14.4: Gene duplications and deletions

While meiotic alignment generally occurs accurately, there are times were mis-alignment happens. Consider the effects of repeated sequences within a chromosome. If the homologous chromosomes misalign, crossing over can lead to haploid cells that emerge from meiosis with either a gene duplication or a deletion (→). Such duplication events can have a liberating effect on subsequent evolutionary pathways.^[489] Most obviously, having two copies of a previously single copy gene means that it is possible to make twice as many transcripts per unit time per cell. This extra activity can be useful. For example, imagine that the original gene product was involved in inactivating an environmental toxin; one copy of the gene might not make enough protein to allow the cell/organism to survive and grow, whereas two copies might. When one analyzes bacterial (or cancer) cells that can grow in the presence of a toxic compound, it is common to find that a gene(s) that encodes a protein(s) involved in degrading or exporting the toxin from the cell has been duplicated one or more times.^[490]

Another adaptive mechanism depends upon the fact (noted above) that while a particular gene product typically has a clear “primary” activity, it can also have one or more weaker secondary activities.^[491] Assuming that a gene product’s primary function is essential for survival or reproductive success, changes that negatively influence survival or reproductive success will be selected against, even if they improve valuable secondary activities. The duplication of the gene allows the original activity to be preserved, while the duplicated gene can evolve freely, and may provide useful, off-target activities or alter when and where the gene is expressed.

---