Thoughts about how to group fungi into species, genera and so on have changed over time, though always with some idea of similarity involved but the earlier history of those changing ideas is irrelevant here. The later 1800s saw the beginnings of modern evolutionary theory with the core idea of descent with modification. Organisms pass on traits to their descendents but over time inherited traits may change and so organisms alive today may possess features markedly different from those of their ancestors many generations ago. If we knew how inherited traits had changed over time we could use such historical knowledge to produce a classification of organisms that would be consistent with evolutionary theory and from the later 1800s the goal has been to produce such a classification.

Let's illustrate this with a hypothetical example. Suppose that white, pink, green, rust brown, dark brown and black are the only spore colours found in mushrooms today. Suppose we know that all mushrooms had white spores until ten million years ago when green spores appeared in one population and all the descendents of this green-spored population have been green-spored. Some time later a population of the white spored mushrooms then extant gave rise to mushrooms with black spores. Later still some of the descendents of these mushrooms with black spores developed dark brown spores, while the other descendents continued to have black spores. Then some of the mushrooms with dark brown spores gave rise to mushrooms with rust brown spores while the other descendants had dark brown spores. Finally suppose that more recently still some of mushrooms still with white spores gave rise to mushrooms with pink spores and the descendents remained pink-spored. We could present this hypothetical scenario by the following diagram, where the arrow on the left indicates time.

If, in this hypothetical scenario, today's mushrooms were divided into six basic groups based on spore colour it's clear that such a division would be consistent with evolutionary history. Another mycologist could argue that the shades of brown are insignificant and the brown-spored mushrooms should be considered as just the one basic group. There would then be five, not six, basic groups but both views are consistent with evolutionary history. Going back to six basic groups, two examples of higher level groups (or higher level taxonomic ranks) would be (rust brown, dark brown) and (rust brown, dark brown, black). Both are consistent with evolutionary history for the first of these arose from one offshoot from black-spored ancestors and the other from one offshoot from white-spored ancestors.

In the hypothetical example we had the benefit of perfect knowledge about the evolutionary history of spore colour but in real life perfect knowledge is never the case. In many other types of organisms there are fossils that can be used to help devise an evolutionary history but the fungal fossil evidence is very limited. Hence, if an evolutionary classification is to be produced, evolutionary history needs to be inferred, somehow, largely from what is found in fungi alive today. Fungi possess a variety of features that have been used to develop classification schemes and a number of these features will be presented a little later. For an evolutionary classification scheme it is necessary to know which form of each feature is the original form and which the changed forms. Moreover, it is necessary to know original forms for all fungi and also for various subgroups. In the hypothetical example white is the original form when we consider all the mushrooms but within the subgroup of brown and black spored mushrooms, black is the original form.

Sometimes there has been strong evidence for what was the original form of a trait but at other times the evidence has been ambiguous, leading to considerable debate. Not surprisingly, in such circumstances it is possible to devise more than one classification scheme, each using evolutionary principles but based on different hypotheses.

Changes in species names

If it were desired that species' names reflect an evolutionary classification then species' names would need to change as ideas about evolutionary relationships changed and that is what happens. Today Amanita xanthocephala is one of many species in the genus Amanita and Mycena interrupta is one of many species in the genus Mycena. All those organisms with species names beginning with Amanita are considered to be evolutionarily closer to each other than any is to an organism that belongs to a species that has a name beginning with Mycena. Naturally another valid statement would be produced if the previous sentence were changed by swapping the words Amanitaand Mycena. Had you been alive in 1860 you would not