Is bird flu evolving? ask many readers. Numerous warnings that the Avian Influenza virus will mutate and cause a pandemic have been in the scientific news services over the last few months. As a result we have received many questions about whether the virus is evolving.
Editorial Comment: Bird flu is one of a number of variants of the Influenza A virus. This virus does undergo regular and sometimes rapid changes that enable it to cause epidemics in birds, animals and people, but these are not evolution.
The most important way that viruses can change in a way that enables them to move to different hosts is reassortment. This occurs when two varieties of virus infect the same host. When viruses infect cells the virus is disassembled and then re-made. If two viruses infect one cell, genes from each virus can be mixed during the reassembly process. This type of re-mixing probably caused the recent SARS outbreak. Viral reassortment facilitates the movement of a virus between hosts. For example, some flu viruses can infect birds and pigs. Others can infect pigs and humans. Therefore, a bird virus and a human virus may meet in a pig and mix some genes, and the recombined bird virus can now infect humans. This does not involve making new genes, just re-distributing existing genes, so it is not evolution in any Darwinian, Neo Darwinian or Gouldian sense. They start and finish the same Kind of virus, but receive a new label as “Influenza A Virus variation (new number or letter)”.
Viruses can also change due to mutation, i.e. copying errors of their genes. Flu viruses are rather prone to this because their genetic information is stored on single stranded RNA instead of DNA, and RNA is not copied as accurately as DNA. DNA has two complementary strands of code letters, so each strand can act as a means of cross checking the stored information, but flu virus RNA has only one strand, so it has only one copy of each gene. When DNA is replicated an enzyme named DNA polymerase not only makes new copies, but also checks them for accuracy and edits any mistakes. The RNA copying enzyme does not have such a Check/Edit function, so any mistakes are left uncorrected.
Some virus mutations change the shape of their surface proteins so that the immune systems of an infected host does not recognise them straight away, and so the virus successfully invades, reproduces and therefore makes them ill until their immune system can respond. It also may mean the virus can attach to cells of other previously non infectable hosts.
Another type of mutation changes the activity level of genes involved in replicating the virus, so they replicate faster or slower. Again they start and finish the same Kind of Virus, but receive a new label as Influenza A Virus variation (new number or letter).
None of these changes are evolution. They simply cause small changes in already existing proteins. They do not explain the origin of the proteins, and the virus containing them has not changed into another type of virus.
So the brief answer is: flu viruses do change, and this may result in epidemic of flu type whatever, but no known change in viruses is an example of evolution.
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