I think it's more likely that the imbalance between the sexes will correct itself through any of the myriad feedback mechanisms before it has a significant effect on Chinese evolution.
For example, parental preference could shift back to girls (or at least even out) since couples will probably be expected to support all four parents in their old age, and eligible bachelorettes will become a valuable commodity. Having a boy becomes an evolutionary gamble: a daughter will surely bring grandchildren but a son may die childless.
The problem has been 30 years in the making. Undoing that is going to take approximately as long.
The big question is whether China will remain stable long enough for the feedback mechanisms do their job. What with the shrinking labor force in what is currently the land of cheap labor (labor costs will rise) and the social obligations (cost) that come with an aging population, it is a pretty scary picture.
For example, parental preference could shift back to girls (or at least even out) since couples will probably be expected to support all four parents in their old age, and eligible bachelorettes will become a valuable commodity. Having a boy becomes an evolutionary gamble: a daughter will surely bring grandchildren but a son may die childless.