Shifts in bilateral asymmetry within a distribution range: The case of thechukar partridge

Three major types of bilateral asymmetry (fluctuating asymmetry, directiona l asymmetry, and antisymmetry) have long been recognized in the literature. Little, however, is known about transitions between asymmetry types, espec ially in natural populations. It is often assumed that directional asymmetr y and antisymmetry have a larger Genetic basis than fluctuating asymmetry. This leads many scientists to exclude traits or populations showing either directional asymmetry or antisymmetry from developmental instability studie s, focusing attention on fluctuating asymmetry alone. This procedure may bi as the findings and thus our understanding of patterns of bilateral asymmet ry and the factors influencing it. To examine changes in bilateral asymmetr y across the distribution range of a species, I studied the length of the t hird toe in I I chukar partridge (Alectoris chukar) populations across a st eep environmental gradient of 320 km within the species' range in Israel. T his trait was selected due to its adaptive value in the chukar, a species t hat spends much of its activity walking, and due to its high measurement re peatability. Moving front the core toward the very extreme periphery of the range, the following four trends are detected: ( I) the expression of the directional asymmetry component significantly increases; (2) the frequency of symmetrical individuals in the population significantly decreases, with a sharp decline at the steepest part of the climatic and environmental grad ient studied, within the Mediterranean-desert ecotone; (3) mean asymmetry l evels, as estimated using the unsigned difference between the right and lef t toe, significantly increases; and (4) the range of asymmetry increases su ch that the most asymmetrical individuals originate from the very edge of t he range. These findings provide primary evidence that substantial shifts i n asymmetry may occur across short geographical distances within a species' distribution range. They show a continuum between asymmetry types and supp ort the notion that all three types of asymmetry can reflect developmental instability. Further studies of developmental instability should be designe d so that they enable detection of transitions between asymmetry types acro ss natural populations. Such a procedure may partly resolve some of the con tradictions seen in the literature regarding the relationship between bilat eral asymmetry and environmental stress.