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.

Table: Summary of Sexual Characteristics of Different Organism Types.

Organism typeDiscussion
Eukaryotes (meiosis-associated sex) •  Sexual cycles involve alternations between meiotic division and fertilization (syngamy) with mitosis intervening between steps

•  Involves most of the nuclear genome

•  Involves a combination of molecular recombination and independent assortment to achieve genetic recombination

•  Both parents typically contribute DNA more or less equally

•  Can occur every generation particularly in multicellular organisms or episodically in single-celled organisms (keeping in mind that in both cases substantial mitosis intervenes between meiotic divisions)

•  Only a fraction of eukaryotes are dependent on these sexual processes for their replication, i.e., production of the progeny organisms

•  Generally associated with low promiscuity, i.e., intra-species matings but not inter-species matings
Eukaryotes (non-meiotic sex) •  Acquisition of endosymbionts, "You are what you eat", movement of endosymbiont DNA to nucleus, virus-mediated, plasmid acquisition, and other mechanisms that result in reproductive cells being associated with exogenous DNA

•  Often involves insertion events, the product of heterologous recombination, where the organism, at least per event, gains DNA

•  Involves relatively small amounts of DNA in comparison with meiotic processes though exceptional is endosymbiont acquisition where substantial amounts of new genetic material is acquired

•  Eukaryotes are not dependent on the occurrence of these sexual mechanisms for their immediate replication

•  Non-meiotic sexual processes generally are associated with substantial promiscuity
Prokaryotes •  As mediated by transformation, transduction (virus-mediated), conjugation (plasmid-mediated), and other mechanisms that may or may not be variations on these three

•  An additional means of DNA acquisition, often not described explicitly as sexual, is prophage insertion in the course of temperate phage lysogenic infection

•  Bacteria are not dependent on these DNA acquisition mechanisms for their replication

•  Amount of DNA acquired tends to be relatively small with a few hundred kilobases of DNA at most

•  Acquired DNA either recombines molecularly with host DNA or is stably maintained as extrachromosomal plasmids

•  Recombination can be homologous or illegitimate (non-homologous, though alternatively, involving micro-homology) depending on similarity of incoming DNA to existing DNA

•  Insertions result in increases in DNA content of cells

•  Generally associated with substantial promiscuity though this is not always the case particularly when a transfer agent such as a phage or plasmid otherwise displays a limited host range, or if transformation mechanisms tend to be biased towards the DNA of conspecifics
Viruses •  There are difference among viruses in terms of the potential to display genetic recombination, molecular recombination, and reassortment

•  For many viruses it is possible to recombine not just with other viruses but also with host DNA

•  Recombination with other viruses typically requires coinfection, though coinfection need not be between two more or less simultaneously infecting viruses but instead between a newly infecting virus and one that is latently infecting (e.g., a prophage or provirus)

•  Typically greater degrees of molecular recombination are associated with dsDNA viruses, lesser but not necessarily non-existent with ssDNA viruses, and even RNA viruses can display molecular recombination

•  Only viruses that display multipartite genomes (segments) display reassortment

•  With only a few exceptions, virus replication does not require coinfection so therefore is not dependent on sex (exceptions include defective viruses which require helper viruses or instead viruses that package what essentially is a single genome across more than one virus particle)

•  The fraction of genomes that can be involved in genetic recombination, whether reassortment or molecular recombination, can be substantial including equal contribution from two parents

•  It is possible with many viruses for more than two parents to contribute to a single hybrid progeny given coinfection by more than two viruses of a single cell, though presumably two-parent events are more common than more-than-two events

•  Dependent on the degree of divergence of the parents involved, recombination can be between essentially identical individuals (clonal) or instead between substantially divergent individuals (including between virus and host DNA)

•  Likelihood of survival of recombinant progeny presumably depends on the degree of change from parental types that has occurred with lower likelihoods associated with substantial change, particularly that involving gene disruption or loss (as well as disruption of epistatic interactions), as is true for prokaryotes and eukaryotes as well

•  The enormous number of viruses in existence results in a substantial amount of survival of hybrid progeny despite expectations that most "experiments" will fail the test of natural selection

•  Virus sex can be exceptionally promiscuous, though is still limited in promiscuity by what cell types a virus can infect and most genetic recombination probably is between relatively closely related viruses