Organisms vary in the degree of gene exchange that they experience, ranging from zero or nearly zero to at least once per generation (with the concept of generation, in fact, delineated in these latter organisms by the occurrence of gene exchange, i.e., sex). Organisms for which gene exchange is less likely typically are those that are physically sequestered from other organisms during most or all of their life cycles. On the other hand, organisms that are obligately sexual for their replication actively seek out others for the sake of gene exchange.

The kinds of individuals with which an individual commonly exchanges genes also may be well defined. This is seen particularly with obligately sexual organisms, such as most animals, where gene exchange occurs effectively solely with conspecifics and serves to define the concept of species (i.e., biological species concept). Organisms that are not obligately sexual, however, can be much more promiscuous, such as in terms of the source of the genes that they receive. The latter appears to be the case with many gene-exchange mechanisms associated with bacteria and also can be the case for certain eukaryotic organisms, such as phagotrophic protists. For most eukaryotes, foreign-gene acquisition also occurs by a mechanism that is the result of the proximity of the eukaryotic nucleus with cytoplasmic genetic elements such as endosymbiotic organisms, that is, receiving new nuclear genes from associated endosymbiotic bacteria. The latter simultaneously is a substantial but nonetheless circumscribed promiscuity, where the endosymbiotic source of DNA is in fact not closely related to the nuclear recipient but nonetheless consists of only a single, specific DNA source.

Gene exchange can be intentional versus mostly or entirely accidental. Probably a reasonable generalization is that intentional gene exchange occurs more often over the course of an organism's life cycle and is less promiscuous whereas less- or un-intentional gene exchange occurs less often – especially when measured in terms of success rather than attempts – and can be more promiscuous. In other words, some organisms have specific adaptations that seem to be for the sake of sharing genes with a specific sub-set of other organisms (e.g., adaptions such as sexual organs) whereas other organisms can lack such adaptations. The latter nonetheless typically will experience gene exchange but have much less say in terms of who (or what) they share genes with. Much of my emphasis here will be on this latter, more-accidental acquisition of genes especially from organisms that would not normally be described as conspecifics. These mechanisms may be variously described (with number of September 30, 2014, in-quotes Google hits shown parenthetically) as horizontal gene flow (405k), horizontal gene transfer (HGT; 477k) , horizontal gene exchange (152k), lateral gene flow (1060k), lateral gene transfer (LGT; 157k), or lateral gene exchange (381k). As noted previously, the term introgression (558k) also may be employed as a synonym.