The poor have a greater risk of being erased from the gene pool.

From A Troublesome Inheritance by Nicholas Wade. Page 162.

There were relatively few attacks on Clark's proposed mechanism of evolutionary change, the ability of the rich to leave more surviving children who would spread their genes and behavior through the population as some of them descended in social rank.

Clark has since corroborated this mechanism by devising an independent way to check it, based on the prevalence of surnames. Surnames, being passed from father to son, are effectively propagated like the Y chromosome. They track male genes, provided that wives are faithful and no one is adopted, but cases of nonpaternity and adoption were both rare in medieval England. Clark chose two sets of rare surnames, such as Banbricke, Cheveny, Reddyforde, Spatchet, and TOkelove, from English records of 1560-1640. One set belonged to men rich enough to leave a will, the other to people indicted in Essex courts for burglary, poaching and crimes of violence, and therefore assumed to be among the poorest.

For rare surnames, a large fraction of the holders will typically be related. Clark found that his rich families survived through the generations much better than the poor ones. By 1851, only 8% of the richest surnames from the 1560-1640 period had disappeared, but 21% of the indicted surnames no longer existed. The poor have a greater risk of being erased from the gene pool.

[snip]

"The surname evidence confirms a permanent selection in pre-industrial England for genes of the economically successful, and against the genes of the poor and the criminal," Clark concludes. "Their extra reproductive success had a permanent impact on the genetic composition of later population."