4 ms·

The science is fascinating, the conclusions compelling, but why didn't these mutations appear during Justinian's Plague?[1] And if mutations helping resistance

The science is fascinating, the conclusions compelling, but why didn't these mutations appear during Justinian's Plague?[1] And if mutations helping resistance to plague were already present from 550AD, why was the Black Plague so devastating 800 years later? 800 years is plenty of time for the mutations to have spread to all of Europe, and yet Black Plague killed as much as 60% of the total European population. [1] https://en.wikipedia.org/wiki/Plague_of_Justinian https://en.wikipedia.org/wiki/Plague_of_Justinian

geneticdrift 4y ago
800 years is also a lot of time for genetic drift to occur. If it’s not being actively selected for anymore, it can end up being in increasingly smaller parts of the population.
- kzrdude 4y ago
  Thanks for putting it as 800 years. In fact, we are closer to the black death now than they were to the justinian plague. Just a small thing that was interesting to notice.
- Maursault 4y ago
  Genetic drift occurs due to random chance, unlike natural selection. Presumably, the population that survived Justinian's Plague had beneficial mutations. While it is possible that 40 generations is long enough for genetic drift as populations mix and become heterogeneous, the surviving population was homogeneous in regards to those beneficial mutations. The bacterium Yersinia pestis never actually went away, so plague deaths continued uninterrupted during those 800 years, just not at the level of pandemic. Those mutations would continue to be beneficial and those with them continued to be actively selected for survival.
chrisco255 4y ago
Are we sure that no mutations exist from the Justinian? How genetically similar were the bacteria that caused the two plagues? One theory off the top of my head, maybe Justinian affected elderly but not the young as much? If true, there would be little selective pressure applied.
- Maursault 4y ago
  > How genetically similar were the bacteria that caused the two plagues? Genetic similarity is far too specific, but both plagues were caused by the same organism and bacteria species, Yersinia pestis.
inglor_cz 4y ago
Covid killed a lot of people even though we have at least four other coronaviruses running around causing common cold, and most of the victims had some banal coronavirus infection before. The plague bacteria of 550 AD was probably not completely identical to the one in 1350 AD either.
- vfclists 4y ago
  Covid killed a lot "old people", people well past reproductive age.
  - Maursault 4y ago
    Only women lose their ability to reproduce past a certain biological age (as opposed to chronological age), and I think you simply mean "those that didn't have children," of any age. But that is how natural selection works, random mutation during sexual reproduction in response to environmental pressure allowing differential survival and reproduction of individuals due to differences in inherited phenotypic traits. Those that died with or without children were not selected for survival as they usually would not have mutated beneficially after they were already conceived. We call that cancer.
- Maursault 4y ago
  > The plague bacteria of 550 AD was probably not completely identical to the one in 1350 AD either. Both plagues were caused by the same bacteria, Yersinia pestis. I don't see how being "completely identical" would matter, only that it was the same species of bacteria.
  - inglor_cz 4y ago
    Yes, it matters. Different strains of the same species of bacteria can have different virulence in human (or animal) hosts.
    - Maursault 4y ago
      Different strains of the same bacteria species can produce different immune responses, but if the species is pathogenic to humans, all its strains will also be considered the same pathogen. The Plague of Justinian and the Black Death were both known to be virulent in that they were both rapid, severe, and destructive, but virulence is not an objectively measurable quality.

geneticdrift 4y ago

800 years is also a lot of time for genetic drift to occur. If it’s not being actively selected for anymore, it can end up being in increasingly smaller parts of the population.

kzrdude 4y ago
Thanks for putting it as 800 years. In fact, we are closer to the black death now than they were to the justinian plague. Just a small thing that was interesting to notice.
Maursault 4y ago
Genetic drift occurs due to random chance, unlike natural selection. Presumably, the population that survived Justinian's Plague had beneficial mutations. While it is possible that 40 generations is long enough for genetic drift as populations mix and become heterogeneous, the surviving population was homogeneous in regards to those beneficial mutations. The bacterium Yersinia pestis never actually went away, so plague deaths continued uninterrupted during those 800 years, just not at the level of pandemic. Those mutations would continue to be beneficial and those with them continued to be actively selected for survival.

chrisco255 4y ago

Are we sure that no mutations exist from the Justinian? How genetically similar were the bacteria that caused the two plagues? One theory off the top of my head, maybe Justinian affected elderly but not the young as much? If true, there would be little selective pressure applied.

Maursault 4y ago
> How genetically similar were the bacteria that caused the two plagues? Genetic similarity is far too specific, but both plagues were caused by the same organism and bacteria species, Yersinia pestis.

inglor_cz 4y ago

Covid killed a lot of people even though we have at least four other coronaviruses running around causing common cold, and most of the victims had some banal coronavirus infection before. The plague bacteria of 550 AD was probably not completely identical to the one in 1350 AD either.

vfclists 4y ago
Covid killed a lot "old people", people well past reproductive age.
- Maursault 4y ago
  Only women lose their ability to reproduce past a certain biological age (as opposed to chronological age), and I think you simply mean "those that didn't have children," of any age. But that is how natural selection works, random mutation during sexual reproduction in response to environmental pressure allowing differential survival and reproduction of individuals due to differences in inherited phenotypic traits. Those that died with or without children were not selected for survival as they usually would not have mutated beneficially after they were already conceived. We call that cancer.
Maursault 4y ago
> The plague bacteria of 550 AD was probably not completely identical to the one in 1350 AD either. Both plagues were caused by the same bacteria, Yersinia pestis. I don't see how being "completely identical" would matter, only that it was the same species of bacteria.
- inglor_cz 4y ago
  Yes, it matters. Different strains of the same species of bacteria can have different virulence in human (or animal) hosts.
  - Maursault 4y ago
    Different strains of the same bacteria species can produce different immune responses, but if the species is pathogenic to humans, all its strains will also be considered the same pathogen. The Plague of Justinian and the Black Death were both known to be virulent in that they were both rapid, severe, and destructive, but virulence is not an objectively measurable quality.