Why is the typical evolutionary tree so lopsided?
In other words, why do some descendants of a parent species evolve into hundreds of different species, while others produce so few they seem to be practicing family planning? Natural selection may provide some answers, but simple probability yields a surprisingly elegant solution. Pinelis explains: Say you have a carp and a perch swimming in a pond and its equally likely that one of them will evolve a third species. Say the goldfish evolves from the common carp. Suddenly you have three fish species in your pond. Assume again that it is equally likely for the carp, the goldfish, and the perch to split into two distinct species. The chances that the carp branch will develop a new species are now double that of the perch branch, because the carp family now has two members. And so it goes, until the pond is overrun with carp and their relatives. “If one branch has more species, the chances are greater that it will speciate,” Pinelis says. “The rich get richer; money goes to money.” In real
