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Fig. 1. | Evolution: Education and Outreach

Fig. 1.

From: Evolutionary Trends

Fig. 1.

Evolutionary trends represent directional changes in the average value of a given characteristic, such as body size (e.g., Alroy 1998) or some measure of complexity (e.g., number of cell types or differentiation of serially repeated limbs; Valentine et al. 1994; Adamowicz et al. 2008), among species and their descendants over prolonged periods of time. In this figure, the values of an undefined physical trait (“morphology”) of older species are given in gray, and those of newer species are shown in white. In a, new species that differ from their ancestors in this morphological trait have appeared, but this has included both increases and decreases in the parameter in question in roughly equal measure, which means there has been no net change in the average and thus no trend with regard to this feature. In b, increases have occurred but decreases have not been possible, perhaps because of a physical limitation. In this case, there is an increase in the average of the trait in younger versus older species, but this is because of the fact that diversification was free to happen in only one direction. In c, there is a clear increase in the value of the trait in the whole distribution; in fact, nearly the entire initial distribution with lower values has been replaced over time. Figure from Wagner (1996), reproduced by permission of Blackwell

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