Divergent selection may initiate ecological speciation extremely rapidly. How often and at what pace ecological speciation proceeds to yield strong reproductive isolation is more uncertain. Here, we document a case of extraordinarily rapid speciation associated with ecological selection in the postglacial Baltic Sea. European flounders (Platichthys flesus) in the Baltic exhibit two contrasting reproductive behaviors: pelagic and demersal spawning. Demersal spawning enables flounders to thrive in the low salinity of the Northern Baltic, where eggs cannot achieve neutral buoyancy. We show that demersal and pelagic flounders are a species pair arising from a recent event of speciation. Despite having a parapatric distribution with extensive overlap, the two species are reciprocally monophyletic and show strongly bimodal genotypic clustering and no evidence of contemporary migration, suggesting strong reproductive isolation. Divergence across the genome is weak but shows strong signatures of selection, a pattern suggestive of a recent ecological speciation event. We propose that spawning behavior in Baltic flounders is the trait under ecologically based selection causing reproductive isolation, directly implicating a process of ecological speciation. We evaluated different possible evolutionary scenarios under the approximate Bayesian computation framework and estimate that the speciation process started in allopatry ∼2,400 generations ago, following the colonization of the Baltic by the demersal lineage. This is faster than most known cases of ecological speciation and represents the most rapid event of speciation ever reported for any marine vertebrate.ecological speciation | genomics | evolution | rapid speciation | Baltic Sea

In this study, the genetic variation of perch Perca fluviatilis from 18 different sites along the Swedish coast of the Baltic Sea was assessed. There was a relative strong support for isolation by distance and the results suggest an overall departure from panmixia. The level of genetic divergence was moderate (global F(ST) = 0·04) and indications of differences in the population genetic structure between the two major basins (central Baltic Sea and Gulf of Bothnia) in the Baltic Sea were found. There was a higher level of differentiation in the central Baltic Sea compared to the Gulf of Bothnia, and the results suggest that stretches of deep water might act as barriers to gene flow in the species. On the basis of the estimation of genetic patch size, the results corroborate previous mark--recapture studies and suggest that this is a species suitable for local management. In all, the findings of this study emphasize the importance of considering regional differences even when strong isolation by distance characterize the genetic population structure of species.

Europe has a long tradition of exploiting marine fishes and is promoting marine economic activity through its Blue Growth strategy. This increase in anthropogenic pressure, along with climate change, threatens the biodiversity of fishes and food security. Here, we examine the conservation status of 1,020 species of European marine fishes and identify factors that contribute to their extinction risk. Large fish species (greater than 1.5 m total length) are most at risk; half of these are threatened with extinction, predominantly sharks, rays and sturgeons. This analysis was based on the latest International Union for Conservation of Nature (IUCN) European regional Red List of marine fishes, which was coherent with assessments of the status of fish stocks carried out independently by fisheries management agencies: no species classified by IUCN as threatened were considered sustainable by these agencies. A remarkable geographic divergence in stock status was also evident: in northern Europe, most stocks were not overfished, whereas in the Mediterranean Sea, almost all stocks were overfished. As Europe proceeds with its sustainable Blue Growth agenda, two main issues stand out as needing priority actions in relation to its marine fishes: the conservation of marine fish megafauna and the sustainability of Mediterranean fish stocks.arine fishes exhibit high biodiversity 1,2 and have been culturally and nutritionally important throughout human history 3 . Europe, in particular, has a well-documented history of exploiting marine fish populations, written records of which commence in the classical works of ancient Greece. Although this historical exploitation has undoubtedly altered populations 4,5 and changed many seascapes 6 , marine defaunation in the region has not been as great as in terrestrial systems 7 . However, the use of ocean space and resources is increasing due to Europe's Blue Growth strategy 8 , the nutritional requirements of an expanding human population are growing 9,10 and marine ecosystems will experience unusually rapid changes in future due to climate change 11,12 . Consequently there are imminent threats both to European marine biodiversity and fish resources 13 . It is important, therefore, to assess the threats of extinction to fish species and to ensure consistency in the management approach by the various agencies involved.We analysed data on the conservation status of 1,020 species of Europe's marine fishes from the recent International Union for Conservation of Nature (IUCN) Red List assessments 14 to identify characteristics that make Europe's fishes most susceptible to extinction risk. We then compared the Red List with 115 fish stock assessments (of 31 species) made by intergovernmental agencies charged with providing advice on the exploitation of commercial fishes. Previous comparisons of this sort applied criteria under various modelling assumptions [15][16][17] or limited the comparison to biomass reference points 18 . ResultsOf the 1,020 European marine fish species that were asses...

Estimating patterns of gene flow during the early stages of speciation is central to understanding whether reproductive isolation arises via the gradual erosion of gene flow or through successive stages of strict isolation and secondary contact. Such scenarios can be explicitly tested by comparing the joint allele frequency spectrum (jAFS) of a set of populations to jAFS simulated under scenarios of isolation with migration (IM) and secondary contact (SC). However, the potential effect of unaccounted demographic events (such as population expansions and bottlenecks) on model choice and parameter estimation remains largely unexplored. Using simulations, we demonstrate that under realistic scenarios of population divergence with constant gene flow, failure to account for population size changes in either a daughter population or the ancestral population leads to overestimated divergence time and to a bias towards the choice of SC models. On the other hand, when the simulations included long periods of strict isolation the correct gene flow scenario was usually retrieved. We illustrate these issues reconstructing the demographic history of North Sea and Baltic Sea turbots (Schopthalmus maximus) by testing 16 IM and 16 SC scenarios, modelling changes in effective population sizes as well as the effects of linked selection and heterogeneous migration rates across the genome. As in the simulated data, failure to account for changes in effective population sizes resulted in selecting SC models with a long period of strict isolation and divergence times preceding the formation of the Baltic Sea. In contrast, models accounting for population size changes suggest that the Baltic Sea turbot population originated from a very recent (<6 kya) invasion and has diverged with constant gene flow from the North Sea. The results have broad implications for the study of speciation by high-lighting the potential effects of ancestral size changes and bottlenecks followed by growth on choices between competing scenarios of speciation. In general, extreme caution should be exercised when interpreting results of demographic model comparisons.

We found low, albeit significant, genetic differentiation among turbot (Psetta maxima) in the Baltic Sea but in contrast to earlier findings we found no evidence of isolation by distance. In fact temporal variation among years in one locality exceeded spatial variation among localities. This is an unexpected result since adult turbot are sedentary and eggs are demersal at the salinities occurring in the Baltic. Our findings are most likely explained by the fact that we sampled fish that were born after/during a large influx of water to the Baltic Sea, which may have had the consequence that previously locally and relatively sedentary populations became admixed. These results suggest that populations that colonize relatively variable habitats, like the Baltic, face problems. Any adaptations to local conditions that may build up during stable periods may quickly become eroded when conditions change and/or when populations become admixed. Our results indicate that the ability of turbot to survive and reproduce at the low salinity in the Baltic is more likely due to phenotypic plasticity than a strict genetic adaptation to low salinity.