Abstract. Steep environmental gradients offer important opportunities to study the interaction between natural selection and gene flow. Allele frequency clines are expected to form at loci under selection, but unlinked neutral alleles may pass easily across these clines unless a generalized barrier evolves. Here we consider the distribution of forms of the intertidal gastropod Littorina saxatilis, analyzing shell shape and amplified fragment length polymorphism (AFLP) loci on two rocky shores in Britain. On the basis of previous work, the AFLP loci were divided into differentiated and undifferentiated groups. On both shores, we have shown a sharp cline in allele frequencies between the two morphs for differentiated AFLP loci. This is coincident with a habitat transition on the shore where the two habitats (cliff and boulder field) are immediately contiguous. The allele frequency clines coincide with a cline in shell morphology. In the middle of the cline, linkage disequilibrium for the differentiated loci rises in accordance with expectation. The clines are extremely narrow relative to dispersal, probably as a result of both strong selection and habitat choice. An increase in F ST for undifferentiated AFLPs between morphs, relative to within-morph comparisons, is consistent with there being a general barrier to gene flow across the contact zone. These features are consistent either with an episode of allopatric divergence followed by secondary contact or with primary, nonallopatric divergence. Further data will be needed to distinguish between these alternatives.
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.
Key innovations have played a central role in the origins of biodiversity, but their evolutionary origin and genetic architecture are usually unknown. A recent transition from egg-laying to live-birth in Littorina snails provides a rare opportunity to study the origin and genetic architecture of a young innovation. While recognized as one species, live-bearing individuals do not form a single clade in a genome-wide phylogenetic analysis, hinting at two independent origins. However, local genealogical analysis identified numerous genomic regions where samples group according to their reproductive mode. These regions are widespread across the genome, show clear evidence for live-bearer-specific positive selection, and are enriched for genes that are differentially expressed between egg-laying and live-bearing reproductive tissues. Our results show that key innovations can have a polygenic basis, and that their historical origins can be obscured by a complex demographic history.
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helped with the collection and processing of samples. Petri Kemppainen kindly provided samples from Trondheim Fjord. Mark Dunning helped with the development of bioinformatic pipelines. The analysis of genomic data was conducted on the University of Sheffield highperformance computing cluster, ShARC.
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