What is indirect vs direction selection of genes?
Direct selection is when the fitness is directly associated with a certain gene. The beneficial alleles of this gene get selected.

When an allele gets selected not because of its direct effect on fitness but because of other reasons for e.g. strong linkage with other genes which in turn are being selected, then it is said to be undergoing indirect selection. 

From Kirkpatrick and Barton (1997)


  Female preferences have caused the evolution of extreme male mating
  displays throughout the animal kingdom (1–3). One mechanism that might
  establish these preferences is indirect selection, which occurs when
  preference genes are associated (that is, in linkage disequilibrium)
  with other genes that are spreading under selection. Indirect
  selection is invoked by two types of theories (2). The first are “good
  genes” theories, which interpret male mating displays as indicators of
  high viability. In this view, the expression of the male display is
  correlated with the presence of alleles that increase viability.
  Because associations naturally develop between preference genes and
  male trait genes, as the high viability alleles spread by natural
  selection, preference alleles for extreme displays also spread. The
  second class of theories that postulates indirect selection involves a
  “runaway process.” Here alleles that exaggerate a male mating display
  decrease survival but nevertheless spread because they enhance male
  mating success. Genes for extreme preferences can then be established
  as the result of their associations with these spreading male trait
  genes.