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Evolution on the X chromosome: unusual patterns and processes

Nature Reviews Genetics volume 7, pages 645–653 (2006)Cite this article

Key Points

  • Differences in mammalian oogenesis and spermatogenesis lead to a lower mutation rate on the X chromosome than on the autosomes.

  • If new mutations are on average partially recessive, adaptive evolution will be more widespread on the X chromosome, whereas deleterious mutations will accumulate faster on the autosomes.

  • Divergence and polymorphism data in Drosophila melanogaster and mammals suggest that, indeed, selection is more efficient on the X chromosome.

  • Sexual-antagonism models predict that the X chromosome is a hot spot for sex-biased genes, as it will accumulate both dominant female-beneficial mutations and recessive male-beneficial mutations.

  • Microarray data have shown a non-random distribution of sex-biased genes on the X chromosome and the autosomes, but the patterns differ between Caenorhabditis elegans, D. melanogaster and mammals.

  • There is an excess of gene retroposition from the X chromosome to the autosomes in mammals and D. melanogaster.

  • These new genes might be preserved by selection, as they are expressed in late spermatogenesis, when the X chromosome is inactivated.

Abstract

Although the X chromosome is usually similar to the autosomes in size and cytogenetic appearance, theoretical models predict that its hemizygosity in males may cause unusual patterns of evolution. The sequencing of several genomes has indeed revealed differences between the X chromosome and the autosomes in the rates of gene divergence, patterns of gene expression and rates of gene movement between chromosomes. A better understanding of these patterns should provide valuable information on the evolution of genes located on the X chromosome. It could also suggest solutions to more general problems in molecular evolution, such as detecting selection and estimating mutational effects on fitness.

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References

  1. Charlesworth, D., Charlesworth, B. & Marais, G. Steps in the evolution of heteromorphic sex chromosomes. Heredity 95, 118–128 (2005).

    Article  CAS  PubMed  Google Scholar 

  2. Richards, S. et al. Comparative genome sequencing of Drosophila pseudoobscura: chromosomal, gene, and cis-element evolution. Genome Res. 15, 1–18 (2005).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  3. The Chimpanzee Sequencing and Analysis Consortium. Initial sequence of the chimpanzee genome and comparison with the human genome. Nature 437, 69–87 (2005).

  4. Parisi, M. et al. Paucity of genes on the Drosophila X chromosome showing male-biased expression. Science 299, 697–700 (2003). A large-scale microarray analysis that reveals a deficit of male-biased genes on the X chromosome of D. melanogaster.

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  5. Ranz, J. M., Castillo-Davis, C. I., Meiklejohn, C. D. & Hartl, D. L. Sex-dependent gene expression and evolution of the Drosophila transcriptome. Science 300, 1742–1745 (2003).

    Article  CAS  PubMed  Google Scholar 

  6. Lercher, M. J., Urrutia, A. O. & Hurst, L. D. Evidence that the human X chromosome is enriched for male-specific but not female-specific genes. Mol. Biol. Evol. 20, 1113–1116 (2003).

    Article  CAS  PubMed  Google Scholar 

  7. Khil, P. P., Smirnova, N. A., Romanienko, P. J. & Camerini-Otero, R. D. The mouse X chromosome is enriched for sex-biased genes not subject to selection by meiotic sex chromosome inactivation. Nature Genet. 36, 642–646 (2004). The genomic distribution of early and late mouse spermatogenesis genes is analysed. This study provides evidence that the mouse X chromosome is indeed enriched in male-biased genes, once late spermatogenesis genes are excluded.

    Article  CAS  PubMed  Google Scholar 

  8. Reinke, V., Gil, I. S., Ward, S. & Kazmer, K. Genome-wide germline-enriched and sex-biased expression profiles in Caenorhabditis elegans. Development 131, 311–323 (2004).

    Article  CAS  PubMed  Google Scholar 

  9. Tao, Y., Chen, S., Hartl, D. L. & Laurie, C. C. Genetic dissection of hybrid incompatibilities between Drosophila simulans and D. mauritiana. I. Differential accumulation of hybrid male sterility effects on the X and autosomes. Genetics 164, 1383–1397 (2003).

