Ontogenetic patterns and phylogenetic trends in freshwater flatworms (Tricladida); constraint or selection?

Semelparity and iteroparity are unevenly distributed among the families of the Paludicola and this implies that there have been taxonomic restrictions on life-history evolution. Species differ in their investments in reproduction and high levels can be related, causally, to reduced life-spans; i.e. semelparous species invest more in reproduction than iteroparous species. However, there does not appear to be any fundamental reason why the extent and timing of these investments should not be open to modification by natural selection. A major morphological difference between the predominantly semelparous Dendrocoelidae and the predominantly iteroparous Dugesiidae and Planariidae is the presence of an anterior adhesive organ; dendrocoelids have one, but members of the other families do not. A plausible scenario can be formulated relating this structure, causally, to enhanced juvenile survivorship which, in turn, favors the semelparous life history.