They argue that, considering the host organism and its microbes together (a holobiont), evolution can work faster because the microbes evolve more quickly:
A famous example of this concept is the relationship between corals and their symbionts, the zooxanthellae. Researchers have demonstrated that some corals can evolve to tolerate higher water temperatures by changing the makeup of their symbiont communities. Because microbes have much shorter generation times than coral polyps, the genetic composition of the symbiont populations can evolve much more rapidly than that of their hosts, and these changes can confer higher tolerance on the holobiont unit.Izhak Mizrahi, Fotini Kokou, “Opinion: Individuals Are Greater Than the Sum of Their Parts” at The Scientist
Okay, but then aren’t the microorganisms the unit of selection rather than the host’s genes? This might work for adaptations to change in habitats (they describe one), but it won’t be Darwinism.
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See also: How much evolution can symbiosis account for?
ID Predictions On Orphan Genes And Symbiosis