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The new coronavirus strains and evolution's "I told you so" moment

February 2021

Unfettered spread has boosted the chances of SARS-CoV-2 acquiring mutations that make it more contagious. Click on image for larger version. Graph courtesy of Johns Hopkins University

It seems like new coronavirus strains are suddenly popping up everywhere. These include the "UK" strain, a variant discovered in South Africa, two in Brazil, and now one in California, inspiring many anxious questions. Do these new strains spread more easily?  At least some of them seem to. Are they more deadly? Evidence is just coming in but the British variant, at least, may be.  Will our vaccines work against them?  To varying degrees. Moderna is developing a booster shot to make its vaccine more effective against the strain from South Africa. As researchers try to sort out answers to these key questions, it's worth taking a step back to ask how we got here. What's the force behind all these new strains? Of course, the answer is evolution.

Where's the evolution?

In short, the new strains evolved. When a virus replicates, the viral particles produced are not all perfect copies of their "parent." Some have small, random differences in their genomes. These mutations are simply mistakes made when the viral RNA was copied. (Coronaviruses like SARS-CoV-2, the virus that causes COVID-19, carry RNA, not DNA, as their genetic material.)  When a virus particle with a new mutation leaves behind a lot of descendants by spreading to many people, we recognize it as a distinct strain.

That was all inevitable.  There is nothing inherently scary about new viral strains carrying mutations.  The vast majority of mutations that occur either harm the virus (slowing its ability to spread and reproduce) or have no effect on it.  Viruses with harmful mutations will leave behind fewer descendants, and their lineages will rapidly die out. Viruses carrying neutral mutations don't pose a greater threat than any other strain.

What was not inevitable was SARS-CoV-2 accumulating beneficial mutations, mutations that help the virus evade our immune systems and boost its ability to spread. SARS-CoV-2 is not a particularly speedy evolver. Coronaviruses have a slow rate of mutation compared to many other viruses like HIV and influenza. But our failure to control the spread of the virus has given it an evolutionary edge: volume. The chance of any individual coronavirus particle leaving behind a descendant carrying a beneficial mutation is tiny. But we have let SARS-CoV-2 infect tens of millions of people, and each of those infections involves the virus producing many millions of copies of itself.  It's like playing the lottery.  Each individual ticket might have a one in ten million chance of being a winner...but you are likely to win if you buy ten million tickets. Globally, we've bought out the lottery, and SARS-CoV-2 has hit the jackpot.

Once a beneficial mutation occurs, it spreads through the process of natural selection. The lucky virus particle has an advantage and so will leave behind more descendants than others.  In that way, the strain carrying a beneficial mutation is quickly established and proliferates.  

Evolution not only helps us understand how and why these new strains arose, it also points towards a commonsense strategy for slowing the evolution of more contagious and virulent strains: stop the spread. Wear a mask. Avoid gathering. Stay home when you can.  All the same things that help protect ourselves and our loved ones from getting sick will also reduce the chances of even more threatening viral variants evolving.  It's elementary evolutionary theory.

Read more about it

Primary literature:

  • Greaney, A., J., Loes, A. N., Crawford, K. H. D., Starr, T. N., Malone, K. D., Chu, J. Y, and Bloom, J. D. January 4, 2021. Comprehensive mapping of mutations to the SARS-CoV-2 receptor-binding domain that affect recognition by polyclonal human serum antibodies. Preprint retrieved from bioRxiv, January 28, 2021.  read it
  • Volz, E., Mishra, S., Chand, M., Barrett, J. C., Johnson, R., Geidelberg, L., ... and Fergusen, N. M. January 4, 2021. Transmission of SARS-CoV-2 lineage B.1.1.7 in England: insights from linking epidemiological and genetic data. Preprint retrieved from medRxiv, January 28, 2021. read it
  • Zhang, W, Davis, B. D., Chen, S. S., Martinez, J. M. S., Plummer, J. T., and Vail, E. January 20, 2021. Emergence of a novel SARS-CoV-2 strain in Southern California, USA. Preprint retrieved from medRxiv, January 28, 2021. read it
News articles:

Understanding Evolution resources:

Discussion and extension questions

  1. The article above describes three possible effects that mutations can have on the virus; they can be harmful to the virus, helpful to the virus, or neutral.
    • Of the three types of mutation, which occur most frequently?
    • Of the three types of mutation, which are least likely to remain in the viral population?
    • Of the three types of mutation, which are most likely to remain in the viral population?
  2. This article explains how three elements (variation, differential reproduction, and heredity) result in evolution by natural selection. Which of those three elements is most directly affected by the uncontrolled spread and replication of the virus that causes COVID-19? Explain how viral replication affects that element.
  3. In your own words, explain how wearing a mask and avoiding contact with people outside your household affects the chances of SARS-CoV-2 acquiring mutations that help it spread even more easily?
  4. Imagine a mutation that causes SARS-CoV-2 to be more deadly but also makes it more difficult for the virus to be transmitted from one person to another. Is this mutation likely to spread or not? Explain your reasoning.
Related lessons and teaching resources

  • Teach about natural selection: This game for high school and college students simulates natural selection. It is suitable for an introductory biology class and for more advanced classes where students can explore principles such as the role of variation and mutation.
  • Teach about random mutations and natural selection: In this activity for grades 9-12, students build and evolve and modify paper-and-straw "birds" to simulate natural selection acting on random mutations.

References

  • Greaney, A., J., Loes, A. N., Crawford, K. H. D., Starr, T. N., Malone, K. D., Chu, J. Y, and Bloom, J. D. January 4, 2021. Comprehensive mapping of mutations to the SARS-CoV-2 receptor-binding domain that affect recognition by polyclonal human serum antibodies. Preprint retrieved from bioRxiv, January 28, 2021. DOI: https://doi.org/10.1101/2020.12.31.425021
  • Volz, E., Mishra, S., Chand, M., Barrett, J. C., Johnson, R., Geidelberg, L., ... and Fergusen, N. M. January 4, 2021. Transmission of SARS-CoV-2 lineage B.1.1.7 in England: insights from linking epidemiological and genetic data. Preprint retrieved from medRxiv, January 28, 2021. DOI: https://doi.org/10.1101/2020.12.30.20249034
  • Zhang, W, Davis, B. D., Chen, S. S., Martinez, J. M. S., Plummer, J. T., and Vail, E. January 20, 2021. Emergence of a novel SARS-CoV-2 strain in Southern California, USA. Preprint retrieved from medRxiv, January 28, 2021. DOI: https://doi.org/10.1101/2021.01.18.21249786