Abstract
The Quaternary has been described as an important time for genetic diversification and speciation. This is based on the premise that Quaternary climatic conditions fostered the isolation of populations and, in some instances, allopatric speciation. However, the 'Quaternary Ice-Age speciation model' rests on two key assumptions: (i) that biotic responses to climate change during the Quaternary were significantly different from those of other periods in Earth's history; and (ii) that the mechanisms of isolation during the Quaternary were sufficient in time and space for genetic diversification to foster speciation. These assumptions are addressed by examining the plant fossil record for the Quaternary (in detail) and for the past 410 Myr, which encompasses previous intervals of icehouse Earth. Our examination of the Quaternary record indicates that floristic responses to climate changes during the past 1.8 Myr were complex and that a distinction has to be made between those plants that were able to withstand the extremes of glacial conditions and those that could not. Generation times are also important as are different growth forms (e.g. herbaceous annuals and arborescent perennials), resulting in different responses in terms of genetic divergence rates during isolation. Because of these variations in the duration of isolation of populations and genomic diversification rates, no canonical statement about the predominant floristic response to climatic changes during the Quaternary (i.e. elevated rates of speciation or extinction, or stasis) is currently possible. This is especially true because of a sampling bias in terms of the fossil record of tree species over that of species with non-arborescent growth forms. Nevertheless, based on the available information, it appears that the dominant response of arborescent species during the Quaternary was extinction rather than speciation or stasis. By contrast, our examination of the fossil record of vascular plants for the past 410 Myr indicates that speciation rates often increased during long intervals of icehouse Earth (spanning up to 50 Myr). Therefore, longer periods of icehouse Earth than those occurring during the Quaternary may have isolated plant populations for sufficiently long periods of time to foster genomic diversification and allopatric speciation. Our results highlight the need for more detailed study of the fossil record in terms of finer temporal and spatial resolution than is currently available to examine the significance of intervals of icehouse Earth. It is equally clear that additional and detailed molecular studies of extant populations of Quaternary species are required in order to determine the extent to which these 'relic' species have genomically diversified across their current populations.
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Selected References
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- Frogley MR, Tzedakis PC, Heaton TH. Climate variability in northwest greece during the last interglacial . Science. 1999 Sep 17;285(5435):1886–1889. doi: 10.1126/science.285.5435.1886. [DOI] [PubMed] [Google Scholar]
- Hays J. D., Imbrie J., Shackleton N. J. Variations in the Earth's Orbit: Pacemaker of the Ice Ages. Science. 1976 Dec 10;194(4270):1121–1132. doi: 10.1126/science.194.4270.1121. [DOI] [PubMed] [Google Scholar]
- Hewitt G. M. Genetic consequences of climatic oscillations in the Quaternary. Philos Trans R Soc Lond B Biol Sci. 2004 Feb 29;359(1442):183–195. doi: 10.1098/rstb.2003.1388. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Hewitt G. The genetic legacy of the Quaternary ice ages. Nature. 2000 Jun 22;405(6789):907–913. doi: 10.1038/35016000. [DOI] [PubMed] [Google Scholar]
- Kadereit Joachim W., Griebeler Eva Maria, Comes Hans Peter. Quaternary diversification in European alpine plants: pattern and process. Philos Trans R Soc Lond B Biol Sci. 2004 Feb 29;359(1442):265–274. doi: 10.1098/rstb.2003.1389. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Lascoux Martin, Palmé Anna E., Cheddadi Rachid, Latta Robert G. Impact of Ice Ages on the genetic structure of trees and shrubs. Philos Trans R Soc Lond B Biol Sci. 2004 Feb 29;359(1442):197–207. doi: 10.1098/rstb.2003.1390. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Peltier W. R. Ice age paleotopography. Science. 1994 Jul 8;265(5169):195–201. doi: 10.1126/science.265.5169.195. [DOI] [PubMed] [Google Scholar]
- Stewart John R. Comment on "Buffered Tree Population Changes in a Quaternary Refugium: Evolutionary Implications". Science. 2003 Feb 7;299(5608):825–825. doi: 10.1126/science.1079388. [DOI] [PubMed] [Google Scholar]
- Tzedakis P. C., Lawson I. T., Frogley M. R., Hewitt G. M., Preece R. C. Buffered tree population changes in a quaternary refugium: evolutionary implications. Science. 2002 Sep 20;297(5589):2044–2047. doi: 10.1126/science.1073083. [DOI] [PubMed] [Google Scholar]
- Willis K. J., Whittaker R. J. Perspectives: paleoecology. The refugial debate. Science. 2000 Feb 25;287(5457):1406–1407. doi: 10.1126/science.287.5457.1406. [DOI] [PubMed] [Google Scholar]
- Willis KJ, Kleczkowski A, Briggs KM, Gilligan CA. The role of sub-milankovitch climatic forcing in the initiation of the northern hemisphere glaciation . Science. 1999 Jul 23;285(5427):568–571. doi: 10.1126/science.285.5427.568. [DOI] [PubMed] [Google Scholar]
- Zachos J., Pagani M., Sloan L., Thomas E., Billups K. Trends, rhythms, and aberrations in global climate 65 Ma to present. Science. 2001 Apr 27;292(5517):686–693. doi: 10.1126/science.1059412. [DOI] [PubMed] [Google Scholar]