Such transfer normally involves a symbiotic relationship of some kind:
- Parasitism and mutualism;
- Direct injection of genes;
- Sexual recombination;
Direct injection of genes is common among bacteria - where it is called bacterial conjugation.
In symbiology, when gene transfer is vertical (i.e. symbiont and host generations are synchronized) the interests of symbionts and hosts tend to become aligned - what is good for the host is good for the symbiont. By contrast, when gene transfer is horizontal the interests of symbionts and hosts are not aligned. This can result in stress and problems for the host. One classic (and topical) example of this is the ebola parasite. This cares little about the welfare of its host, and treats it as a reservoir of resources to be turned into more parasites as soon as possible - often killing the host in the process.
Horizontal gene transfer produces a "reticulated" (networked, web-like) phylogeny - rather than a classic tree-like shape.
Horizontal meme transfer works in a similar way to horizontal gene transfer. The symbionts involved are cultural, and their heritable material is memes. The hosts involved are often human beings - but they can also be computer systems or organizations.
Sometimes it is claimed that cultural evolution features more horizontal transfer than than is seen in the organic realm. However, actual studies of the topic tend to show that cultural evolution and DNA-based evolution are roughly similarly reticulated. Alex Mesoudi devoted a section to this topic in his 2011 book. It starts on page 90 and is titled: "But is cultural evolution treelike". He reports on work comparing the "retention index" of 21 phylogenetic trees and 21 phylomemetic trees. The cultural trees had a retention index of 0.59, whole the organic trees managed 0.61. His citation for this work is: Collard, M; Shennan, S.J.; Tehrani, J.J.; (2006) Branching versus blending in macroscale cultural evolution: A comparative study.
Another surprisingly-common claim is that cultural evolution features horizontal and oblique meme transfer - while the organic realm, gene transfer is always vertical. For examples of this claim, see here and here. This claim is patently false.
We are currently seeing a shift from vertical meme transfer (as seen in many traditional religions) to horizontal meme transfer. Human population densities are increasing, and additionally, new meme-transfer routes are appearing - in the form of wires, optical cables, and radio waves. Consequently meme interests and the interests of the host DNA genes are likely to become increasingly different - according to standard evolutionary theory.