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Molecular evolution

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Title: Molecular evolution  
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Subject: Gene duplication, Genetics, Mutation, Models of DNA evolution, Evolutionary biology
Collection: Molecular Evolution
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Molecular evolution

Molecular evolution is a change in the sequence composition of cellular molecules such as DNA, RNA, and proteins across generations. The field of molecular evolution uses principles of evolutionary biology and population genetics to explain patterns in these changes. Major topics in molecular evolution concern the rates and impacts of single nucleotide changes, neutral evolution vs. natural selection, origins of new genes, the genetic nature of complex traits, the genetic basis of speciation, evolution of development, and ways that evolutionary forces influence genomic and phenotypic changes.


  • Forces in molecular evolution 1
    • Mutation 1.1
    • Recombination 1.2
      • Gene conversion 1.2.1
    • Genetic drift 1.3
    • Selection 1.4
  • Genome architecture 2
    • Genome size 2.1
    • Repetitive elements 2.2
    • Chromosome number and organization 2.3
    • Gene content and distribution 2.4
    • Organelles 2.5
  • Origins of new genes 3
  • Molecular phylogenetics 4
  • The driving forces of evolution 5
  • Discordance with morphological evolution 6
  • Journals and societies 7
  • See also 8
  • References 9
  • Further reading 10

Forces in molecular evolution

The content and structure of a genome is the product of the molecular and population genetic forces which act upon that genome. Novel genetic variants will arise through mutation and will spread and be maintained in populations due to genetic drift or natural selection.


Mutations are permanent, transmissible changes to the genetic material (DNA or RNA) of a cell or virus. Mutations result from errors in DNA replication during cell division and by exposure to radiation, chemicals, and other environmental stressors, or viruses and transposable elements. Most mutations that occur are single nucleotide polymorphisms which modify single bases of the DNA sequence. Other types of mutations modify larger segments of DNA and can cause duplications, insertions, deletions, inversions, and translocations.

Most organisms display a strong bias in the types of mutations that occur with strong influence in GC-content. Transitions (A ↔ G or C ↔ T) are more common than transversions (purinepyrimidine)[1] and are less likely to alter amino acid sequences of proteins.

Mutations are Genetic drift, and others will be detrimental and will be eliminated from the genome by natural selection.

Because mutations are extremely rare, they accumulate very slowly across generations. While the number of mutations which appears in any single generation may vary, over very long time periods they will appear to accumulate at a regular pace. Using the mutation rate per generation and the number of nucleotide differences between two sequences, divergence times can be estimated effectively via the molecular clock.


Recombination is a process that results in genetic exchange between chromosomes or chromosomal regions. Recombination counteracts physical linkage between adjacent genes, thereby reducing genetic hitchhiking. The resulting independent inheritance of genes results in more efficient selection, meaning that regions with higher recombination will harbor fewer detrimental mutations, more selectively favored variants, and fewer errors in replication and repair. Recombination can also generate particular types of mutations if chromosomes are misaligned.

Gene conversion

Gene conversion is a type of recombination that is the product of DNA repair where nucleotide damage is corrected using orthologous genomic regions as a template. Damaged bases are first excised, the damaged strand is then aligned with an undamaged homolog, and DNA synthesis repairs the excised region using the undamaged strand as a guide. Gene conversion is often responsible for homogenizing sequence of duplicate genes over long time periods, reducing nucleotide divergence.

Genetic drift

Genetic drift is the change of allele frequencies from one generation to the next due to stochastic effects of random sampling in finite populations. Some existing variants have no effect on fitness and may increase or decrease in frequency simply due to chance. "Nearly neutral" variants whose selection coefficient is close to a threshold value of 1 / the effective population size will also be affected by chance as well as by selection and mutation. Many genomic features have been ascribed to accumulation of nearly neutral detrimental mutations as a result of small effective population sizes.[2] With a smaller effective population size, a larger variety of mutations will behave as if they are neutral due to inefficiency of selection.


