Genetic assimilation
Genetic assimilation is a process described by
The classic example of genetic assimilation was a pair of experiments in 1942 and 1953 by Waddington. He exposed
Waddington's explanation has been controversial, and has been accused of being Lamarckian. More recent evidence appears to confirm the existence of genetic assimilation in evolution; in yeast, when a stop codon is lost by mutation, the reading frame is preserved much more often than would be expected.[3] Genetic assimilation has been incorporated into the extended evolutionary synthesis.[4][5][6][7]
History
Waddington's experiments
Conrad H. Waddington's classic experiment (1942) induced an extreme environmental reaction in the developing embryos of Drosophila. In response to ether vapor, a proportion of embryos developed a radical phenotypic change, a second thorax. At this point in the experiment bithorax is not innate; it is induced by an unusual environment. Waddington then repeatedly selected Drosophila for the bithorax phenotype over some 20 generations. After this time, some Drosophila developed bithorax without the ether treatment.[8]
Waddington carried out a similar experiment in 1953, this time inducing the cross-veinless phenocopy in Drosophila with a heat shock, with 40% of the flies showing the phenotype prior to selection. Again he selected for the phenotype over several generations, applying heat shock each time, and eventually the phenotype appeared even without heat shock.[9][10]
Waddington's explanation
Waddington called the effect he had seen "genetic assimilation". His explanation was that it was caused by a process he called "
A Darwinian explanation
Other evolutionary biologists have agreed that assimilation occurs, but give a different, purely
Perturbations can be genetic or epigenetic rather than environmental. For example, Drosophila fruit flies have a heat shock protein, Hsp90, which protects the development of many structures in the adult fly from heat shock. If the protein is damaged by a mutation, then just as if it were damaged by the environmental effects of drugs, many different phenotypic variants appear; if these are selected for, they quickly establish without further need for the mutant Hsp90.[12]
A mutational explanation
In 2017, L. Fanti and colleagues replicated Waddington's experiments, but included DNA sequencing, revealing that the wing phenotypes were due to mutational events, small deletions and the insertions of transposable elements that were mobilised by the heat exposure.[13]
Neo-Darwinism or Lamarckism
Waddington's theory of genetic assimilation was controversial.
Relationship to adaptation
Mathematical modeling suggests that under certain circumstances, natural selection favours the evolution of canalization that is designed to fail under extreme conditions.
A 2023 transcriptomic analysis revealed that genetic assimilation in environmental adaptations is rare.[26]
In natural populations
Several instances of genetic assimilation have been documented contributing to natural selection in the wild. For example, populations of the island tiger snakes (
In another example, patterns of left-right asymmetry or "handedness", when present, can be determined either genetically or plastically. During evolution, genetically determined directional asymmetry, as in the left-oriented human heart, can arise either from a nonheritable (phenotypic) developmental process, or directly by
A third example has been seen in yeast. Evolutionary events in which stop codons are lost preserve the reading frame much more often than would be expected from mutation bias. This finding is consistent with the role of the yeast prion [PSI+] in epigenetically facilitating stop codon readthrough, followed by genetic assimilation via the permanent loss of the stop codon.[3]
See also
- Evolutionary developmental biology
- List of genetics-related topics
References
- ^ a b Pocheville, Arnaud; Danchin, Etienne (January 1, 2017). "Chapter 3: Genetic assimilation and the paradox of blind variation". In Huneman, Philippe; Walsh, Denis (eds.). Challenging the Modern Synthesis. Oxford University Press.
- ISBN 978-0306438424.
- ^ PMID 17099057.
- ^ PMID 16731812.
- ^ Pigliucci, Massimo. Phenotypic Plasticity. In Massimo Pigliucci, and Gerd B. Müller (eds), Evolution: The Extended Synthesis (Cambridge, MA, 2010; online edn, MIT Press Scholarship Online, 22 Aug. 2013).
- PMID 29763656.
- doi:10.1086/714999.
- S2CID 4127926.
- JSTOR 2405747.
- ^ S2CID 11050630.
- ISBN 978-0-582-24302-6.
- S2CID 204996106.
- PMID 28576865.
- ISBN 978-1-4615-6823-0.
- JSTOR 2405747.
- PMID 25788723.
- S2CID 84217300.
- S2CID 19632484.
- S2CID 83910453.
- ^ S2CID 9292273.
- ^ PMID 9691063.
- PMID 12940351.
- JSTOR 3883742.
- ^ Rendel, J. M. (1968). R. C. Lewinton (ed.). Genetic control of developmental processes. Syracuse University Press. pp. 47–68.
- ISBN 978-1-4008-2010-8.
- PMID 37756399.
- ^ S2CID 205091.
- S2CID 32054147.