Darwinian Selection at Large Law Firms

Everett Bellamy, Review: Dirty secrets of legal profession (May 5, 2013):

"[Stephen J. Harper, LAWYER BUBBLE: A PROFESSION IN CRISIS (2013)] argues that the role top law schools play in putting their graduates in large law firms is misguided, particularly in light of the high turnover rate of associates at big firms (the fifth-year associate turnover rate exceeds 80%)."
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Is it even clear that large law firms generally select the "fittest" associates for partnerships?
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