    CAS  PubMed  PubMed Central  Google Scholar 

  10. Coyne, J. A. & Orr, H. A. Speciation (Sinauer Associates, Sunderland, 2004).

    Google Scholar 

  11. Skuse, D. H. X-linked genes and mental functioning. Hum. Mol. Genet. 14, R27–R32 (2005).

    Article  CAS  PubMed  Google Scholar 

  12. Saifi, G. M. & Chandra, H. S. An apparent excess of sex- and reproduction-related genes on the human X chromosome. Proc. Biol. Sci. 266, 203–209 (1999).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  13. Drake, J. W., Charlesworth, B., Charlesworth, D. & Crow, J. F. Rates of spontaneous mutation. Genetics 148, 1667–1686 (1998).

    CAS  PubMed  PubMed Central  Google Scholar 

  14. Keightley, P. D. & Eyre-Walker, A. Deleterious mutations and the evolution of sex. Science 290, 331–333 (2000).

    Article  CAS  PubMed  Google Scholar 

  15. Haldane, J. The mutation rate of the gene for haemophilia, and its segregation ratios in males and females. Ann. Eug. 13, 262–271 (1947).

    Article  CAS  Google Scholar 

  16. Miyata, T., Hayashida, H., Kuma, K., Mitsuyasu, K. & Yasunaga, T. Male-driven molecular evolution: a model and nucleotide sequence analysis. Cold Spring Harb. Symp. Quant. Biol. 52, 863–867 (1987).

    Article  CAS  PubMed  Google Scholar 

  17. Charlesworth, B. Evolution in Age-Structured Populations (Cambridge Univ. Press, Cambridge, 1994).

    Book  Google Scholar 

  18. Kimura, M. Evolutionary rate at the molecular level. Nature 217, 624–626 (1968).

    Article  CAS  PubMed  Google Scholar 

  19. Li, W.-H. Molecular Evolution (Sinauer Associates, Sunderland, 1997).

    Google Scholar 

  20. Drost, J. B. & Lee, W. R. Biological basis of germline mutation: comparisons of spontaneous germline mutation rates among Drosophila, mouse, and human. Environ. Mol. Mutagen. 25 (Suppl. 26), 48–64 (1995).

    Article  CAS  PubMed  Google Scholar 

  21. Drost, J. B. & Lee, W. R. The developmental basis for germline mosaicism in mouse and Drosophila melanogaster. Genetica 102/103, 421–443 (1998).

    Article  Google Scholar 

  22. Bauer, V. L. & Aquadro, C. F. Rates of DNA sequence evolution are not sex-biased in Drosophila melanogaster and D. simulans. Mol. Biol. Evol. 14, 1252–1257 (1997).

    Article  CAS  PubMed  Google Scholar 

  23. Ebersberger, I., Metzler, D., Schwarz, C. & Paabo, S. Genomewide comparison of DNA sequences between humans and chimpanzees. Am. J. Hum. Genet. 70, 1490–1497 (2002).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  24. Mcvean, G. T. & Hurst, L. D. Evidence for a selectively favourable reduction in the mutation rate of the X chromosome. Nature 386, 388–392 (1997).

    Article  CAS  PubMed  Google Scholar 

  25. Malcom, C. M., Wyckoff, G. J. & Lahn, B. T. Genic mutation rates in mammals: local similarity, chromosomal heterogeneity, and X-versus-autosome disparity. Mol. Biol. Evol. 20, 1633–1641 (2004).

    Article  CAS  Google Scholar 

  26. Lercher, M. J., Williams, E. J. B. & Hurst, L. D. Local similarity in evolutionary rates extends over whole chromosomes in human–rodent and mouse–rat comparisons: implications for understanding the mechanistic basis of the male mutation bias. Mol. Biol. Evol. 18, 2032–2039 (2001).

    Article  CAS  PubMed  Google Scholar 

  27. Huttley, G. A., Jakobsen, I. B., Wilson, S. R. & Easteal, S. How important is DNA replication for mutagenesis? Mol. Biol. Evol. 17, 929–937 (2000).

