, when temperatures are too low for pollinators and seed development to ensure the passage of genes for greater growth. This delicate balance gives optimal flowering time, which tracks changes in the length of the growing season.

Fast spread

So how do natural selection and evolutionary constraint shape the flowering time of the purple vine as it spreads in North America? We cannot travel back in time, but natural history collections provide a tangible connection to the past.

Dried specimens of purple herbarium are stored in Fowler’s herbarium at Queen’s University and in dozens of other herbarium collections in North America. Recorded with each carefully preserved copy is the place and date of collection.

Using historical weather data, we reconstructed the local growing conditions of each specimen in order to estimate what each plant would look like if grown under the same growing conditions – a virtual common garden.

No longer limited by viable seed collections, we will use the virtual shared garden to reconstruct 150 years of evolution in North America.

The results are astounding. Early flowering has evolved many times in response to shorter growing seasons in North America. But after about a century, the pace of evolution seems to have stopped, limited by a trade-off between flowering time and size. This type of evolutionary stagnation is also observed in fossils for much longer time rocks. It seems to be a common feature of evolution.

Restrictions are a good reason to be skeptical that evolution will save species from extinction in a stressful environment. But constraints also make evolution more predictable, at least in the shorter time scales that are most appropriate for human civilization.

And this is just the beginning – a species among millions. How is the balance between natural selection and restrictions in other invasive species or in endangered species manifested? Natural history collections help us to understand the past, to make predictions about our future. It’s time to get the attention they deserve.

Robert I Kolauti is an Assistant Professor of Biology and Research in Canada (Tier II) at Rapid Evolution, Queen’s University, Ontario. Disclosure Statement: Robert I Callouty receives funding from the Queen’s University, the Government of Ontario and three federal grant agencies: NSERC, SSHRC and CIHR.

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