Seems to me that these two sources (M. Whitlock, B. Wallace) use different definitions of soft and hard selection. M. Whitlock: Soft selection occurs when the relative fitness of an individual is ...
This article cites a bunch of articles (I haven't been through them) that the effect of the two-fold cost of sex is "reduced" in stable environment or in K-selected environment. It says: [..] the ...
"Two species that reduce one another's fitness on microevolutionary (short-term) timescales can increase each other's macroevolutionary (long-term) fitness" Opinions? Intuitive Examples: ...