For definition social darwinism is the application of Charles Darwin's theories of evolution and natural selection to society, especially regarding the alleged genetic inferiority of the lower classes. There is no such thing, as we now know, and has 19th century upper classes leaned with the rise of industrial revolution millionaires. Championed by William Graham Sumner (1840-1910) and British philosopher Herbert Spencer (1820-1902), who is generally credited with fusing Darwinian theories with biology and sociology in his book 'Principles of Biology.' "The poverty of the incapable, the distresses that come upon the imprudent, the starvation of the idle... are the decrees of a large, farseeing benevolence." (Spencer)
His suggestion was that if incapable people died out their incapable genes would also, however the connection between genetics and inability is not clear. Many children of Spencers' target groups have refused to accept their lot and taken charge of their own lives.
In my opinion this is where social darwinism leaves off and liberty with pvte property begins. There is no pretense at genetic superiority of groups, there is only each case for its own merits. There is no denial that some descendants have the advantage of wealth creators in their ascendants (even thieves sons are actually advantaged by the original wealth creators). Most proponents of laissez faire capitalism view mankind as a largely benevolent people unlikely to walk past the dying, the view is based upon analysis of charitable contributions, of social interaction, of emergency situation case studies and common sense observation such as 'mankind couldnt have gotten this far at eachothers throats'. My opinion is mixed, as expressed in my post earlier about why most people will spend extra income on goodies rather than all of it on charity. I think people are benevolent, that that benevolence is reduced by "we'll take care of it" authoritarian governments and that self interest and distance from the poor is also a factor in reducing charitable contribution against other ways to dispose of income.
Therefore, capitalism does not espouse the genetic 'pruning' of the race, in its ideal form the philosophy of liberty and individualism does not seek to group people together, nor tie one person coercicely to another under the name of 'duty'. I would say that modern mixed economies of the west, regardless of intent (and I doubt the intent was to be helpful), have tended to create growing state dependant 'underclasses' with attendant all round antagonism - which to me would seem to be a reverse midas touch, a serious backfire!