Botanical Journal of the Linnean Society, 2003, 141, 465–470. With 2 figures © 2003 The Linnean Society of London, Botanical Journal of the Linnean Society, 2003, 141, 465–470 465 Blackwell Science, LtdOxford, UKBOJBotanical Journal of the Linnean Society0024-4074The Linnean Society of London, 2003? 2003 141? •••• Original Article MORPHOLOGICAL and GEOGRAPHIC VARIATION IN DIOON EDULE J. GONZÁLEZ-ASTORGA, A. P. VOVIDES AND C. IGLESIAS *Corresponding author. E-mail: vovidesa@ecologia.edu.mx Morphological and geographic variation of the cycad Dioon edule Lindl. (Zamiaceae): ecological and evolutionary implications J. GONZÁLEZ-ASTORGA 1 , ANDREW P. VOVIDES 2, * and CARLOS IGLESIAS 1 1 Laboratorio de Genética de Poblaciones, Jardín Botánico Clavijero, Instituto de Ecologia A. C., km 2.5 Antigua Carretera a Coatepec no. 351, Xalapa 91070, Veracruz, Mexico 2 Departamento de Sistemática Vegetal. Instituto de Ecología, A. C., km 2.5 Antigua Carretera a Coatepec no. 351, Xalapa 91070, Veracruz, Mexico Received October 2002; accepted for publication January 2003 The relationship in geographical distribution and morphological variation of leaflet width and length (diagnostic trait), between and within populations of Dioon edule Lindl., has been investigated throughout its known range in eastern Mexico (from the states of Nuevo León to Veracruz, north to south, respectively). A total of 1832 leaflets were measured for width and length from 154 plants distributed amongst five populations using four leaflet replicas from each of three leaves per plant. For leaflet width and length the variation among populations indicated significant stat- istical differences (F 4,147 = 125.83; P < 0.0001; R 2 = 92.17% and F 4,147 = 9.04; P < 0.001; R 2 = 26.8%), respectively. With respect to leaflet width, the multiple range test showed three groups with a north to south distributional relationship along the range of the species. The correlation coefficient among paired populations, respect to geographical distance and the absolute value of the mean difference of leaflet width in each population, was positive, and different from zero (r = 0.82; P = 0.013). A great variation of important ecological and evolutionary parameters was shown. © 2003 The Linnean Society of London, Botanical Journal of the Linnean Society, 2003, 141, 465–470. ADDITIONAL KEYWORDS: Cycadales – Mexico – phenotypic plasticity – populations – species complex – speciation. INTRODUCTION Cycads are the most primitive living seed plants with a fossil history going back to the Permian and possibly the Carboniferous (Mamay, 1976; Norstog & Nicholls, 1997). They are regarded popularly as ‘living fossils’ and thought to be an evolutionary ‘cul de sac’ awaiting their final extinction. This we believe to be dogma, as recent research appears to confirm. Molecular studies appear to indicate that some extant genera originated more recently during Miocene and Pleistocene times (Schneider et al., 2002). Schneider et al. (2002) also suggested that the cycad–beetle association related to entomophily is recent and not a result of an old Meso- zoic coevolution. In the genus Ceratozamia, some spe- cies appear to be of more recent origin (Pleistocene), and undergoing adaptive radiation, whilst others are found in Cenozoic floristic refuges of southern Mexico (González & Vovides, 2002). The genus appears to form species complexes showing great morphological variation (Vovides et al. in press). The study of natural variation in ecologically impor- tant traits has a long history in evolutionary ecology (Mayr, 1963; Grant, 1971; Endler, 1977, 1986). The initial motivation for studies of this nature is to further our understanding of the roles of natural selection, gene flow, inbreeding and gene drift in shaping levels of adaptation, and in affecting the distribution and abundance of populations and species. The phenotypic and genetic differentiation between populations of plant species occupying an array of environments was first experimentally demonstrated by Turesson (1922), and plant ecotypes along broad environmental gradients have been detected in many studies (e.g. Grant & Wilken, 1988; Macdonald &