“Gene duplication followed by divergence creates genes with new functions.”
§1 The claim, as taught
Gene duplication provides raw material for evolution. One copy can continue to perform the original function while the other accumulates mutations and may eventually take on a new function.Representative summary of introductory-text treatments (e.g. Campbell Biology, “Evolution of genomes”; Futuyma & Kirkpatrick Evolution). PLACEHOLDER: to be replaced with exact quotations and page numbers.
§2 The strongest evidence offered
These are the examples most often cited. We present each one fairly and say what it actually shows.
New genes in real time
In Salmonella, a gene with a weak secondary activity was copied many times under selection. The copies then specialised, one for each function. The process was complete in fewer than 3,000 generations.Antifreeze from a digestive enzyme
Antarctic notothenioid fish make antifreeze glycoproteins from a gene built partly from pieces of a trypsinogen (digestive enzyme) gene, plus a short repeat amplified many times. The authors date the conversion to roughly 5–14 million years ago.Duplicates are common, and short-lived
Comparing genomes, Lynch and Conery estimated that gene duplicates arise often but that most are silenced within a few million years. Duplication supplies plenty of spare copies; few of them last.§3 The grade: two claims hiding in one
The claim as taught combines a modest claim with a sweeping one. Graded separately, they look very different. Hover or tap a rung to see why it got its score. How the grading works →
Duplicated genes can specialise or boost an activity the ancestor already had
BStrongly supportedDuplication and divergence explain the origin of new enzymes and folds in general
DExtrapolation§4 What it would take
Here is what would move the claim up a grade. These are not impossible demands. They are the observations the claim itself implies.
- A truly new function, observedObserve a duplicate gaining a function its ancestor lacked entirely, not just a boosted side-activity.
- A passable road between enzymesShow, by experiment, a step-by-step path between two existing enzyme functions where every step works and is selectable.
- Gain that outpaces lossShow that useful divergence happens faster than duplicates are silenced, in realistic population sizes.
§5 Another reading of the same data
The best experimental cases share a feature: the new function was already there, faintly. The Salmonella gene could already do both jobs, weakly; duplication let each copy do one job well. That is a real and elegant mechanism, but it explains how an existing capability is sharpened, not where it came from.
The antifreeze gene is a remarkable story, and the sequence evidence for its trypsinogen origin is strong. But the protein it makes is mostly a simple repeat of three amino acids carrying sugars. Building it required recruiting a signal sequence and amplifying a short repeat, not inventing a new folded enzyme. It shows that something simple can be assembled from existing parts. It is a long way from showing the same for a complex enzyme.
When Gauger and Axe asked how hard it would be to convert one enzyme into a close structural relative with a different function, they estimated that seven or more coordinated changes would be needed, more than they judged feasible by mutation and selection. Mainstream responses, such as Lynch (2005), argue that neutral drift can ease such paths. That debate is ongoing.
Read through a design lens, gene families look like variations on a theme, built with built-in flexibility (weak side-activities, regulatory switches) that lets organisms adapt. Duplication then becomes a way of adjusting dosage and specialising what exists, which is exactly what the experiments show.
§6 Reasoning check
The grade weighs the evidence. This check looks at the reasoning: the step from the evidence to the conclusion. The same tests are applied to both readings. How the reasoning check works →
- Clear case
The experiments show a duplicated gene specialising in an activity the ancestor already had. The claim as taught covers the origin of new functions in general.
- Possible
“A new function” can mean a weak side-activity made strong, or a function with no precursor at all. The evidence supports the first meaning.
- Possible
Gauger and Axe studied one pair of related enzymes. Treating their estimate as a general limit on enzyme evolution generalises from a single case, as their critics point out.
§7 Questions to ponder
- Is amplifying a weak activity that already exists the same as inventing a new one?
- Antifreeze glycoprotein is a simple repeating protein. What would a comparable case look like for a complex enzyme?
- If most duplicates decay within a few million years, how often must a beneficial change occur to build a gene family?
§8 Sources
- Ohno S. Evolution by Gene Duplication. Springer (1970).C
- Näsvall J, Sun L, Roth JR, Andersson DI. “Real-time evolution of new genes by innovation, amplification, and divergence.” Science 338:384–387 (2012).B
- Chen L, DeVries AL, Cheng C-HC. “Evolution of antifreeze glycoprotein gene from a trypsinogen gene in Antarctic notothenioid fish.” PNAS 94:3811–3816 (1997).B
- Lynch M, Conery JS. “The evolutionary fate and consequences of duplicate genes.” Science 290:1151–1155 (2000).B
- Zhang J. “Evolution by gene duplication: an update.” Trends in Ecology & Evolution 18:292–298 (2003).C
- Lynch M. “Simple evolutionary pathways to complex proteins.” Protein Science 14:2217–2225 (2005).C
- Behe MJ, Snoke DW. “Simulating evolution by gene duplication of protein features that require multiple amino acid residues.” Protein Science 13:2651–2664 (2004).C
- Gauger AK, Axe DD. “The evolutionary accessibility of new enzyme functions: a case study from the biotin pathway.” BIO-Complexity 2011(1):1–17 (2011).C
Source grades are provisional. In production each links to a full published evaluation that readers can challenge.