Background: Theory predicts that speciation can be quite rapid. Previous examples comprise a wide range of organisms such as sockeye salmon, polyploid hybrid plants, fruit flies and cichlid fishes. However, few studies have shown natural examples of rapid evolution giving rise to new species in marine environments.

Asexual reproduction by cloning may affect the genetic structure of populations, their potential to evolve, and, among foundation species, contributions to ecosystem functions. Macroalgae of the genus Fucus are known to produce attached plants only by sexual recruitment. Recently, however, clones of attached plants recruited by asexual reproduction were observed in a few populations of Fucus radicans Bergström et L. Kautsky and F. vesiculosus L. inside the Baltic Sea. Herein we assess the distribution and prevalence of clonality in Baltic fucoids using nine polymorphic microsatellite loci and samples of F. radicans and F. vesiculosus from 13 Baltic sites. Clonality was more common in F. radicans than in F. vesiculosus, and in both species it tended to be most common in northern Baltic sites, although variation among close populations was sometimes extensive. Individual clonal lineages were mostly restricted to single or nearby locations, but one clonal lineage of F. radicans dominated five of 10 populations and was widely distributed over 550 × 100 km of coast. Populations dominated by a few clonal lineages were common in F. radicans, and these were less genetically variable than in other populations. As thalli recruited by cloning produced gametes, a possible explanation for this reduced genetic variation is that dominance of one or a few clonal lineages biases the gamete pool resulting in a decreased effective population size and thereby loss of genetic variation by genetic drift. Baltic fucoids are important habitat-forming species, and genetic structure and presence of clonality have implications for conservation strategies.

Parallel evolution has been invoked as a forceful mechanism of ecotype and species formation in many animal taxa. However, parallelism may be difficult to separate from recently monophyletically diverged species that are likely to show complex genetic relationships as a result of considerable shared ancestral variation and secondary hybridization in local areas. Thus, species' degrees of reproductive isolation, barriers to dispersal and, in particular, limited capacities for long-distance dispersal will affect demographical structures underlying mechanisms of divergent evolution. Here, we used nine microsatellite DNA markers to study intra-and interspecific genetic diversity of two recently diverged species of brown macroalgae, Fucus radicans (L. Bergstr€ om & L. Kautsky) and F. vesiculosus (Linnaeus), in the Baltic Sea. We further performed biophysical modelling to identify likely connectivity patterns influencing the species' genetic structures. For each species, we found intraspecific contrasting patterns of clonality incidence and population structure. In addition, strong genetic differentiation between the two species within each locality supported the existence of two distinct evolutionary lineages (F ST = 0.15-0.41). However, overall genetic clustering analyses across both species' populations revealed that all populations from one region (Estonia) were more genetically similar to each other than to their own taxon from the other two regions (Sweden and Finland). Our data support a hypothesis of parallel speciation. Alternatively, Estonia may be the ancestral source of both species, but is presently isolated by oceanographic barriers to dispersal. Thus, a limited gene flow in combination with genetic drift could have shaped the seemingly parallel structure.

In dioecious species with both sexual and asexual reproduction, the spatial distribution of individual clones affects the potential for sexual reproduction and local adaptation. The seaweed Fucus radicans, endemic to the Baltic Sea, has separate sexes, but new attached thalli may also form asexually. We mapped the spatial distribution of clones (multilocus genotypes, MLGs) over macrogeographic (>500 km) and microgeographic (<100 m) scales in the Baltic Sea to assess the relationship between clonal spatial structure, sexual recruitment, and the potential for natural selection. Sexual recruitment was predominant in some areas, while in others asexual recruitment dominated. Where clones of both sexes were locally intermingled, sexual recruitment was nevertheless low. In some highly clonal populations, the sex ratio was strongly skewed due to dominance of one or a few clones of the same sex. The two largest clones (one female and one male) were distributed over 100–550 km of coast and accompanied by small and local MLGs formed by somatic mutations and differing by 1–2 mutations from the large clones. Rare sexual events, occasional long‐distance migration, and somatic mutations contribute new genotypic variation potentially available to natural selection. However, dominance of a few very large (and presumably old) clones over extensive spatial and temporal scales suggested that either these have superior traits or natural selection has only been marginally involved in the structuring of genotypes.

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