Mosaic evolution

Mosaic evolution (or modular evolution) is the concept that evolutionary change takes place in some body parts or systems without simultaneous changes in other parts.^[1] Another definition is the "evolution of characters at various rates both within and between species".^[2]408 Its place in evolutionary theory comes under long-term trends or macroevolution.^[2]

By its very nature, the evidence for this idea comes mainly from palaeontology. It is not claimed that this pattern is universal, but there is now a wide range of examples from many different taxa. Some examples:

• Hominid evolution: the early evolution of bipedalism in Australopithecines, and its modification of the pelvic girdle took place well before there was any significant change in the skull, or brain size.^[3][4]
• Archaeopteryx. Nearly 150 years ago Thomas Henry Huxley compared Archaeopteryx with a small theropod dinosaur, Compsognathus. These two fossils came from the Solnhofen limestone in Bavaria. He showed that the two were very similar, except for the front limbs and feathers of Archaeopteryx. Huxley's interest was in the basic affinity of birds and reptiles, which he united as the Sauropsida.^[5] The interest here is that the rest of the skeleton had not changed.
• Meadow voles during the last 500,000 years.^[6]
• The pterosaur Darwinopterus. The type species, D. modularis was the first known pterosaur to display features of both long-tailed (rhamphorhynchoid) and short-tailed (pterodactyloid) pterosaurs.^[7]
• Evolution of the horse, in which the major changes took place at different times, not all simultaneously.^[8][9]
• Mammalian evolution, especially during the Mesozoic provides a clear and well-understood example.^[10][11][12]

Background

In the neodarwinist theory of evolution, as postulated by Stephen Jay Gould, there is room for differing development, when a life form matures earlier or later, in shape and size. This is due to allomorphism. Organs develop at differing rhythms, as a creature grows and matures. Thus a "heterochronic clock" has three variants: 1) time, as a straight line; 2) general size, as a curved line; 3) shape, as another curved line.^[13]

When a creature is advanced in size, it may develop at a smaller size; alternatively, it may maintain its original size or, if delayed, it may result in a larger sized creature. That is insufficient to understand heterochronic mechanism. Size must be combined with shape, so a creature may retain paedomorphic features if advanced in shape or present recapitulatory appearance when retarded in shape. These names are not very indicative, as past theories of development were very confusing.^[13]

A creature in its ontogeny may combine heterochronic features in six vectors, although Gould considers that there is some binding with growth and sexual maturation. A creature may, for example, present some neotenic features and retarded development, resulting in new features derived from an original creature only by regulatory genes. Most novel human features (compared to closely related apes) were of this nature, not implying major change in structural genes, as was classically considered.^[13]

See also

• Co-adaptation
• Coevolution
• Coextinction
• Convergent evolution
• Evolutionary biology
• Parallel evolution

References

1. King R.C. Stansfield W.D. & Mulligan P.K. 2006. A dictionary of genetics. 7th ed, Oxford University Press. p286 ISBN 0-19-530761-5
2. Carroll R.L. 1997. Patterns and processes of vertebrate evolution. Cambridge University Press. ISBN 0-521-47809-X
3. Stanley S.M. 1979. Macroevolution: pattern and process. Freeman, San Francisco. p154 ISBN 0-7167-1092-7
4. Jurmain, Robert. et al. 2008. Introduction to Physical Anthropology. Thompson Wadsworth. p479
5. Foster, Michael and Lankester, E. Ray (eds )1898–1903. The scientific memoirs of Thomas Henry Huxley. 4 vols and supplement, Macmillan, London ISBN 1-4326-4011-9
6. Barnovsky A.D. 1993. Mosaic evolution at population level in Microtus pennsylvanicus. In Morphological changes in Quaternary mammals of North America. ed R.A. Martin & A.D. Barnovsky. Cambridge University Press. pp24–59
7. Lü J., Unwin D.M., Jin X., Liu Y. and Ji Q. 2010. Evidence for modular evolution in a long-tailed pterosaur with a pterodactyloid skull. Proceedings of the Royal Society B, 277(1680): 383-389. doi:10.1098/rspb.2009.1603 PMID 19828548
8. MacFadden, Bruce J (2003 digitization of 1999 reprint). "Fossil horses: systematics, paleobiology, and evolution of the Family Equidae". Cambridge: Cambridge University Press. ISBN 0-521-47708-5. Retrieved 6 June 2010. Check date values in: |date= (help)
9. Maynard Smith, John 1993. The theory of evolution. Cambridge University Press. 3rd ed + new Introduction. pp285–290 ISBN 0-521-45128-0
10. Kermack, D.M.; Kermack, K.A. (1984). The evolution of mammalian characters. Croom Helm. ISBN 0-7099-1534-9.
11. Kemp T.S. 2005. The origin and evolution of mammals. Oxford University Press, Oxford. ISBN 0-19-850761-5
12. Kielan-Jaworowska, Zofia; Richard L. Cifelli and Zhe-Xi Luo 2004. Mammals from the Age of Dinosaurs: origins, evolution, and structure, Columbia University Press, New York. ISBN 0-231-11918-6
13. Jay Gould, S. 1977. Ontogeny and phylogeny. Belknap Press of Harvard University Press.