Selection occurs when organisms with greater sexual selection is a product of mate choice and can favor the spread of genetic variants which act counter to natural selection but increase desirability to the opposite sex or increase mating success. Artificial selection, also known as selective breeding, is imposed by an outside entity, typically humans, in order to increase the frequency of desired traits.

The principles of population genetics apply similarly to all types of selection, though in fact each may produce distinct effects due to clustering of genes with different functions in different parts of the genome, or due to different properties of genes in particular functional classes. For instance, sexual selection could be more likely to affect molecular evolution of the sex chromosomes due to clustering of sex specific genes on the X,Y,Z or W.

Selection can operate at the gene level at the expense of organismal fitness, resulting in a selective advantage for selfish genetic elements in spite of a host cost. Examples of such selfish elements include transposable elements, meiotic drivers, killer X chromosomes, selfish mitochondria, and self-propagating introns. (See Intragenomic conflict.)

Genome architecture

Genome size

Genome size is influenced by the amount of repetitive DNA as well as number of genes in an organism. The birds likely have experienced strong selection for reduced genome size, in response to changing energetic needs for flight. Birds, unlike humans, produce nucleated red blood cells, and larger nuclei lead to lower levels of oxygen transport. Bird metabolism is far higher than that of mammals, due largely to flight, and oxygen needs are high. Hence, most birds have small, compact genomes with few repetitive elements. Indirect evidence suggests that non-avian theropod dinosaur ancestors of modern birds [3] also had reduced genome sizes, consistent with endothermy and high energetic needs for running speed. Many bacteria have also experienced selection for small genome size, as time of replication and energy consumption are so tightly correlated with fitness.

Repetitive elements

Transposable elements are self-replicating, selfish genetic elements which are capable of proliferating within host genomes. Many transposable elements are related to viruses, and share several proteins in common.

DNA transposons are cut and paste transposable elements which excise DNA and move it to alternate sections of the genome.

non-LTR retrotransposons

LTR retrotransposons


Alu elements comprise over XX % of the human genome. They are short non-autonomous repeat sequences.

Chromosome number and organization


  • A. Meyer (Editor), Y. van de Peer, "Genome Evolution: Gene and Genome Duplications and the Origin of Novel Gene Functions", 2003, ISBN 978-1-4020-1021-7
  • T. Ryan Gregory, "The Evolution of the Genome", 2004, YSBN 978-0123014634

