1978), and has pervasive effects on nearly all biological traits, from the size of other body parts (Lande 1979 Huxley 2011), to key life history traits such as fecundity (Honěk 1993) and lifespan (Peters 1986 Speakman 2005). Our results suggest that the methods routinely used by biologists to account for covariation between focal traits and body size may be prone to producing misleading estimates that result in widespread inferential errors with respect to the underpinning evolutionary processes.īody size varies widely across species (Blueweiss et al. We find that, within the field of brain size evolution, most studies show sign reversals, and thus inferences from these studies may be inconclusive or incorrect. Given that body size and brain size are usually strongly correlated, studies on the evolution of brain size provide an excellent case study to investigate the challenges associated with drawing evolutionary inferences in the presence of strong collinearity. To examine the occurrence of such sign changes in the empirical literature, we reviewed studies that have investigated the putative role of various ecological variables in driving the evolution of relative brain size. We used simulations to demonstrate that these sign reversal occurs over a wide range of parameter values common in the biological sciences. However, when these traits are strongly correlated to body size, then the partial (i.e., relative) regression coefficients can spuriously change sign compared to the original coefficients, something that could lead to erroneous evolutionary inferences. Multiple regression analysis is a commonly used method in biology to identify the ecological factors that drive evolutionary responses in key morphological or life‐history traits that are independent of associated changes in body size. Finally, we propose some approaches to mitigating this issue. We find that most studies show sign reversals and thus that the inferences of many studies in this field may be inconclusive.
Further, as a case‐in‐point, we review the literature on brain size evolution a field that explores how ecological traits relate to the evolution of relative brain size (brain size relative to body size). Here, we use simulations to demonstrate that sign reversal occurs over a wide range of parameter values common in the biological sciences. Such sign reversals are difficult to interpret in a biologically meaningful way, and could lead to erroneous evolutionary inferences if the true mechanism underlying the sign reversal differed from the proposed mechanism. However, it is not well appreciated that in the presence of strongly correlated variables, the partial (i.e., relative) regression coefficients often change sign compared to the original coefficients. Studies that seek to uncover the ecological factors that drive evolutionary responses in traits typically examine these responses relative to associated changes in body size using multiple regression analysis.
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