    Article  CAS  PubMed  Google Scholar 

  28. Taylor, J., Tyekucheva, S., Zody, M., Chiaromonte, F. & Makova, K. D. Strong and weak male mutation bias at different sites in the primate genomes: insights from the human–chimpanzee comparison. Mol. Biol. Evol. 23, 565–573 (2005). Shows that, in hominoids, male-driven evolution is the main cause of deceased neutral X-linked divergence when CpG sites are excluded.

    Article  CAS  PubMed  Google Scholar 

  29. Axelsson, E., Smith, N. G. C., Sundstrom, H., Berlin, S. & Ellegren, H. Male-biased mutation rate and divergence in autosomal, Z-linked and W-linked introns of chicken and turkey. Mol. Biol. Evol. 21, 1538–1547 (2004).

    Article  CAS  PubMed  Google Scholar 

  30. Sundstrom, H., Webster, M. T. & Ellegren, H. Reduced variation on the chicken Z chromosome. Genetics 167, 377–385 (2004).

    Article  PubMed  PubMed Central  Google Scholar 

  31. Montell, H., Fridolfsson, A.-K. & Ellegren, H. Contrasting levels of nucleotide diversity on the avian Z and W sex chromosomes. Mol. Biol. Evol. 18, 2010–2016 (2001).

    Article  CAS  PubMed  Google Scholar 

  32. Haldane, J. B. S. A mathematical theory of natural and artificial selection. Part I. Trans. Camb. Philos. Soc. 23, 19–41 (1924).

    Google Scholar 

  33. Rice, W. R. Sex chromosomes and the evolution of sexual dimorphism. Evolution 38, 735–742 (1984). This is the theoretical framework on which most of the work on the genomic distribution of sex-biased genes is based.

    Article  PubMed  Google Scholar 

  34. Charlesworth, B., Coyne, J. A. & Barton, N. H. The relative rates of evolution of sex-chromosomes and autosomes. Am. Nat. 130, 113–146 (1987). The rates of evolution at X-linked and autosomal sites are modelled and the conditions for faster X-chromosome evolution are determined.

    Article  Google Scholar 

  35. Kirkpatrick, M. & Hall, D. W. Male-biased mutation, sex linkage, and the rate of adaptive evolution. Evolution 58, 437–440 (2004).

    Article  PubMed  Google Scholar 

  36. Orr, H. A. & Betancourt, A. J. Haldane's sieve and adaptation from the standing genetic variation. Genetics 157, 875–884 (2001).

    CAS  PubMed  PubMed Central  Google Scholar 

  37. Fay, J. C., Wyckoff, G. J. & Wu, C.-I. Positive and negative selection on the human genome. Genetics 158, 1227–1234 (2001).

    CAS  PubMed  PubMed Central  Google Scholar 

  38. Smith, N. G. & Eyre-Walker, A. Adaptive protein evolution in Drosophila. Nature 415, 1022–1024 (2002).

    Article  CAS  PubMed  Google Scholar 

  39. Bierne, N. & Eyre-Walker, A. The genomic rate of adaptive amino acid substitution in Drosophila. Mol. Biol. Evol. 21, 1350–1360 (2004).

    Article  CAS  PubMed  Google Scholar 

  40. Sawyer, S. A., Kulathinal, R. J., Bustamante, C. D. & Hartl, D. L. Bayesian analysis suggests that most amino acid replacements in Drosophila are driven by positive selection. J. Mol. Evol. 57 (Suppl. 1), S154–S164 (2003).

    Article  CAS  PubMed  Google Scholar 

  41. Betancourt, A. J., Presgraves, D. C. & Swanson, W. J. A test for faster X evolution in Drosophila. Mol. Biol. Evol. 19, 1816–1819 (2002).

    Article  CAS  PubMed  Google Scholar 

  42. Thornton, K. & Long, M. Rapid divergence of gene duplicates on the Drosophila melanogaster X chromosome. Mol. Biol. Evol. 19, 918–925 (2002).

    Article  CAS  PubMed  Google Scholar 

  43. Thornton, K. & Long, M. Excess of amino acid substitutions relative to polymorphism between X-linked duplications in Drosophila melanogaster. Mol. Biol. Evol. 22, 273–284 (2005).

