 Holobionts can also be reproducers, where the host usually reproduces vertically and the microbiota reproduce either vertically, horizontally, or both. This situation has provoked discussion among philosophers (Godfrey-Smith 2009, 2011; Sterelny 2011; Griesemer 2014, 2016; Booth 2014; Lloyd forthcoming). Holobionts’ microbiota can reproduce outside the context of the original host organism, so some holobionts, e.g., the Hawaiian bobtail squid and its luminescent bacteria, are not “Darwinian populations” (Godfrey-Smith 2009, 2011), and therefore not units of selection (see Booth 2014). This approach contrasts with that of the original reproducer approach which would include the squid-bacteria system and also retroviruses excluded under the “Darwinian populations” account (Griesemer2000a, 2016). 