A current Nature paper describes removal of selective pressure in the lab, and the genetic results after a few hundred generations:
Nature. 2004 Aug 5;430(7000):679-82.
High mutation rate and predominance of insertions in the Caenorhabditis elegans nuclear genome.
Denver DR, Morris K, Lynch M, Thomas WK.
From the 'New & Views' article on the paper:
Denver et al. bypassed the pheno-type-bias problem by directly sequencing randomly chosen stretches of DNA in laboratory-grown worms. They also minimized selection against harmful mutations by maintaining many lines of worms, separating a single worm from each progeny and allowing it to produce the next generation by self-fertilization, without competing with other worms. Rapid and severe loss of fitness occurs in these worms because, when their numbers are reduced to one repeatedly, random mutations become fixed a phenomenon known as Muller's ratchet.
In other words, the authors removed selective pressure by removing competitors, as opposed to the scenario proposed in the opening post, which provided an 'ideal environment' to prevent selective pressure.
The primary result described in the above paper is a ten-fold increase in accumulated mutations when compared to other worms raised conventionally in the presence of other worms. The conclusion is not that the mutation rate increased in the selection-free worms, but rather that in the absence of selective pressure, mutations were not selected against, and so accumulated in the genome - resulting in low fitness.
A simple yet elegant experiment - it gets around the impossibility of providing an ideal environment, although at the cost of examination of population structure.