I Contain Multitudes: The Microbes Within Us and a Grander View of Life

by Ed Yong

made no sense to think of these collections of DNA separately. He believed that they work as a single entity – a holo- genome, which ‘should be considered as the unit of natural selection in evolution’.23 To understand what that means, remember that evolution by natural selection depends on just three things: individuals must vary; those variations must be heritable; and those variations must have the potential to affect their fitness – that is, their ability to survive and reproduce. Variation, inheritance, fitness: if all three boxes are ticked, the engine of evolution whirrs into action, pumping out generations that are successively better adapted to their environment. An animal’s genes certainly meet this trinity of criteria. But Rosenberg noted that an animal’s microbes do, too. Different individuals can carry different communities, species, or strains of microbes – so, there’s variation. As we have seen, there are many ways in which animals can pass microbes down to their offspring – so, there’s inheritance. And as we shall see, the microbes bestow important abilities that influence the success of their host – so, they can affect fitness. Tick, tick, tick, and the engine starts up. Over time, the holobionts that can best meet life’s challenges will pass their hologenomes – the total of their genes plus those of their microbes – to the next generation. The animals and their microbes evolve as one. It’s a more holistic take on evolution, one that redefines what it mean...