Intergradation

Abstract: The taxonomic rank of subspecies remains highly contentious, largely because traditional subspecies boundaries have sometimes been contradicted by molecular phylogenetic data. The most complete meta-analysis to date, for instance, found that only 3% of traditional avian subspecies represented distinct phylogenetic lineages. However, the global generality of this phenomenon remains unclear due to this previous study's narrow geographic focus on continental Nearctic and Palearctic subspecies. Here, we present a new global analysis of avian subspecies and show that 36% of avian subspecies are, in fact, phylogenetically distinct. Among biogeographic realms we find significant differences in the proportion of subspecies that are phylogenetically distinct, with Nearctic/Palearctic subspecies showing significantly reduced levels of differentiation. Additionally, there are differences between island and continental subspecies, with continental subspecies significantly less likely to be genetically distinct. These results indicate that the overall level of congruence between taxonomic subspecies and molecular phylogenetic data is greater than previously thought. We suggest that the widespread impression that avian subspecies are not real arises from a predominance of studies focusing on continental subspecies in North America and Eurasia, regions which show unusually low levels of genetic differentiation. The broader picture is that avian subspecies often provide an effective short-cut for estimating patterns of intraspecific genetic diversity, thereby providing a useful tool for the study of evolutionary divergence and conservation.

Abstract: Apis mellifera is composed of three evolutionary branches including mainly African (branch A), western and northern European (branch M), and southeastern European (branch C) populations. The existence of morphological clines extending from the equator to the Polar Circle through Morocco and Spain raised the hypothesis that the branch M originated in Africa. Mitochondrial DNA analysis revealed that branches A and M were characterized by highly diverged lineages implying very remote links between both branches. It also revealed that mtDNA haplotypes from lineages A coexisted with haplotypes M in the Iberian Peninsula and formed a south-north frequency cline, suggesting that this area could be a secondary contact zone between the two branches. By analyzing 11 populations sampled along a France-Spain/Portugal-Morocco-Guinea transect at 8 microsatellite loci and the DraI RFLP of the COI-COII mtDNA marker, we show that Iberian populations do not present any trace of "africanization" and are very similar to French populations when considering microsatellite markers. Therefore, the Iberian Peninsula is not a transition area. The higher haplotype A variability observed in Spanish and Portuguese samples compared to that found in Africa is explained by a higher mutation rate and multiple and recent introductions. Selection appears to be the best explanation to the morphological and allozymic clines and to the diffusion and maintenance of African haplotypes in Spain and Portugal.

Abstract: Environmental gradients are very common and many plant species respond to them through adaptive genetic change. This can be a first step along a continuum of change that leads ultimately to the origin of fully reproductively isolated forms, i.e., ‘biological species’. Before complete reproductive isolation is achieved, hybrid zones may form between divergent lineages either through primary intergradation or secondary contact. Here, I review the literature on plant hybrid zones between native species and highlight: mode of origin (primary intergradation versus secondary contact); distribution among plant families, genera and life form; type and genotypic composition related to strength and type of reproductive isolation between parental lineages; nature of prezygotic and postzygotic reproductive barriers; level and direction of gene flow; and the stability of hybrid zones in the face of climate change. The total number of plant hybrid zones detected in a literature search was surprisingly small (137). This was the case even for areas of the world with a long history of research into plant evolution, ecology and systematics. Reasons for this are discussed, including the possibility that plant hybrid zones are naturally rare in the wild. Only for a few hybrid zones have attempts been made to distinguish between formation by primary intergradation or secondary contact, and it is assumed that most hybrid zones originate through secondary contact. From the limited information available, it appears that plant hybrid zones may frequently move in response to climate change, but long-term studies are required to substantiate this.

Abstract: Zones of secondary contact between allopatrically evolved populations have long been of particular interest to students of evolution. Former isolates often have not evolved fully efficient isolating mechanisms before rejoining and hybridization results. With the acquisition of electrophoretic techniques, the genetic aspects of natural hybridization can be examined in detail. Patterns of introgression of alleles are expected to differ depending largely on the extent to which the populations' gene pools have diverged and on the intensity of selection against \"the infiltration of genes from one balanced complex into another\" (Mayr, 1963). Theoretically, situations should be found representing a continuum from hybridization with no introgression, to free gene exchange resulting in the eventual fusion of gene pools. Genic and/or chromosomal imbalance may lead to hybrid sterility and restrict allelic exchange to the F 1 generation. Patton et al. (1972) found no evidence of genic introgression between two hybridizing species of gophers in Arizona. Lowered fitness of certain recombinant types may greatly limit allelic introgression. Hall and Selander (1973) found evidence for a very low level of introgression between karyotypically distinct \"F6\" and \"PI\" populations formerly placed in the lizard species Sceloporus grammicus. Selander et al. (1969) and Hunt and Selander (1973), working with semispecies of house mice, found steep gradients of transition in genic character across an area of contact, and differential extents of introgression of alleles among loci. They