Decline of survival to sexual maturity in C. Elegans shielded from selective forces

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My Biology text, Evolutionary Analysis, claims that survival to sexual maturity decreased in C. elegans that originated from a single progenitor and were kept in a benign environment to decrease selective forces. This is cited from a paper by Vassilieva in 2000. My book claims that this observation indicates that the majority of mutations are deleterious. My question is, if these organisms were kept in a purely benign environment, why would their survivability decrease? Perhaps I am misunderstanding, and the survivability of the descendants of the original progenitor are lower when reintroduced to their natural environment, but my book makes it seem as though it's saying that survivability would decrease even in a benign environment. Perhaps I am just misunderstanding, but any mutations that did decrease survivability in a benign environment should still have selective forces working against them, correct?
 

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The majority of mutations are silent, not deleterious. Whether or not a mutation is beneficial, neutral or deleterious also depends on the environment. For instance, Nachman and Crowell [1] estimate that around 3 deleterious mutations out of 175 per generation in humans

[1] Nachman, M. W. and S. L. Crowell. 2000. Estimate of the mutation rate per nucleotide in humans. Genetics 156(1): 297-304.
 
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The text Evolutionary Analysis asserts otherwise. In general it states that the selection coefficients are generally (over 70% of the time) less than .02, (http://www.jstor.org/pss/2640707) but that most mutations are slightly deleterious.
 
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