There is a species of bamboo that flowers all at once, everywhere in the world, precisely every 120 years. They last bloomed in the late 1960's; we have historical records of their regular flowering going back to the year 999. Another species flowers precisely every 32 years; another, every 60.
How does this happen? A group of computational biologists did a very clever analysis and seem to have found why this pattern is selected. Synchronized flowering comes from a need to defend oneself against seed-eaters: if there aren't enough plants blooming at once to overwhelm the seed-eaters, it's likely that all of the seeds will be eaten, and everyone will die. And mutations that increase the period of flowering by a small integer – and a small integer alone! – will help a bamboo grove outbreed others. So over time, the most successful species end up with periods which are multiples of a lot of small integers – such as 32 (=2*2*2*2*2), 60 (=2*2*3*5), or 120 (=2*2*2*3*5).
How does this happen? +Carl Zimmer
gives an excellently clear explanation below. It's one of those things that, once you see it, looks obvious in retrospect.
h/t +Valdis Klētnieks
and +Bill Ries-Knight