The statistics of detecting positional fluctuating asymmetry

The detection of asymmetry can be difficult because the signals indicating right–left differences are frequently weak. When bilaterally symmetric organisms can be further inspected for positional asymmetry (differences between body sides in the placement of meristic traits) anterior–posterior allocation may provide additional evidence for fluctuating asymmetry. Statistical power comparisons were made for indices used to measure fluctuating (FA) and positional fluctuating asymmetry (PFA) in meristic traits. Monte Carlo simulations of developmental and random processes were used to generate null and alternative distributions of a hypothetical meristic trait (such as bristle counts). The counts were used to calculate the corresponding distributions of six indices used to detect fluctuating and positional fluctuating asymmetry. Power comparisons show that there is no single best index. If positional deviations are strong relative to bilateral errors then pfa1 is generally the most powerful, while fa, the G-statistic, and in some instances the procrustes-distance measure do best when there is little positional deviation relative to side-to-side deviations. The decision on what index is best is thus influenced by knowledge of the likely sources of error. As a consequence, all three indices (fa, G and pfa1) should be explored when little a priori knowledge is available. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 77 , 491–498.     

Environs: The Superniches of Ecosystems

Evolution proceeds by natural selection of heritable variationsof individual organisms based on direct influences of environment.However, indirect effects probably vastly outweigh direct onesin ecosystems. Therefore, why is evolution based on direct effectsonly? The ecological niche represents the point of direct contactbetween organisms and their environments. To encompass indirectinfluences, niches are extended to new structures, environs,which are units of organism-environment coevolution. The motiveforce for coevolution is closure of outputs back upon inputsof the organism members of ecosystems. Closure is achieved bybiogeochemical cycling and feedback interactions, direct andindirect, between organisms. To the extent that closure doesnot occur, there is no imperative for organism-environment coevolution.Coevolution at the system level based on indirect effects iscompatible with normal evolution at the individual organismlevel based on direct effects. The organism is the unit of thelatter, but environs are the unit of coevolution.