> In many (most?) cases there is a tendency for the invasive organism to become less virulent.

We have discussed this at length. In a scenario where the parasite's survival depended on the survival of the host, a symbiotic relationship *could* evolve. But if the parasite is always hopping to a new host, there is no need to be symbiotic as long as there is a ready supply of new healthy hosts. 

Furthermore, species do not *always adapt*. More species have gone extinct than have survived long term. Many of the long term survivors are symbionts, but many are virulent pests that wipe out other species and adapt to prey on new host species.

In short, there is evolution does not produce inevitable results. Some species undergo modifications, some find niches where they can remain the same for millions of years, and some go extinct. 

The more successful species such as humans, canines, honey bees, etc are extremely adaptable and can adjust to varying conditions. Migration is one such adaptation. Tropical honey bees frequently migrate, whereas the temperate honey bee tends to hoard and stay put. Humans fall into migrators and hunker-downers, too. Dogs, on the other hand, tag along. 

So, anyone who suggests that non-virulence is inevitable in a parasite is mistaken. Evolution has no foresight, so a species can run rampant through its supply of hosts and then go extinct itself, not having "fore-seen" this eventuality.

PLB

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