Further reading

  1. ^
  2. ^  
  3. ^ Organ, C. L.; Shedlock, A. M.; Meade, A.; Pagel, M.; Edwards, S. V. (2007). "Origin of avian genome size and structure in nonavian dinosaurs". Nature 446: 180–184.  
  4. ^ Crosland, M.W.J., Crozier, R.H. (1986). "Myrmecia pilosula, an ant with only one pair of chromosomes". Science 231 (4743): 1278.  
  5. ^ Gerardus J. H. Grubben (2004). Vegetables. PROTA. p. 404.  
  6. ^ Nikolai P. Kandul, Vladimir A. Lukhtanov, Naomi E. Pierce (2007), "KARYOTYPIC DIVERSITY AND SPECIATION IN AGRODIAETUS BUTTERFLIES", The Society for the Study of Evolution, 61(3):546-559,  
  7. ^ Tautz, Diethard and Domazet-Lošo, Tomislav (2011). "The evolutionary origin of orphan genes". Nature Reviews Genetics 12 (10): 692–702.  
  8. ^ Levine MT, Jones CD, Kern AD, et al. (2006). "Novel genes derived from noncoding DNA in Drosophila melanogaster are frequently X-linked and exhibit testis-biased expression". Proc Natl Acad Sci USA 103 (26): 9935–9939.  
  9. ^ Zhou Q, Zhang G, Zhang Y, et al. (2008). "On the origin of new genes in Drosophila". Genome Res 18 (9): 1446–1455.  
  10. ^ Cai J, Zhao R, Jiang H, et al. (2008). "Saccharomyces cerevisiae"De novo origination of a new protein-coding gene in . Genetics 179 (1): 487–496.  
  11. ^ Xiao W, Liu H, Li Y, et al. (2009). El-Shemy HA, ed. "A rice gene of de novo origin negatively regulates pathogen- induced defense response". PLoS ONE 4 (2): e4603.  
  12. ^ Knowles DG, McLysaght A (2009). "Recent de novo origin of human protein-coding genes". Genome Res 19 (10): 1752–1759.  
  13. ^ Wilson, Ben A.; Joanna Masel (2011). "Putatively Noncoding Transcripts Show Extensive Association with Ribosomes". Genome Biology & Evolution 3: 1245–1252.  
  14. ^ Graur, D. and Li, W.-H. (2000). Fundamentals of molecular evolution. Sinauer.  
  15. ^ Hahn, Matthew W. (February 2008). "TOWARD A SELECTION THEORY OF MOLECULAR EVOLUTION". Evolution 62 (2): 255–265.  
  16. ^ Hershberg, Ruth; Petrov, Dmitri A. (December 2008). "Selection on Codon Bias". Annual Review of Genetics 42 (1): 287–299.  
  17. ^  
  18. ^ Kimura, Motoo (1968). "Evolutionary rate at the molecular level" (PDF). Nature 217 (5129): 624–626.  
  19. ^ King, J.L. and Jukes, T.H. (1969). "Non-Darwinian Evolution" (PDF).  
  20. ^ Nachman M. (2006). C.W. Fox and J.B. Wolf, ed. ""Detecting selection at the molecular level" in: Evolutionary Genetics: concepts and case studies". pp. 103–118. 
  21. ^ The nearly neutral theory expanded the neutralist perspective, suggesting that several mutations are nearly neutral, which means both random drift and natural selection is relevant to their dynamics.
  22. ^ Ohta, T (1992). "The nearly neutral theory of molecular evolution". Annual Review of Ecology and Systematics 23 (1): 263–286.  
  23. ^  
  24. ^ Sueoka, N. (1964). "On the evolution of informational macromolecules". In In: Bryson, V. and Vogel, H.J. Evolving genes and proteins. Academic Press, New-York. pp. 479–496. 
  25. ^ Lahr, D. J.; Laughinghouse, H. D.; Oliverio, A. M.; Gao, F.; Katz, L. A. (2014). "How discordant morphological and molecular evolution among microorganisms can revise our notions of biodiversity on Earth". BioEssays 36 (10): 950–959.  


See also

The Society for Molecular Biology and Evolution publishes the journals "Molecular Biology and Evolution" and "Genome Biology and Evolution" and holds an annual international meeting. Other journals dedicated to molecular evolution include Journal of Molecular Evolution and Molecular Phylogenetics and Evolution. Research in molecular evolution is also published in journals of genetics, molecular biology, genomics, systematics, and evolutionary biology.

Journals and societies

There are sometimes discordances between molecular and morphological evolution, which are reflected in molecular and morphological systematic studies, especially of bacteria, archaea and eukaryotic microbes. These discordances can be categorized as two types: (i) one morphology, multiple lineages (e.g. morphological convergence, cryptic species) and (ii) one lineage, multiple morphologies (e.g. phenotypic plasticity, multiple life-cycle stages). Neutral evolution possibly could explain the incongruences in some cases.[25]

Discordance with morphological evolution

Mutationists hypotheses emphasize random drift and biases in mutation patterns.[23] Sueoka was the first to propose a modern mutationist view. He proposed that the variation in GC content was not the result of positive selection, but a consequence of the GC mutational pressure.[24]

Neutralist hypotheses emphasize the importance of mutation, purifying selection and random genetic drift.[17] The introduction of the neutral theory by Kimura,[18] quickly followed by King and Jukes' own findings,[19] led to a fierce debate about the relevance of neodarwinism at the molecular level. The Neutral theory of molecular evolution states that most mutations are deleterious and quickly removed by natural selection, but of the remaining ones, the vast majority are neutral with respect to fitness while the amount of advantageous mutations is vanishingly small. The fate of neutral mutations are governed by genetic drift, and contribute to both nucleotide polymorphism and fixed differences between species.[20][21][22]

