AP Syllabus focus:
‘Genetic variation also increases through horizontal gene transfer in prokaryotes and recombination events in viruses and sexually reproducing organisms.’
Genetic variation is not produced only by mutation. Many organisms also gain new allele combinations or entirely new genes through DNA exchange between individuals, species, or genomes, accelerating evolutionary change.
Horizontal gene transfer (HGT) in prokaryotes
Horizontal gene transfer moves DNA between organisms without parent-to-offspring inheritance, rapidly introducing novel traits into a population (e.g., new metabolic pathways or antibiotic resistance).
Horizontal gene transfer (HGT): Transfer of genetic material between organisms by mechanisms other than reproduction, producing heritable genetic change in the recipient.
HGT is especially important in bacteria and archaea because they can acquire and use foreign DNA quickly.
Major HGT mechanisms
Transformation: uptake of free DNA fragments from the environment
DNA often comes from dead cells; successful uptake requires competence (cell state enabling DNA import).
Incorporated DNA can replace a homologous region or persist if it becomes part of a stable DNA element.
Transduction: DNA transfer via bacteriophages (viruses that infect bacteria)
A phage can mistakenly package host DNA and deliver it to a new bacterial cell.
Recipient DNA can recombine with the chromosome, altering genotype.
Conjugation: direct DNA transfer between cells through cell-to-cell contact
This diagram illustrates bacterial conjugation, in which a donor cell forms a pilus to contact a recipient cell and transfer a plasmid strand through a mating bridge. It emphasizes that plasmids can move horizontally between unrelated cells and then be replicated to restore double-stranded DNA in both donor and recipient. This mechanism is a major route for rapid spread of traits such as antibiotic resistance within bacterial populations. Source
Typically involves plasmids that encode transfer machinery.
Can spread multi-gene traits (e.g., resistance genes clustered on plasmids).
Consequences of HGT for genetic variation
Creates genetic mosaics: genomes assembled from different sources.
Allows rapid adaptation under strong selection (e.g., antibiotic exposure).
Can move genes across distantly related lineages, complicating “tree-like” evolutionary patterns.
Recombination as a source of variation
HGT often becomes permanent only when incoming DNA is integrated or maintained. This depends on recombination, which reshuffles DNA to create new allele combinations.
Genetic recombination: Rearrangement of DNA sequences that produces new combinations of alleles, commonly through exchange between DNA molecules or within a genome.
Recombination is a key driver of variation in viruses and sexually reproducing organisms, and it also stabilizes many HGT events in prokaryotes.
Recombination in viruses
Viruses generate variation by exchanging or mixing genetic material during infection, especially when multiple viral genomes are present in the same host cell.
Homologous recombination can occur when related viral genomes align and exchange segments.
Reassortment (in segmented viruses) mixes genome segments from different strains, producing novel combinations in one replication cycle.
These processes can change traits such as host range or immune recognition, influencing viral evolution.
Recombination in sexually reproducing organisms
Sexual reproduction increases variation largely by producing new allele combinations rather than creating new genes.
Meiotic recombination (crossing over) exchanges corresponding DNA segments between homologous chromosomes, creating recombinant chromatids.
This figure shows crossing over during prophase I of meiosis, where non-sister chromatids of homologous chromosomes exchange corresponding DNA segments. The result is a pair of recombinant chromatids alongside chromatids that retain the original (non-recombinant) arrangement. It visually connects the physical exchange event to the creation of new allele combinations passed to gametes. Source
Independent assortment separates maternal and paternal homologs into gametes in varied combinations.
Random fertilization combines two independently produced gametes, further increasing genotype diversity.
Connecting the syllabus statement
Genetic variation increases through:
HGT in prokaryotes, which can introduce entirely new genes and traits into a lineage.
Recombination events in viruses, which can rapidly create novel genotypes.
Recombination in sexually reproducing organisms, which generates unique allele combinations each generation.
FAQ
Bacteria use multiple “genome defence” systems that reduce uptake or persistence of foreign DNA.
Restriction–modification systems cut unmodified incoming DNA.
CRISPR-Cas can target sequences from previous infections.
Costs of extra DNA can be reduced by silencing or deleting unhelpful genes over time.
Natural competence is regulated and varies by species and conditions.
It can be triggered by stress, nutrient limitation, or high cell density, and depends on membrane transport proteins that bind and import DNA. Some species rarely become competent, limiting transformation-driven variation.
Host range is the set of species in which a plasmid can replicate and persist.
Broad-host-range plasmids can spread resistance across diverse bacterial groups, while narrow-host-range plasmids mostly circulate within related species. Compatibility with replication proteins and partitioning systems strongly influences this.
Segmented genomes allow whole segments to be swapped when two strains co-infect the same cell.
This can produce large genetic shifts in a single generation, rapidly creating new combinations of traits (e.g., antigenic profiles) without needing many point mutations.
Recombination can assemble beneficial allele combinations and separate harmful mutations from useful ones.
However, it can also break up co-adapted gene complexes, reducing fitness in some environments. The net effect depends on selection strength, linkage between genes, and the frequency of recombination.
Practice Questions
State two ways horizontal gene transfer can occur in prokaryotes. (2 marks)
Any two of: transformation; transduction; conjugation. (1 mark each)
Explain how horizontal gene transfer and recombination can increase genetic variation, referring to prokaryotes and one of viruses or sexually reproducing organisms. (5 marks)
HGT transfers DNA between organisms without reproduction / not parent-to-offspring. (1)
In prokaryotes, HGT can introduce new genes/alleles (e.g., antibiotic resistance) into a recipient cell. (1)
Incoming DNA can become heritable by recombining with the chromosome or being maintained on a plasmid. (1)
Viral recombination via exchange of genome segments or homologous recombination generates novel viral genotypes. (1)
OR sexual recombination (crossing over and/or independent assortment) produces new allele combinations in gametes/offspring. (1)
