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Folmer Bokma, Time, Species, and Separating Their Effects on Trait Variance in Clades, Systematic Biology, Volume 59, Issue 5, October 2010, Pages 602–607, https://doi.org/10.1093/sysbio/syq029
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It remains largely unknown how phenotypic differences between species develop over time (Eldredge and Gould 1972; Gould and Eldredge 1993; Stanley 1998; Gould 2002; Reznick and Ricklefs 2009). From his early writings, it seems that Darwin initially envisioned macroevolution (evolution on a geological timescale) as the long-term consequence of microevolutionary changes in subsequent generations within populations. Darwin's view of gradual, anagenetic evolution has been widely adopted by evolutionary biologists although paleontologists have repeatedly pointed out that several fossil records suggest a different pattern; of rapid evolution when a species appears, followed by morphological stasis during its subsequent lifetime (Eldredge and Gould 1972; Gould and Eldredge 1993; Stanley 1998).
In an attempt to estimate tempo and mode of phenotypic evolution for a large group of species, Ricklefs (2004) estimated by multiple regression the separate contributions of clade age and species number to phenotypic variance in subclades of passerine birds (Aves: Passeriformes). The idea of this statistical exercise was that the multiple regression would partition the morphological variance into components uniquely related to each of the independent variables (clade age and species number) and a component associated with the correlation between age and species number. Because species number but not clade age had a unique statistical influence on morphological variance, Ricklefs (2004) initially concluded that diversification was associated with speciation events.
