The evolution of host‐associated sympatric populations in phytophagous insects (so called “host races”) connects adaptive divergence to barriers to gene flow. Pea aphid ( Acyrthosiphon pisum ) host races specialise on legume species, and because host plant choice leads to assortative mating, the genetic basis of host plant acceptance is key to understanding speciation. Aphids use smell and taste in their host plant selection....

Speciation is rarely observable directly. A way forward is to compare pairs of ecotypes that evolved in parallel in similar contexts but have reached different degrees of reproductive isolation. Such comparisons are possible in the marine snail Littorina saxatilis by contrasting barriers to gene flow between parallel ecotypes in Spain and Sweden. In both countries, divergent ecotypes have evolved to withstand either crab preda...

Inversions are thought to play a key role in adaptation and speciation, suppressing recombination between diverging populations. Genes influencing adaptive traits cluster in inversions, and changes in inversion frequencies are associated with environmental differences. However, in many organisms, it is unclear if inversions are geographically and taxonomically widespread. The intertidal snail, Littorina saxatilis , is one such...

Sandy beaches are biogeochemical hotspots that bridge marine and terrestrial ecosystems via the transfer of organic matter, such as seaweed (termed wrack). A keystone of this unique ecosystem is the microbial community, which helps to degrade wrack and re‐mineralize nutrients. However, little is known about this community. Here, we characterize the wrackbed microbiome as well as the microbiome of a primary consumer, the seawee...

Combining hybrid zone analysis with genomic data is a promising approach to understanding the genomic basis of adaptive divergence. It allows for the identification of genomic regions underlying barriers to gene flow. It also provides insights into spatial patterns of allele frequency change, informing about the interplay between environmental factors, dispersal and selection. However, when only a single hybrid zone is analyse...

Speciation underlies the generation of novel biodiversity. Yet, there is much to learn about how natural selection shapes genomes during speciation. Selection is assumed to act against gene flow at barrier loci, promoting reproductive isolation. However, evidence for gene flow and selection is often indirect and we know very little about the temporal stability of barrier loci. Here we utilize haplodiploidy to identify candidat...

Hybrid zone movement may result in substantial unidirectional introgression of selectively neutral material from the local to the advancing species, leaving a genetic footprint. This genetic footprint is represented by a trail of asymmetric tails and displaced cline centres in the wake of the moving hybrid zone. A peak of admixture linkage disequilibrium is predicted to exist ahead of the centre of the moving hybrid zone. We t...

Both classical and recent studies suggest that chromosomal inversion polymorphisms are important in adaptation and speciation. However, biases in discovery and reporting of inversions make it difficult to assess their prevalence and biological importance. Here, we use an approach based on linkage disequilibrium among markers genotyped for samples collected across a transect between contrasting habitats to detect chromosomal re...

In many diploid species, the sex chromosomes play a special role in mediating reproductive isolation. In haplodiploids, where females are diploid and males haploid, the whole genome behaves similarly to the X/Z chromosomes of diploids. Therefore, haplodiploid systems can serve as a model for the role of sex chromosomes in speciation and hybridization. A previously described population of Finnish Formica wood ants displays geno...

Aim Community assembly is traditionally assumed to result from speciation and colonization mediated by available niche space. This paradigm is expanded by the theory that niche space can also be saturated by intersexual adaptive divergence (ecological sexual dimorphism) when interspecific competition is relaxed. This theory (here termed ‘niche‐packing equivalence’) predicts that the evolution of ecological sexual dimorphism co...

Host‐associated races of phytophagous insects provide a model for understanding how adaptation to a new environment can lead to reproductive isolation and speciation, ultimately enabling us to connect barriers to gene flow to adaptive causes of divergence. The pea aphid ( Acyrthosiphon pisum) comprises host races specializing on legume species and provides a unique system for examining the early stages of diversification along...

Host‐race formation in phytophagous insects is thought to provide the opportunity for local adaptation and subsequent ecological speciation. Studying gene expression differences amongst host races may help to identify phenotypes under (or resulting from) divergent selection and their genetic, molecular and physiological bases. The pea aphid ( Acyrthosiphon pisum ) comprises host races specializing on numerous plants in the Fab...

Parallel divergence and speciation provide evidence for the role of divergent selection in generating biological diversity. Recent studies indicate that parallel phenotypic divergence may not have the same genetic basis in different geographical locations – ‘outlier loci’ (loci potentially affected by divergent selection) are often not shared among parallel instances of phenotypic divergence. However, limited sharing may be du...

Parallel speciation occurs when selection drives repeated, independent adaptive divergence that reduces gene flow between ecotypes. Classical examples show parallel speciation originating from shared genomic variation, but this does not seem to be the case in the rough periwinkle ( L ittorina saxatilis ) that has evolved considerable phenotypic diversity across Europe, including several distinct ecotypes. Small ‘wave’ ecotype...

Seasonally changing environments at high latitudes present great challenges for the reproduction and survival of insects, and photoperiodic cues play an important role in helping them to synchronize their life cycle with prevalent and forthcoming conditions. We have mapped quantitative trait loci ( QTL ) responsible for the photoperiodic regulation of four life history traits, female reproductive diapause, cold tolerance, egg‐...

The number and demographic history of colonists can have dramatic consequences for the way in which genetic diversity is distributed and maintained in a metapopulation. The bed bug ( C imex lectularius ) is a re‐emerging pest species whose close association with humans has led to frequent local extinction and colonization, that is, to metapopulation dynamics. Pest control limits the lifespan of subpopulations, causing frequent...

The role of natural selection in promoting reproductive isolation has received substantial renewed interest within the last two decades. As a consequence, the study of ecological speciation has become an extremely productive research area in modern evolutionary biology. Recent innovations in sequencing technologies offer an unprecedented opportunity to study the mechanisms involved in ecological speciation. Genome scans provid...

Poleward range expansions are widespread responses to recent climate change and are crucial for the future persistence of many species. However, evolutionary change in traits such as colonization history and habitat preference may also be necessary to track environmental change across a fragmented landscape. Understanding the likelihood and speed of such adaptive change is important in determining the rate of species extinctio...

How common is speciation‐with‐gene‐flow? How much does gene flow impact on speciation? To answer questions like these requires understanding of the common obstacles to evolving reproductive isolation in the face of gene flow and the factors that favour this crucial step. We provide a common framework for the ways in which gene flow opposes speciation and the potential conditions that may ease divergence. This framework is cent...

Ecology Letters (2010) 13: 485–494 Abstract All species are restricted in their distribution. Currently, ecological models can only explain such limits if patches vary in quality, leading to asymmetrical dispersal, or if genetic variation is too low at the margins for adaptation. However, population genetic models suggest that the increase in genetic variance resulting from dispersal should allow adaptation to almost any ecolo...