    Article  CAS  PubMed  Google Scholar 

  44. Counterman, B. A., Ortiz-Barrientos, D. & Noor, M. A. Using comparative genomic data to test for fast-X evolution. Int. J. Org. Evolution 58, 656–660 (2004).

    Article  CAS  Google Scholar 

  45. Thornton, K., Bachtrog, D. & Andolfatto, P. X chromosomes and autosomes evolve at similar rates in Drosophila: no evidence for faster-X protein evolution. Genome Res. 16, 498–504 (2006).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  46. Zhang, Z., Hambuch, T. M. & Parsch, J. Molecular evolution of sex-biased genes in Drosophila. Mol. Biol. Evol. 21, 2130–2139 (2004).

    Article  CAS  PubMed  Google Scholar 

  47. Lu, J. & Wu, C.-I. Weak selection revealed by the whole-genome comparison of the X chromosome and autosomes of human and chimpanzee. Proc. Natl Acad. Sci. USA 102, 4063–4067 (2005). By evaluating the rates of evolution for a large number of human–chimpanzee gene comparisons, this study provides evidence that both weak negative selection and positive selection are more efficient at X-linked loci.

    Google Scholar 

  48. Torgerson, D. G., Kulathinal, R. J. & Singh, R. S. Mammalian sperm proteins are rapidly evolving: evidence of positive selection in functionally diverse genes. Mol. Biol. Evol. 19, 1973–1980 (2002).

    Article  CAS  PubMed  Google Scholar 

  49. Swanson, W. J., Nielsen, R. & Yang, Q. Pervasive adaptive evolution in mammalian fertilization proteins. Mol. Biol. Evol. 20, 18–20 (2003).

    Article  CAS  PubMed  Google Scholar 

  50. Torgerson, D. G. & Singh, R. S. Sex-linked mammalian sperm proteins evolve faster than autosomal ones. Mol. Biol. Evol. 20, 1705–1709 (2003).

    Article  CAS  PubMed  Google Scholar 

  51. Torgerson, D. G. & Singh, R. S. Enhanced adaptive evolution of sperm-expressed genes on the mammalian X chromosome. Heredity 96, 39–44 (2006).

    Article  CAS  PubMed  Google Scholar 

  52. Khaitovich, P. et al. Parallel patterns of evolution in the genomes and transcriptomes of humans and chimpanzees. Science 309, 1850–1854 (2005).

    Article  CAS  PubMed  Google Scholar 

  53. Powell, J. R. & Moriyama, E. N. Evolution of codon usage bias in Drosophila. Proc. Natl Acad. Sci. USA 94, 7784–7790 (1997).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  54. Hambuch, T. M. & Parsch, J. Patterns of synonymous codon usage in Drosophila melanogaster genes with sex-biased expression. Genetics 170, 1691–1700 (2005).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  55. Singh, N. D., Davis, J. C. & Petrov, D. A. X-linked genes evolve higher codon bias in Drosophila and Caenorhabditis. Genetics 171, 145–155 (2005).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  56. Mcvean, G. T. & Charlesworth, B. A population genetic model for the evolution of synonymous codon usage: patterns and predictions. Genet. Res. 74, 145–158 (1999).

    Article  Google Scholar 

  57. Gordo, I. & Charlesworth, B. Genetic linkage and molecular evolution. Curr. Biol. 11, R684–R686 (2001).

    Article  CAS  PubMed  Google Scholar 

  58. Betancourt, A. J., Kim, Y. & Orr, H. A. A pseudohitchhiking model of X vs autosomal diversity. Genetics 168, 2261–2269 (2004).

    Article  PubMed  PubMed Central  Google Scholar 

  59. Begun, D. J. & Whitley, P. Reduced X-linked nucleotide polymorphism in Drosophila simulans. Proc. Natl Acad. Sci. USA 97, 5960–5965 (2000).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  60. Andolfatto, P. Contrasting patterns of X-linked and autosomal nucleotide variation in Drosophila melanogaster and Drosophila simulans. Mol. Biol. Evol. 18, 279–290 (2001).

    Article  CAS  PubMed  Google Scholar 

  61. Kauer, M., Zangerl, B., Dieringer, D. & Schlotterer, C. Chromosomal patterns of microsatellite variability contrast sharply in African and non-African populations of Drosophila melanogaster. Genetics 160, 247–256 (2002).

