4. Conclusions and Future Research Directions
Based on the literature search of 253 studies documenting speciation inDaphnia genus, we find that the majority of studies examine
geographic barriers (55%; 139 studies). While geographic barriers
clearly facilitate divergence between species of Daphnia (Hebert
& Wilson, 1994; Ishida & Taylor, 2007b), ecological barriers are also
likely to be prominent due to ecological opportunity and habitat
differences shaped by the interaction of biotic and abiotic factors.
Despite the presence of ecological barriers in many young species pairs
of Daphnia , interspecific hybridization is frequent in species
complexes within the genus, suggesting that reproductive isolating
barriers between closely related species are permeable. Moreover,
genetic studies suggest that in several instances, hybridization was the
driver of speciation rather than the impediment to diversification.
Based on experimental studies (Table 2; Figure 3), we find that none of
the reproductive isolating barriers are able to completely restrict gene
flow completely between closely related species. However, when examining
ecological and non-ecological isolating barriers, we find that the
majority of studies examine ecological isolating barriers (68%),
focusing mostly on two species complexes, Daphnia pulex andDaphnia longispina , especially when examining non-ecological
isolating barriers (93%; Figure 1). There is a need for more
experimental studies examining the relative strength of reproductive
isolating barriers between closely related species of Daphnia,particularly to establish the importance of prezygotic and postzygotic
isolating barriers. Specifically, there is a lack of studies that
examine behavioural, mechanical, and gametic isolation between closely
related species. Moreover, the increasing availability of genomic tools
and resources, the identification of loci or regions that are associated
with reproductive isolation between closely related species inDaphnia , as well as comparative studies of different sister
species across the speciation continuum, make it increasingly possible
to infer speciation processes.
Factors responsible for the maintenance of freshwater diversity are
receiving increased attention. The rich biodiversity heritage of
freshwater systems has been exploited by our civilization for millennia,
perhaps more than any other natural systems and understanding the forces
that shape and maintain this diversity continues to be of great interest
(Dudgeon et al., 2006). With the increase of human activity in
freshwater systems (Foley et al., 2005), identification of the major
drivers of speciation in Daphnia provides an opportunity to
identify the impact of human influence on the keystone species of
freshwater ecosystems.