Showing papers by "Roger K. Butlin published in 2001"

Sexual selection and speciation.

Abstract: The power of sexual selection to drive changes in mate recognition traits gives it the potential to be a potent force in speciation. Much of the evidence to support this possibility comes from comparative studies that examine differences in the number of species between clades that apparently differ in the intensity of sexual selection. We argue that more detailed studies are needed, examining extinction rates and other sources of variation in species richness. Typically, investigations of extant natural populations have been too indirect to convincingly conclude speciation by sexual selection. Recent empirical work, however, is beginning to take a more direct approach and rule out confounding variables.

Differential gene exchange between parapatric morphs of Littorina saxatilis detected using AFLP markers

Abstract: Speciation requires the acquisition of reproductive isolation, and the circumstances under which this could evolve are of great interest. Are new species formed after the acquisition of generalized incompatibility arising between physically separated populations, or may they arise as a result of the action of disruptive selection beginning with the divergence of a rather restricted set of gene loci? Here we apply the technique of amplified fragment length polymorphism (AFLP) analysis to an intertidal snail whose populations display a cline in shell shape across vertical gradients on rocky shores. We compare the FST values for 306 AFLP loci with the distribution of FST estimated from a simulation model using values of mutation and migration derived from the data. We find that about 5% of these loci show greater differentiation than expected, providing evidence of the effects of selection across the cline, either direct or indirect through linkage. This is consistent with expectations from nonallopatric speciation models that propose an initial divergence of a small part of the genome driven by strong disruptive selection while divergence at other loci is prevented by gene flow. However, the pattern could also be the result of differential introgression after secondary contact.

Spatial structure and habitat variation in a grasshopper hybrid zone

Abstract: A hybrid zone between the grasshoppers Chorthippus brunneus and C. jacobsi (Orthoptera: Acrididae) in northern Spain has been analyzed for variation in morphology and ecology. These species are readily distinguished by the number of stridulatory pegs on the hind femur. Both sexes are fully winged and inhabit disturbed habitats throughout the study area. We develop a maximum-likelihood approach to fitting a two-dimensional cline to geographical variation in quantitative traits and for estimating associations of population mean with local habitat. This method reveals a cline in peg number approximately 30 km south of the Picos de Europa Mountains that shows substantial deviations in population mean compared with the expectations of simple tension zone models. The inclusion of variation in local vegetation in the model explains a significant proportion of the residual variation in peg number, indicating that habitat-genotype associations contribute to the observed spatial pattern. However, this association is weak, and a number of populations continue to show strong deviations in mean even after habitat is included in the final model. These outliers may be the result of long-distance colonization of sites distant from the cline center or may be due to a patchy pattern of initial contact during postglacial expansion. As well as contrasting with the smooth hybrid zones described for Chorthippus parallelus, this situation also contrasts with the mosaic hybrid zones observed in Gryllus crickets and in parts of the hybrid zone between Bombina toad species, where habitat-genotype associations account for substantial amounts of among-site variation.

Evolutionary biology. Searching for speciation genes.

Abstract: The formation of new animal species often results from divergence in male sexual behaviours and female preferences The genetic basis of this sexual isolation in fruitflies is gradually being revealed

Speciation in Chitaura grasshoppers (Acrididae: Oxyinae) on the island of Sulawesi: colour patterns, morphology and contact zones

Abstract: We investigate colour pattern and morphological variation in Chitaura grasshoppers on the Indonesian island of Sulawesi, and examine the relationship between divergence in these two sets of characters and population history, as reflected by variation in mitochondrial DNA. Analysis of colour pattern variation identifies a total of 21 distinct Operational Taxonomic Units in Sulawesi, the majority of which have parapatric distributions. Patterns of phenotypic variation at two contact zones in north Sulawesi suggest genetic independence between three of these colour forms, indicating that speciation has occurred. Despite this, colour pattern divergence is only coincident with morphological differentiation at one of these contact zones. In addition, neither type of phenotypic divergence is associated with geographical structuring in mitochondrial DNA, suggesting that historical isolation has had little influence on evolutionary diversification. Instead, divergence in colour pattern and morphology appears to have occurred rapidly or under conditions of continued gene flow, possibly in response to spatially variable natural selection. This result has implications for the identification of conservation units based solely on molecular markers.

Amplified fragment length polymorphism for the analysis of genetic structure in grasshopper populations

Abstract: A recently developed PCR-based assay, amplified fragment length polymorphism (AFLP), has been used for assessing phylogenetic relationships and the genetic structure of populations in many organisms including insects. Preliminary studies show that the procedure does not work well in acridid grasshoppers since it leads to very large numbers of indistinct bands, probably due to the large size of the genome. To overcome this drawback, we developed long primer sets with four selective nucleotides, and used these for the grasshoppers Chorthippus brunneus and C. jacobsi in northern Spain. This approach has allowed us to reduce the number of bands, so that we can compare homologous bands easily. Using the revised method, it appears that the degree of interspecific genetic differentiation of C. brunneus and C. jacobsi is relatively small.