    CAS  PubMed  PubMed Central  Google Scholar 

  62. Mousset, S. & Derome, N. Molecular polymorphism in Drosophila melanogaster and D. simulans: what have we learned from recent studies? Genetica 120, 79–86 (2004). A review of the polymorphism data that have recently been collected in Drosophila species, with a discussion on the evolutionary forces that might cause the patterns observed for the X chromosome.

    Article  CAS  PubMed  Google Scholar 

  63. Schofl, G. & Schlotterer, C. Patterns of microsatellite variability among X chromosomes and autosomes indicate a high frequency of beneficial mutations in non-African D. simulans. Mol. Biol. Evol. 21, 1384–1390 (2004).

    Article  CAS  PubMed  Google Scholar 

  64. Charlesworth, B. The effect of life-history and mode of inheritance on neutral genetic variability. Genet. Res. 77, 153–166 (2001).

    Article  CAS  PubMed  Google Scholar 

  65. Haddrill, P. R., Thornton, K. R., Charlesworth, B. & Andolfatto, P. Multilocus patterns of nucleotide variability and the demographic and selection history of Drosophila melanogaster populations. Genome Res. 15, 790–799 (2005).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  66. Wang, E. T., Kodama, G., Baldi, P. & Moyzis, R. K. Global landscape of recent inferred Darwinian selection for Homo sapiens. Proc. Natl Acad. Sci. USA 103, 135–140 (2006).

    Article  CAS  PubMed  Google Scholar 

  67. Wang, P. J., Mccarrey, J. R., Yang, F. & Page, D. C. An abundance of X-linked genes expressed in spermatogonia. Nature Genet. 27, 422–426 (2001).

    Article  CAS  PubMed  Google Scholar 

  68. Lifschytz, E., Lindsley, D. L. The role of X-chromosome inactivation during spermatogenesis (Drosophila-allocycly-chromosome evolution-male sterility-dosage compensation). Proc. Natl Acad. Sci. USA 69, 182–186 (1972).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  69. Oliver, B. & Parisi, M. Battle of the Xs. Bioessays 26, 543–548 (2004).

    Article  PubMed  Google Scholar 

  70. Mueller, J. L. et al. Cross-species comparison of Drosophila male accessory gland protein genes. Genetics 171, 131–143 (2005).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  71. Gupta, V. et al. Global analysis of X-chromosome dosage compensation. J. Biol. 5, 3 (2006).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  72. Connallon, T. & Knowles, L. L. Intergenomic conflict revealed by patterns of sex-biased gene expression. Trends Genet. 21, 495–499 (2005).

    Article  CAS  PubMed  Google Scholar 

  73. Brudno, M. et al. Automated whole-genome multiple alignment of rat, mouse, and human. Genome Res. 14, 685–692 (2004).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  74. Wu, C. I. & Xu, E. Y. Sexual antagonism and X inactivation — the SAXI hypothesis. Trends Genet. 19, 243–247 (2003).

    Article  CAS  PubMed  Google Scholar 

  75. Zechner, U. et al. A high density of X-linked genes for general cognitive ability: a run-away process shaping human evolution? Trends Genet. 17, 697–701 (2001).

    Article  CAS  PubMed  Google Scholar 

  76. Wilda, M. et al. Do the constraints of human speciation cause expression of the same set of genes in brain, testis, and placenta? Cytogenet. Cell. Genet. 91, 300–302 (2000).

    Article  CAS  PubMed  Google Scholar 

  77. Arnold, P. A. Sex chromosomes and brain gender. Nature Rev. Neurosci. 5, 701–708 (2004).

    Article  CAS  Google Scholar 

  78. Nguyen, D. K. & Disteche, C. M. Dosage compensation of the active X chromosome in mammals. Nature Genet. 38, 47–53 (2006).

    Article  CAS  PubMed  Google Scholar 

  79. Guo, J. et al. In silico analysis indicates a similar gene expression pattern between human brain and testis. Cytogenet. Genome Res. 103, 58–62 (2003).