Selectionist hypotheses argue that selection is the driving force of molecular evolution. While acknowledging that many mutations are neutral, selectionists attribute changes in the frequencies of neutral alleles to linkage disequilibrium with other loci that are under selection, rather than to random genetic drift.[15] Biases in codon usage are usually explained with reference to the ability of even weak selection to shape molecular evolution.[16]

Depending on the relative importance assigned to the various forces of evolution, three perspectives provide evolutionary explanations for molecular evolution.[14]

The driving forces of evolution

Molecular systematics has been made possible by the availability of techniques for chromosome. Typical molecular systematic analyses require the sequencing of around 1000 base pairs.

Molecular systematics is a product of the traditional field of systematics and molecular genetics. It uses DNA, RNA, or protein sequences to resolve questions in systematics, i.e. about their correct scientific classification or taxonomy from the point of view of evolutionary biology.

Molecular phylogenetics

may cause an extended protein that includes a previously non-coding sequence. frameshift to a regular codon or a stop codon Mutation of a [13] De novo genes may evolve from transcripts that are already expressed at low levels.[12] and humans.[11] rice[10] Novel genes can also arise from previously

Chimeric genes form when duplication, deletion, or incomplete retrotransposition combine portions of two different coding sequences to produce a novel gene sequence. Chimeras often cause regulatory changes and can shuffle protein domains to produce novel adaptive functions.

Retrotransposition creates new genes by copying mRNA to DNA and inserting it into the genome. Retrogenes often insert into new genomic locations, and often develop new expression patterns and functions.

In gene duplication, a gene sequence is copied to create redundancy. Duplicated gene sequences can then mutate to develop new functions or to specialize so that each new gene performs a subset of the original ancestral functions. In addition to duplicating whole genes, sometimes only a domain or part of a protein is duplicated so that the resulting gene is an elongated version of the parental gene.

New genes arise from several different genetic mechanisms including gene duplication, de novo origination, retrotransposition, chimeric gene formation, recruitment of non-coding sequence, and gene truncation.

Origins of new genes

In addition to the nuclear genome, endosymbiont organelles contain their own genetic material typically as circular plasmids. Mitochondrial and chloroplast DNA varies across taxa, but membrane-bound proteins, especially electron transport chain constituents are most often encoded in the organelle. Chloroplasts and mitochondria are maternally inherited in most species, as the organelles must pass through the egg. In a rare departure, some species of mussels are known to inherit mitochondria from father to son.


Different organisms house different numbers of genes within their genomes as well as different patterns in the distribution of genes throughout the genome. Some organisms, such as most bacteria, Drosophila, and Arabidopsis have particularly compact genomes with little repetitive content or non-coding DNA. Other organisms, like mammals or maize, have large amounts of repetitive DNA, long introns, and substantial spacing between different genes. The content and distribution of genes within the genome can influence the rate at which certain types of mutations occur and can influence the subsequent evolution of different species. Genes with longer introns are more likely to recombine due to increased physical distance over the coding sequence. As such, long introns may facilitate ectopic recombination, and result in higher rates of new gene formation.

Gene content and distribution

Changes in chromosome number can play a key role in speciation, as differing chromosome numbers can serve as a barrier to reproduction in hybrids. Human chromosome 2 was created from a fusion of two chimpanzee chromosomes and still contains central telomeres as well as a vestigial second centromere. Polyploidy especially allopolyploidy, which occurs often in plants, can also result in reproductive incompatibilities with parental species. Agrodiatus blue butterflies have diverse chromosome numbers ranging from n=10 to n=134 and additionally have one of the highest rates of speciation identified to date.[6]

genomes house each gene in individual chromosomes, resulting in a genome which is not physically linked. Reduced linkage through creation of additional chromosomes should effectively increase the efficiency of selection. Cilliate [5] has up to 1260 chromosomes.Ophioglossum reticulatum whereas the Adders-tongue fern [4]

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