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.