Perhaps the most enduring debate in reptile systematics has involved the giant Galá pagos tortoises (Geochelone nigra), whose origins and systematic relationships captivated Charles Darwin and remain unresolved to this day. Here we report a phylogenetic reconstruction based on mitochondrial DNA sequences from Galá pagos tortoises and Geochelone from mainland South America and Africa. The closest living relative to the Galá pagos tortoise is not among the larger-bodied tortoises of South America but is the relatively small-bodied Geochelone chilensis, or Chaco tortoise. The split between G. chilensis and the Galá pagos lineage probably occurred 6 to 12 million years ago, before the origin of the oldest extant Galá pagos island. Our data suggest that the four named southern subspecies on the largest island, Isabela, are not distinct genetic units, whereas a genetically distinct northernmost Isabela subspecies is probably the result of a separate colonization. Most unexpectedly, the lone survivor of the abingdoni subspecies from Pinta Island (''Lonesome George'') is very closely related to tortoises from San Cristó bal and Españ ola, the islands farthest from the island of Pinta. To rule out a possible recent transplant of Lonesome George, we sequenced DNA from three tortoises collected on Pinta in 1906. They have sequences identical to Lonesome George, consistent with his being the last survivor of his subspecies. This finding may provide guidance in finding a mate for Lonesome George, who so far has failed to reproduce.

Analyses of inversions in polytene chromosomes indicate that, in West Africa, Anopheles gambiae (sensu stricto) may be a complex of more than a single taxonomic unit, and these units have been called chromosomal forms. In order to determine whether this genetic discontinuity extends to the rest of the genome, as would be expected if reproductive isolation exists, we have sequenced several regions of both the nuclear and mitochondrial genomes. With one exception, we were unable to identify any nucleotide sites that differentiate the chromosomal forms. The exception was the internal transcribed spacer (ITS) of the ribosomal DNA (rDNA). Three sites in this region distinguish Mopti chromosomal form from Savanna and Bamako in Mali and Burkina Faso. However, outside these two countries, the association between chromosomal form and rDNA type does not always hold. Together with the variants in the rDNA intergenic spacer (IGS) described in the accompanying papers (della Torre et al., 2001; Favia et al., 2001), we can recognize two major types of rDNA, Type I and Type II (corresponding to molecular forms S and M in della Torre et al., 2001). Type I is widespread in West Africa and is the only type found outside of West Africa (i.e. Tanzania and Madagascar). Type II is confined to West Africa. We were unable to detect any heterozygosity for the ITS types even in five collections containing both types. A sample from the island of São Tomé could not be classified into either Type I or Type II as the rDNA had characteristics of both. In general, our results confirm that An. gambiae is not a single pan-mictic unit, but exactly how to define any new taxa remains problematic. Finally, we have found minor variants of the major rDNA types fixed in local populations; contrary to most previous studies, this suggests restricted gene flow among populations of this species.

The subterranean Isopods belonging to the genus Stenasellus have an interesting disjunct distribution in the peri-Tyrrhenian area with morphologically closely related taxa occurring in Sardinia, Corsica, Tuscany and in the Pyrenees phreatic and interstitial waters. Because the dispersal capacities of these organisms are limited, their distribution has been associated traditionally with the tectonic events leading to the separation of the Sardinia-Corsica microplate from the Pyrenees and its subsequent movement towards the Italian peninsula. We sequenced a fragment of the mtDNA cytochrome oxidase I gene (COI) for multiple populations of the S. racovitzai species-group (Corsica, Sardinia, Tuscany) and S. virei (Pyrenees). We found that multiple phylogenetic analyses always gave the same topology, which is consistent with the genetic relations found using allozyme data, and with the palaeogeography of the area. The molecular data suggest that a combination of vicariance and dispersal events explain most effectively the present distribution pattern of these organisms. We also calculated COI rates and calibrated them against absolute time, taking advantage of the availability of two geologically based time estimates. Rates on all substitutions are similar to those published for other crustaceans for the same COI fragment, including taxonomically and ecologically distant groups. Rates on third codon positions or on transversions are generally lower than those found in other crustaceans.