Organisms that cause disease illustrate the power of evolution
Bacteria, single-cell pathogens, have one huge advantage in the evolutionary game. While it takes humans decades to reproduce, bacteria can do it in minutes. So a chance change, which gives a bacterium just a tiny advantage, can rapidly spread.
Now, because of our lax use of antibiotics, resistant microbes are a big problem. The use of antibiotics when they are not needed (eg for viral infections) and in agriculture (they are widely used as animal growth promoters), and a tendency for people to not stick to their prescribed dosages, creates a situation where low levels of drug exist.
Partially resistant bacteria have a survival advantage and spread. Now, multidrug-resistant TB and other drug-resistant bacteria are a major threat in the UK and globally.
Hijacking the cell
Viruses, tiny particles that infect and take over a cell, also have numbers on their side. A single infected cell can spew out millions of new virus particles.
Once again it is diversity that underpins their success. In most organisms, heritable material such as DNA is copied extremely carefully. In many viruses, though, genome copying is extraordinarily haphazard. But with millions of copies produced in each cell, plenty will be ﬁne – and among the millions of ‘defective’ ones there may be some promoting drug resistance.
This is one reason why HIV is so dangerous and ﬂu is so difﬁcult to treat. They are moving targets: no sooner has the immune system learned to recognise one form than it has changed into something else.
Virus genomes can also swap bits of their genome with one another. This may create defective viruses, but occasionally something with brand new powers can arise. This is probably how the Spanish ﬂu epidemic of 1918 began. An avian ﬂu virus mixed with a human ﬂu virus, possibly in a pig (pigs can be infected by both avian and human strains of ﬂu); this new virus could infect and be spread between people, and it was lethal.
This could happen again, with avian ﬂu strain H5N1. Currently, this strain can be spread from birds to people but not between people. But if it swaps genes with another virus and becomes transmissible between people, the potential for a global pandemic is very real.
In addition, there are concerns that other avian flu strains – such as H7N9, first detected in humans in 2013 – could also cause widespread severe disease if they evolve the capacity to be spread between people.Lead image:
Christian Lundh/Flickr CC BY NC
- Chief Medical Officer annual report 2011 (published 2013) Chief Medical Officer annual report 2011: volume 2
- RNA replication errors and the evolution of virus pathogenicity and virulence (2014)
- HIV: the ultimate evolver
- Influenza, an ever-evolving target for vaccine development
- Wikipedia: H5N1
- WHO FAQ on H7N9
Questions for discussion
- Which is the most successful pathogen – Ebola, HIV or Mycobacterium tuberculosis (the bacterium that causes TB)? Research each of them and compare how evolution has shaped their biology.