The geographical distribution is an important concept in biological sciences, however in some cases it has been interpreted inadequately. We consider that the unified species concept can help to clarify the geographical distribution concept, since incorporate the spatial dimension because the species are a separately evolving metapopulation lineage. The geographical distribution is defined as that fraction of the geographical space where a species is present and interacts in unephemeral way with the ecosystem. The presence or absence of the species in the geographic space is defined by biogeography, physiology, and ecological factors. Geographical distribution is an attribute difficult to estimate, nevertheless, it has described some patterns concerning to size, shape and boundaries. Currently, there are some methods that allow to carry out spatially explicit accurate hypothesis about the geographical distribution of the species.

• Background and AimsDioon angustifolium was considered within D. edule. Recent morphometric and allozyme studies on D. edule have shown that D. angustifolium has originated from geographic isolation and is therefore considered to be a separate species. This cycad is endemic to north-eastern Mexico and is known only from three populations in the Sierra Madre Oriental mountain chain. Its populations are small when compared with its southern relative D. edule. In this study, genetic variation was determined within and between populations of D. angustifolium and the genetic consequences of habitat fragmentation and isolation of populations of this species were assessed. • Methods Allozyme electrophoresis of 14 presumptive loci was used. The data were analysed with statistical approximations for estimating genetic diversity, structure, gene flow and recent genetic bottlenecks. • Key Results Means and standard deviations of genetic diversity estimators were: number of alleles per locus (A = 1·67 ± 0·23), percentage of polymorphic loci (P = 52·4 ± 23 %) and expected heterozygosity (H_E = 0·218 ± 0·093). The genetic variation attributable to differences among populations was 16·7 %. Mean gene flow between paired populations was Nm = 1·55 ± 0·67, which is similar to that reported for endemic plant species of narrow geographical distribution and species with gravity-dispersed seed. A recent bottleneck is detected in the populations studied. • ConclusionsDioon angustifolium presents high levels of genetic diversity compared with other cycad species, in spite of small population sizes. The recent bottleneck effect did not effectively reduce the genetic variation to the extent of eliminating these populations. The distribution of D. angustifolium appears to be the result of historical biogeographical effects related to the Pleistocene glaciations. It is recommended that this species be catalogued in the IUCN Red List of Threatened Species and conservation efforts be made to preserve it.

The genetic diversity and structure of four populations of the cycad Zamia loddigesii were studied throughout its range in Mexico. Allozyme electroph