Mutually dependent species could not have evolved (Talk.Origins)
Many pairs of species are mutually and completely dependent upon each other. For example, fig trees require fig gall wasps to pollinate them, and the wasps require the figs to live. Another such example, among many, is the yucca and Pronuba yucca moth. If one species evolved first, it could not have survived on its own; the mutualistic species must have come into existence at essentially the same time.
(Talk.Origins quotes in blue)
1. Obligate mutualism can evolve gradually from nonobligate associations. For example, a yucca can be pollinated by many insects and gradually specialize to attract just one moth, and a moth may live off many species of yucca and gradually specialize on just one. In fact, some yucca species are still pollinated by more than one moth, and the yucca moth Tegeticula yuccasella pollinates several species of yucca.
This may be true of some cases of obligate mutualism, but there are other cases that defy such an explanation.