Biomedical subjects

Jon R Bridle

Publications and source records attributed to Jon R Bridle.

5 recordsLinked to original sources

PubMed2006

Limits to evolution at range margins: when and why does adaptation fail?

What stops populations expanding into new territory beyond the edge of a range margin? Recent models addressing this problem have brought together population genetics and population ecology, and some have included interactions among species at range edges. Here, we review these models of adaptation at environmental or parapatric margins, and discuss the contrasting effects of migration in either swamping local adaptation, or supplying the genetic variation that is necessary for adaptation to continue. We illustrate how studying adaptation at range margins (both with and without hybridization) can provide insight into the genetic and ecological factors that limit evolution more generally, especially in response to current rates of environmental change.

Adaptation, Biological↗

PubMed2004

Speciation in the apple maggot fly: a blend of vintages?

The importance of speciation without the complete geographical separation of diverging populations (sympatric speciation) has become increasingly accepted. One of the textbook examples of recent speciation in sympatry is the apple maggot fly Rhagoletis pomonella, in which genetically differentiated host races feed on either hawthorn or apple. Three recent articles by Feder and collaborators show that the history of these host races is more complicated than was previously realized. Genes that differentiate races of flies that feed on either apple or hawthorn are located in chromosomal rearrangements. This variation forms a latitudinal cline that must have been established long before apples were available as hosts. Furthermore, there is also new evidence for the very recent evolution of a derived preference for volatile chemicals that are typical of apple fruits among apple-feeding flies. These results show that adaptation to apple populations has involved both the sorting of ancestral adaptive variation and the selection of novel mutations.

Journal Article↗

PubMed2004

Habitat fragmentation and biodiversity: testing for the evolutionary effects of refugia.

Concordant areas of endemism among taxa have important implications both for understanding mechanisms of speciation and for framing conservation priorities. Here we discuss the need for careful testing of phylogeographic data for evidence of such concordance, with particular reference to the Indonesian island of Sulawesi. This is because there are good reasons to question whether concordance between taxa is likely to be a common pattern, and because of the serious implications of incorrectly concluding that the biodiversity of a given area can be partitioned in this way.

Biodiversity↗

PubMed2002

Low levels of chromosomal differentiation between the grasshoppers Chorthippus brunneus and Chorthippusjacobsi (Orthoptera; Acrididae) in northern Spain.

The grasshopper species Chorthippus brunneus and C. jacobsi (Orthoptera: Acrididae) form a hybrid zone in northern Spain. These species probably diverged while isolated in southern refugia during one of the recent ice ages, and are clearly distinguished by morphology and male calling song. However, in contrast to other Chorthippus taxa that form hybrid zones in Europe, these two species cannot be reliably distinguished on the basis of characteristics of the karyotype such as heterochromatin banding patterns and composition, as revealed by C-banding and fluorochrome staining. Silver staining also reveals the presence of two autosomal nucleolar organiser regions (NORs) in both species. However, differentiation between C. brunneus and C. jacobsi was revealed on the X chromosome using fluorescent in situ hybridisation (FISH). C. brunneus individuals showed additional rDNA sequences on the X chromosome that were not observed in any C. jacobsi individuals. These sequences are not transcribed, indicating either mutational silencing of an ancestral NOR on the X chromosome, or the transposition of non-functional sequences from the autosomes. The implications of these results for the evolution of NOR number in Chorthippus are discussed.

Animals↗

PubMed2002

Mating signal variation and bimodality in a mosaic hybrid zone between Chorthippus grasshopper species.

The grasshoppers Chorthippus brunneus and Chorthippus jacobsi are easily distinguished by male calling song and the number of stridulatory pegs on the hind femur, and form a mosaic hybrid zone in northern Spain. In this paper, we fit a two-dimensional cline to variation in male calling song characters, which are of particular interest as they are likely to be involved in mate choice by females. As with variation in peg number, local habitat makes only a small contribution in explaining deviations in mean song score from clinal expectations. However, the fitted width of the cline for song characters is significantly narrower than for peg number, suggesting that mating signals may be associated with reduced hybrid fitness in the field and that recombination rates are sufficient to allow clines for different characters to diverge in width. Despite this, estimates for the overall elevation in linkage disequilibrium at the zone center, based on covariance between peg and song characters, reveal a substantial overrepresentation of parental genotypes at the cline center relative to the expectations of a tension zone of similar width. Examination of covariance at individual sites reveals that this inflated estimate of linkage disequilibrium is caused by several sites where the distribution of phenotypes is effectively bimodal. This substantial variation in linkage disequilibrium at the cline center could result from local variation in the strength of assortative mating or selection against hybrids, or may reflect the long-distance colonization of empty habitat from outside the hybrid zone, which would continually create new contacts between parental genotypes at the cline center. Hybrid zones like this, in which strong linkage disequilibrium occurs in some situations but not in others, are of particular relevance to speciation research and allow investigation of the spread of combinations of alleles through different genetic and ecological backgrounds.

Animals↗