Data from: Parallel speciation or long-distance dispersal? Lessons from seaweeds (Fucus) in the Baltic Sea

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 demographic 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. Bergström & 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 (FST =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 supports 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.