Why variation within species increases likelihood that some species memer survive under changed conditions.?

Answer 1

If everyone had the exact same resistance to disease, then any disease which could kill one of them would kill all of them. Bummer.

If people have different resistances to different things, then there's a possibility that even if something can kill of most people, it's less likely to kill them all off.

A good example of this is the HIV virus. It eventually kills most people who get it... but there are some few who are just completely immune to the thing. If we had never discovered any treatments, then it might have been those few people who carried on for the rest of us. Or at least their genes would probably have spread wildly until most of us had a version.

So it goes.

Answer 2

In-adaptability

Answer 3

those that have the variation that is best suited to the 'changed environment' will be the fittest. it follows the idea of the 'strongest survive'.