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Increase in number of nucleotides found in genetic material due either to mutation or recombination.
Insertions can be both detrimental and beneficial. They are detrimental when they disrupt adaptations such as when they occur within genes. They are particularly disruptive when they lead to frameshifts.
When insertions are the products of horizontal gene transfer then they can be quite beneficial by supplying not just new genetic material but genetic material that has survived the test of natural selection within other organisms. Even then, however, this genetic material may still be detrimental or at least neutral if insertion is into existing genes or genetic constructs, acquired gene products are metabolically disruptive, or if the new genetic material is more energetically costly than it otherwise is beneficial.
Insertions are detectable particularly via comparison. Just as with deletions, however, it is not always easy to distinguish an insertion in the focus lineage from a deletion in the comparison lineage. Alternatively, if the insertion is a consequence of horizontal gene transfer, then it can possess signatures of that nature such as variance in codon usage or base composition from the rest of the genome, association with known virus genes, or sequence similarity of insertions with that associated with yet additional organisms.
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