The evolution of reproductive isolation in Daphnia

Background The process by which populations evolve to become new species
involves the emergence of various reproductive isolating barriers (RIB).
Despite major advancements in understanding this complex process, very
little is known about the order in which RIBs evolve or their relative
contribution to the total restriction of gene flow during various stages
of speciation. This is mainly due to the difficulties of studying
reproductive isolation during the early stages of species formation. This
study examines ecological and non-ecological RIB within and
between Daphnia pulex and Daphnia
pulicaria, two recently diverged species that inhabit distinct
habitats and exhibit an unusual level of intraspecific genetic
subdivision. Results We find that while ecological prezygotic barriers are
close to completion, none of the non-ecological barriers can restrict gene
flow between D. pulex and D.
pulicaria completely when acting alone. Surprisingly, we also
identified high levels of postzygotic reproductive isolation in
‘conspecific’ interpopulation crosses of D. pulex. Conclusions
While the ecological prezygotic barriers are prevalent during the mature
stages of speciation, non-ecological barriers likely dominated the early
stages of speciation. This finding indicates the importance of studying
the very early stages of speciation and suggests the contribution of
postzygotic isolation in initiating the process of speciation.