 As in the other cases of developmental plasticity, the next step was to see if changes in developmental symbionts could produce changes in evolution (see O'Malley 2015). It was shown that changes in symbionts could provide selectable variants for evolution (Zhang et al. 2019), and it could provide the basis for reproductive isolation through cytoplasmic incompatibility or mating preference (Brucker &amp; Bordenstein 2013; Sharon et al. 2010). One of the most interesting possibilities, though, comes from the view that most, if not all, eukaryotic organisms are holobionts, and that symbionts open new evolutionary trajectories. Symbiotic microbes, for instance, have long been known to be responsible for the plant-digesting enzymes in the stomachs of ruminants. Without cellulose-digesting bacteria, cows cannot digest grass or grain. Moreover, the microbes help create the rumen after they colonize the digestive system at birth. Thus, these microbes also induce the formation of the organ that houses them, and allows them to function (Gilbert 2020; Chiu &amp; Gilbert 2020). Developmental symbiosis (sympoiesis) thus has opened evolutionary trajectories for certain mammals. This is an example of both developmental symbiosis and niche construction. Niche construction depends on developmental plasticity (Laland et al. 2008).