By any measure, fungi are diverse, although just how diverse is not known. About 75,000 species have been formally described, but estimates of true fungal diversity run as high as 5 million species. Among eukaryotes, the only organisms more diverse than fungi are the animals. The availability of DNA sequence data has greatly advanced our understanding of phylogenetic relationships within the fungi, which are shown in Fig. 34.15. The tree clearly shows how the characters present in familiar mushrooms accumulated through the course of evolution: first cell walls of chitin, then hyphae, then regularly placed septa and, finally, the complex multicellular reproductive bodies we call mushrooms. Motile single cells are found only in the most ancient groups, while dikaryotic cells and complexity of form are found only on the most recent branches.

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The evolutionary history of fungi can be viewed as the transition from a flagellated ancestral form that lived in aquatic environments to soil-dwelling forms in which hyphae divided by septa develop complex morphologies visible to the naked eye. Moreover, in the earliest diverging fungal group, karyogamy follows quickly on the heels of plasmogamy and there is no heterokaryotic stage. Thus, another evolutionary trend is the progressive separation in time between plasmogamy and karyogamy in fungal life cycles. Many of the distinguishing features of fungi evolved as adaptations to the challenges faced by nonmotile heterotrophs on land that absorb nutrients directly from their substrate.

We also see that the numbers of species are not spread evenly across the phylogeny. Instead, the two dikaryotic groups include more than 98% of known species, while the remaining 2% is divided among all other groups. Clearly, dikaryotic fungi are well adapted to many different habitats, and these include other organisms, both living and dead.

Quick Check 3 From the phylogenetic tree shown in Fig. 34.15, would you say that complex multicellular fruiting bodies evolved once or twice (independently) in fungi?