Biased gene conversion is a recombination-associated process that preferentially transmits G or C alleles over A or T during meiotic repair. This non-Mendelian bias arises when heteroduplex DNA formed during recombination is resolved in favor of GC, shifting the fixation probability of alleles independently of classical natural selection. Adam Eyre-Walker, University of Sussex, emphasized that biased gene conversion can mimic signs of positive selection by increasing fixation of GC-favoring mutations, complicating inferences about adaptive evolution. This distinction matters when interpreting genome-wide patterns of substitution.
Mechanism and genomic patterns
Molecularly, the effect is strongest at sites linked to recombination hotspots where double-strand breaks and repair are frequent. Marta Przeworski, University of Chicago, and collaborators have mapped human recombination hotspots and documented local enrichments of GC-favoring substitutions, supporting the mechanistic link between recombination activity and shifts in base composition. Kevin Ellegren, Uppsala University, reported similar associations in birds, showing that taxa with particular recombination landscapes evolve distinct GC profiles. These empirical findings demonstrate that BGC produces predictable spatial patterns: GC-rich regions often coincide with historically high recombination, producing genomic mosaics rather than uniform change.
Relevance, causes, and consequences
The primary causes of differential GC evolution therefore include the rate and localization of recombination, the molecular bias during mismatch repair, and population genetic context. Effective population size influences the efficacy of biased gene conversion because the process competes with genetic drift; larger populations allow weak biases to shape nucleotide composition more readily. Taxa differences matter: species with stable, high recombination rates develop pronounced GC-rich domains, while others remain AT-biased.
Consequences extend beyond base composition. Because BGC increases fixation of potentially deleterious GC mutations when linked to recombination, it can elevate genetic load and obscure signals used to detect adaptation. Laurent Duret, Institut des Sciences de l'Evolution de Montpellier, and Nicolas Galtier, CNRS, have argued that recognizing BGC is essential for correctly attributing evolutionary forces shaping genomes. Human genomic and cultural research must therefore consider that recombination-driven processes, not only selective pressures tied to environment or lifestyle, can underlie observed sequence variation. In conservation and comparative genomics, appreciating BGC clarifies territorial and lineage-specific genome architectures and prevents misinterpretation of molecular evidence for adaptation. Ignoring biased gene conversion risks conflating molecular mechanisms with ecological or selective narratives.