This is not entirely true if the pressure for changes are removed. For
instance in mammals the bones in the ear are what used to be gills in fish.
When fish became amphibians the gills weren't needed as much and evolution
took a side path.
In nature there is a lot of duplication: several copies of genes can exist
and different copies may have a distinct evolution. There is also a lot of
'junk' DNA that doesn't code for anything (although it may have regulating
functions). In there some copies of genes may remain that are inactivated,
as well as parts of virusses, slowly obtaining random mutations because
there is no pressure on the evolution of them. Some may eventually become
active again and have different functions.
The duplication also often ensures there is fallback when random mutations
are acquired and a protein is knocked out. Besides the two chromosomes
several proteins also can have overlapping functions. The result is more
like a balance.
Evolution in nature and changes in code are different because in code junk
and bugs are constantly removed. In biology junk is allowed and may provide
a pool for future development. Linux development is intended and not
survival.
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