Core of the epigenetic approach to evolution
According to the modern epigenetic concept, the subject of evolution is not individual genes, but integrated developmental systems of the organism. Epigenetic mechanisms significantly influence the historical development of species by allowing phenotype alterations without initial genotype restructuring.
When a population enters unusual living conditions, evolutionary change is triggered. In response to external influences, a whole "fan" of phenotypes—a spectrum of various modifications—arises under the exact same genotype. These new external factors affect ontogeny (individual development), causing the appearance of a significant number of unusual phenotypes called morphoses. It is precisely the bearers of such multidirectional morphogenetic reactions at the individual level that serve as material for natural selection. It is important to note that morphoses are realized by the developmental system only when environmental conditions deviate from normal.
Mechanisms of epigenetic inheritance
For phenotypic changes to be transmitted to offspring without altering the DNA nucleotide sequence, a number of molecular systems function within cells. The following main mechanisms are distinguished:
- DNA methylation — the addition of methyl groups to nitrogenous bases.
- Chromatin remodeling — altering the DNA packing structure to regulate gene accessibility.
- RNA interference — suppression of gene expression using small RNA molecules.
- Protein prionization — transmission of information via changes in the spatial conformation of proteins.
- X-chromosome inactivation — the silencing of one of the sex chromosomes in female mammals.
Genetic assimilation: from phenocopy to genocopy
The emerging phenotypic variations are not random: strictly specific environmental changes cause specific morphoses. Thus, the external environment dictates the direction of evolution.
If the resulting morphose proves to be adaptive (i.e., allowing the organism to successfully survive in new conditions), genetic assimilation occurs during natural selection. This is a large-scale polygenic process implemented through changes in multiple genes of the genotype. The resulting genetic differences affect several loci and can essentially be located on any chromosomes.
As a result of genetic assimilation, an initially non-hereditary change—a phenocopy—turns into a genocopy (a hereditarily fixed trait). The outcome of this process is that the morphose acquires genetic conditioning and begins to be realized in ontogeny regardless of the external conditions that originally induced it.
Examples in natural populations
A distinctive feature of epigenetic variability is its mass nature. Morphoses appear immediately in large groups of individuals rather than spontaneously and singly. Numerous such phenomena are described in nature:
- Albino mice: a classic example described by the geneticist R. Goldschmidt, when a simultaneous mass emergence of a large number of albino mice was recorded in Bavaria.
- Polymelia in frogs: the appearance of extra limbs, which can affect up to 30% of individuals in a population. The nature of this trait is twofold: it can act as a dominant inherited trait or be induced by an external factor. Interestingly, in some individuals, these extra legs remain fully functional.
Briefly: answers to topical questions
Which enzymes carry out DNA methylation?
DNA methyltransferase enzymes participate in DNA methylation.
They add methyl groups to specific DNA sites, usually at cytosine followed by guanine (CpG sites). MeCPs proteins bind to methylated nucleotides, recruiting repressor complexes, which suppresses gene expression.
What types of small RNA molecules are involved in the mechanism of RNA interference?
MicroRNAs (miRNAs) and small interfering RNAs (siRNAs) are involved in the mechanism of RNA interference.
- MicroRNAs (miRNAs) — endogenous, self-complementary, single-stranded molecules 21–22 nucleotides in size. They inhibit translation or remove the poly(A) tail.
- Small interfering RNAs (siRNAs) — double-stranded molecules 21–25 nucleotides in size. They suppress gene activity during and after transcription.
How exactly is information transmitted during protein prionization?
Information transmission during protein prionization occurs due to changes in the spatial structure of normal proteins upon contact with pathological ones.
The contact of a normal protein ($PrP^c$) with an altered infectious prion ($PrP^{sc}$) leads to the transformation of the normal protein into a pathological one. As a result, the protein conformation changes, leading to the development of conformational diseases (dysproteininoses).
Twin terms: A Phenocopy is a "Photo" (temporary, depends on flash/environment), whereas a Genocopy is an "Engraving" (etched into genes forever).
Class and exam questions
What is the triggering mechanism of evolution in this theory?
The process is triggered when a population enters unusual living conditions, which provokes disruptions in ontogenesis.
What is a morphose?
This is an unusual phenotype that arises in response to environmental conditions deviating from normal and serves as material for selection.
What is the essence of genetic assimilation?
This is a process by which a beneficial morphose becomes fixed in the genotype through polygenic changes and begins to form even without the action of the original external stimulus.
What else to study on this topic
- Molecular mechanisms of DNA methylation
- Chromatin remodeling processes
- The role of RNA interference in the regulation of ontogeny
- Prions and protein prionization as a factor of heredity
- The mechanism of X-chromosome inactivation
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