Origin & Evolution of Viruses (HL) (HL IB Biology)

• Viruses can infect nearly all living species and are estimated to have been on Earth 3.5 billion years before humans evolved
• There is evidence that viruses evolved alongside other species, a process known as coevolution
  • Around 8 % of the human genome contains small segments of viral DNA thought to be left over from ancient infections
  • These DNA fragments are called endogenous retroviruses (ERVs) and have been passed along and modified over millions of years of evolution
• The origin of viruses is still under debate in the scientific community and among virologists
  • One issue is that viruses are not found in fossils so there is limited evidence for their evolution

There are three key theories as to the origin of viruses:

Escape theory

• Viruses arose from genetic elements, such as DNA and RNA, that gained the ability to move between cells
• These genetic elements became surrounded by an outer boundary forming a virus particle

Regressive/reduction theory

• Viruses are remnants of cellular organisms or were once small cells that became parasites of larger cells
• Over time the cellular structures that were no longer needed were shed , leaving behind just viral structures

Virus-first theory

• Viruses predate their current cellular hosts
• During evolution we expect simpler organisms to give rise to more complex organisms, so the simple nature of virus particles could indicate that viruses evolved first

Theories of virus origin diagram

There are multiple theories for the origin of viruses

• Viruses are diverse and this diversity suggests that there may have been different origins for different viruses
  • It is possible that all of the above theories or correct, or indeed that none of them are correct and that a different process occurred
• There are some features that are common among many viruses which indicates that convergent evolution may have occurred
  • All viruses have a capsid protein outer boundary and no cytoplasm contained within this boundary
  • All viruses have genetic material, either DNA or RNA
    • The genetic code is the same as that used by other organisms
  • All viruses are parasitic in nature and cannot replicate or or carry out their functions without a host cell

Viral evolution

• Viruses can undergo very evolution extremely rapidly
• Two examples that demonstrate this are:
  • The evolution of influenza viruses
  • The evolution of HIV
• Both of these viruses:
  • Have high mutation rates
    • This is largely due to the fact that these viruses have RNA as their genetic material; mutations can occur during the process of converting viral RNA into DNA during viral replication
  • Have large population sizes
  • Have short generation times
• These features mean that both of these viruses can quickly evolve to evade the immune systems of their hosts

Antigenic drift and antigenic shift

Viruses can undergo genetic change either by antigenic drift or antigenic shift

Antigenic drift

• The accumulation of small changes to viral genetic material over time
• Variation in the surface proteins of the virus appear slowly
• Eventually the host's immune system cannot recognise the virus
• HIV undergoes antigenic drift

Antigenic shift

• A major change occurs in the viral genetic material in a short time period
  • Two or more virus types infect the same cell within the host
  • They combine their genetic material
• Rapid variation is produced in the surface proteins of the virus 
• A new virus is created which is not recognised by the host's immune system
• The influenza virus undergoes antigenic shift

Treating disease caused by rapidly evolving viruses

• Vaccines
  • For rapidly evolving viruses, vaccines need to be changed and updated yearly so that they remain effective
  • This is a successful approach for viruses that undergo antigenic drift because the changes are small and not hugely rapid
    • Although HIV undergoes genetic drift, it does so at an unusually rapid rate so a vaccine has not yet been successful
  • For viruses undergoing antigenic shift vaccines are not so successful because the changes are rapid and not predictable
• Fast-evolving viruses may need to be dealt with by the isolation of infected individuals to stop the spread of infection