Australia didn't have any native rabbits. A few were introduced in the mid-1800s, and by the mid-1900s, Australia had half a billion. That's a lot of rabbits. In 1950, the myxoma virus was introduced to control the rabbit population. The virus, which had an extremely high fatality rate, was very effective. But soon after it was introduced, the virus evolved to be less virulent.
Here's why: The virus was spread from rabbit to rabbit via mosquitoes, which bit infected rabbits and transferred the virus to uninfected rabbits. It just so happened that if the host rabbits had survived longer, a greater chance existed that a mosquito would bite an infected rabbit and transfer the virus to a new host.
That being the case, you'd think that the virus would eventually evolve extremely low virulence to maximize the opportunity for transmission. But this didn't happen either, because mosquito transmission is maximized if the rabbit is covered with virus-filled lesions on which mosquitoes can feed. (If that situation sounds like it's bad for the rabbit, that's because it is.)
The myxoma virus needs a certain level of virulence to be transmitted to a new host. It's not good for the virus to kill its host immediately or to float around in the bloodstream relatively harmlessly. But it is good for the virus if the host is covered with lesions.
The rabbit population wasn't taking this virus lying down; it was evolving to be more resistant to the virus. As the rabbit population became more resistant, the virus population evolved increased virulence to maintain maximum mosquito transmissibility.
Scientists know all these things because the original rabbits (from places other than Australia that had never been exposed to the myxoma virus) and the original virus were still available for study. Using the original virus, scientists showed that the rabbits living in Australia evolved increased resistance. Using the original rabbit population, scientists showed that the virulence of the virus in the Australian rabbit population first decreased but then increased in concert with the increased resistance of the host rabbit population. So there you have it — a classic example of host-disease co-evolution in a natural setting.
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