Levels of selection in biofilms: multispecies biofilms are not evolutionary individuals

Microbes are generally thought of as unicellular organisms, but we know that many microbes live as parts of biofilms—complex, surface-attached microbial communities numbering millions of cells. Some authors have recently argued in favour of reconceiving biofilms as biological entities in their own right. In particular, some have claimed that multispecies biofilms are evolutionary individuals (Doolittle in Biol Philos 28:351–378, 2013; Ereshefsky and Pedroso in PNAS USA 112(33): 10126–10132 2015). Against this view, I defend the conservative consensus that selection acts primarily upon microbial cells.