    Article  CAS  PubMed  Google Scholar 

  80. Son, C. G. et al. Database of mRNA gene expression profiles of multiple human organs. Genome Res. 15, 443–450 (2005).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  81. Betrán, E., Thornton, K. & Long, M. Retroposed new genes out of the X in Drosophila. Genome Res. 12, 1854–1859 (2002).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  82. Emerson, J. J., Kaessmann, H., Betrán, E. & Long, M. Extensive gene traffic on the mammalian X chromosome. Science 303, 537–540 (2004). References 81 and 82 describe the patterns of retroposition to and from the X chromosome in D. melanogaster and mammals, and discuss possible causes.

    Article  CAS  PubMed  Google Scholar 

  83. Vinckenbosch, N., Dupanloup, I. & Kaessmann, H. Evolutionary fate of retroposed gene copies in the human genome. Proc. Natl Acad. Sci. USA 103, 3220–3225 (2006).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  84. Khil, P. P., Oliver, B. & Camerini-Otero, R. D. X for intersection: retrotransposition both on and off the X chromosome is more frequent. Trends Genet. 21, 3–7 (2005).

    Article  CAS  PubMed  Google Scholar 

  85. Wichman, H. A., Van Den Bussche, R. A., Hamilton, M. J. & Baker, R. J. Transposable elements and the evolution of genome organization in mammals. Genetica 86, 287–293 (1992).

    Article  CAS  PubMed  Google Scholar 

  86. Langley, C. H., Montgomery, E. A., Hudson, R. H., Kaplan, N. L. & Charlesworth, B. On the role of unequal exchange in the containment of transposable element copy number. Genet. Res. 52, 223–235 (1988).

    Article  CAS  PubMed  Google Scholar 

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Acknowledgements

We thank H. Ellegren and V. Kaiser for providing us with their unpublished manuscript. B.V. is supported by a postgraduate fellowship from the Fundação de Ciência e Tecnologia of Portugal, and B.C. by the Royal Society (UK). We thank two anonymous reviewers for their comments, which helped to improve the manuscript.

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  1. Institute of Evolutionary Biology, School of Biological Sciences, University of Edinburgh, Edinburgh, EH9 3JT, UK

    Beatriz Vicoso & Brian Charlesworth

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  1. Beatriz Vicoso
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Correspondence to Brian Charlesworth.

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FURTHER INFORMATION

Institute of Evolutionary Biology, University of Edinburgh

Glossary

Haldane's rule

The disproportionate loss of fitness to the heterogametic sex in F1 hybrids between species.

Clade

A group of species which share a common ancestor.

Male heterogamety

Describes the situation in which males carry two heteromorphic sex chromosomes (such as X and Y) and females carry two copies of the same chromosome (XX).

Neutral DNA

DNA that is not subject to selection.

Silent nucleotide sites

Nucleotides where mutations do not change protein sequences.

CpG sites

Adjacent cytosine and guanine bases in a DNA sequence.

Fitness

The expected contribution of an individual to the next generation.

Genetic drift

Random fluctuation of allele frequencies in a population due to sampling effects (as only a subsample of the gametic pool is used in each generation).

Non-synonymous mutations

Mutations that change the protein sequence; these are likely to be under selection.

Fixation

Increase of an allele frequency to 1.

Positive selection

Spread of a mutation through a population, because of increased survival or reproduction of the individuals carrying it.

Purifying selection

Removal of mutations from the population, because of reduced survival or reproduction of the individuals carrying it.

Pseudogene

A gene that has accumulated mutations in its protein-coding sequence or regulatory region, so that it is no longer functional.

Codon bias

Preferred usage of some codons over others that code for the same amino acid, possibly as a result of selection for increased translation efficiency or accuracy.

Polymorphism

Genetic variation within a species or population.

Linkage disequilibrium

Non-independent associations of alleles at different loci in a population.

Ectopic recombination

Recombination between homologous sequences that are located in different genomic locations. It can result in deletions and other types of chromosomal rearrangement.

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Vicoso, B., Charlesworth, B. Evolution on the X chromosome: unusual patterns and processes. Nat Rev Genet 7, 645–653 (2006). https://doi.org/10.1038/nrg1914

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