Brendan tipped me off to the discovery of 4 new glowing mushroom species (and 3 old species that no one knew glowed). These tiny mushrooms (with caps less than 1 cm in width) continuously emit a bright yellow-green light and belong to the genus Mycena, which also contains most other known luminescent mushroom species.
The genus Mycena contains innumerable tiny, dull-colored species that are much more common than anyone would normally notice. Their tiny caps can be found sprouting from decaying vegetable matter wherever it's found. Mycena species (and those of a few other genera) are often referred to as "little brown mushrooms" or "little brown jobs," reflecting their nondescript ubiquity and the near-impossibility of actually identifying them. Back when I worked in redwood forests, I often saw putative Mycenas popping out of fallen Doug-fir cones.
Dennis Desjardin, author of the above discovery, points out that luminescence appears in 16 separate lineages of Mycena species - suggesting that the ancestor of all Mycena species was capable of glowing, but that some species and lineages have since lost this ability. Desjardin came to this conclusion because one of the key tactics in science is to assume that the simplest explanation is most likely to be correct - and should be assumed to be correct until proven otherwise. Because it is more difficult/unlikely for an organism to acquire the ability to glow than to lose the ability, it is more likely that many Mycena species lost the ability (and a few retained it) than that many Mycena species acquired it independently. When shared traits are used to place evolutionary links between related species on a phylogenetic tree, this is known as "parsimony."
Desjardin suggests that the glow of these tiny mushrooms may attract nocturnal animals, which disperse their spores.
So the next time you kick open a rotten log and find the inside glowing, think of Mycena!