In a message dated 11/20/99 3:55:57 PM Pacific Standard Time, firstname.lastname@example.org writes:
> Now, if mice suddenly mutated (or got genetically engineered) so that they
> live 200 years and grow as big as elephants, should we expect them to do
> large numbers, in the wild? Or would something in their natural environment
> interact with the Uber-Mice in such a way as to reduce rather than increase
> their numbers? --J. R.
Longer lifespan with no disadvantages would be selected for, roughly to the point where only a negligeable number of mice would survive that long due to other causes (a lot less than 200 years, I'd think). If the effects of the gene are more complex (only works if a particular set is in the organism, useless or harmful otherwise) then selection is less reliable, hence weaker and the effect "gives out" earlier.
Of course longer lifespan may have costs. In the Rose lab, the long-lived strains take longer to develop and lay fewer eggs in the first part of life.
Big size can be either good or bad, so it's hard to say in general. Big as an elephant would, I think, be very bad. Little mice are doing very well while the only land animal as big as an elephant (the elephant :-) will be extinct in the wild in a few decades if humans don't save the ranges.