Annalee Newitz over at io9:
Most creatures on Earth have one sex that fertilizes and one that gets fertilized. Not so with olive trees. Last week scientists described how these trees evolved a system of males and a third sex which can go both ways.
The sexual system these trees have is called androdioecy: It includes males and a third “hermaphrodite” sex. A group of French researchers last week explained how such a setup could evolve from a pure hermaphrodite system. Initially, the trees were probably all able to pollinate or be pollinated. But over time, some of the trees mutated and lost their female functionality. Now, a very sizable male population exists among the olive trees.
But how? You'd think that males, who can only reproduce by pollinating, would have a strong disadvantage in a system where their competitors can reproduce either by pollinating or being pollinated.
However, among the olive trees these scientists studied, the androdioecy had reached a stable state.