Re: How Drosophilas and oranges can mantain a stable evolutionary relationship?

Dear Claudia,
Think about oranges in their native habitat, where the quality of the 
fruit determines whether it's eaten by an animal, which then disperses the 
seeds.  If oranges had no defenses against fruit flies, then they would be 
completely unable to resist an attack: the fruit flies would destroy the 
fruit, reducing the chance that the seeds would be dispersed.  This in 
turn would lead to smaller populations of orange trees in the next 
generation.  
Now think about an orange tree that had a gene mutation that was a defense 
to fruit flies.  Its fruit would not be attacked, and the seeds would be 
widely dispersed, so in the following generation more of the orange trees 
would carry this gene, as they would be the successful offspring of the 
resistant mutant orange tree.  If the defense mechanism of the oranges 
against the flies was really severe, then you can easily see that fruit 
flies would give up on oranges.  
The piece of information that you are missing is that defenses against 
fruit flies have a cost.  To build up a really strong defense, orange trees 
would have to use food (photosynthate) & energy to make defensive 
compounds (poisons) or barriers against the fruit flies.  These orange 
trees would have less food & energy available for growth or seed 
production and would therefore produce fewer offspring in the next 
generation.  It's a delicate balancing act between having enough defense 
but not so much that your other functions are seriously affected.
Now have a think about humans and disease, if a virus comes along that 
immediately kills its human host without giving the person time to spread 
that virus to others, then the virus won't increase its population very 
much.  A better viral strategy might be to make the person ill and 
infectious but still able to wander around and spread the virus.  This 
might be why extremely deadly viruses are fairly rare: you're much more 
likely to pick up a mild virus that doesn't kill the resource it relies 
upon.
I hope this helps,
Yvonne Buckley