Classification of Lichens: The systematic position of lichens is a controversial matter because it is a combination of two different members of two different groups. Essentially, counting the number of places in which sequences differ gives the basis for a numerical measure of closeness. For example, if you wish to draw sound conclusions from genomic evidence it is important to study both the genomes of a large number of species and also different parts of each genome. At any given time, anyone attempting to classify lichens can use only those features that the available technology reveals. You could say that, with regard to this section of DNA, species 1 is closer to species 2 than it is to species 3 since it takes three changes to transform sequence 1 into sequence 2, but five changes to transform it into sequence 3. Here are some of the hypotheses supported by the evidence behind the above tree. Based on their certain special features, they are classified into different types. I have said that early classification was based on naked eye features. In such analyses one looks at equivalent sections from the genomes of different species and the results are presented as sequences of letters, indicating how the four basic DNA components (adenine, cytosine, guanine, thymine) are arranged. Various groupings of characteristics are used to define species, genera and so on, with fewer characteristics as you go up in taxonomic rank. THE CLASSIFICATION OF LICHENS. Most of the fungal members in the lichens belong to class Ascomycetes, except a few which belong to Basidiomycetes. Sometimes this is simply an improvement in seeing something previously known. This particular classification system is often referred to as the Linnean classification, though the use of some of the elements predates Linnaeus. Lignicolous. Algal members belonging to lichens are mostly the members of class Cyanophyceae, ex­cept a few of Chlorophyceae. Whenever new information has become available it has inevitably supported some existing ideas of lichen classification but contradicted others. Lichens are classified by the fungal partner, so all lichens belong to the fungal kingdom. Lichen species are given the same scientific name (binomial name) as the fungus species in the lichen. Cladonia, Ramalina and Usnea are the common examples of fruticose lichens. Moreover, it is a taxon at species rank. The yellow area in the small tree to the right shows where the large tree fits into the tree of all ascomycetes. Studies in the field showed there to be good evidence that at times the environment could influence thallus morphology, whereas microscopic features remained constant. Since the 1700s the development of tools or techniques such as optical microscopes, chemical testing, growth studies, electron microscopes and DNA analysis have revealed details about aspects such as lichen structure, physiology, metabolic products, ecology and genetics. This diagram is saying that 3 marks the beginning of the divergence between the lineage that gave rise to D and the lineage that gave rise to the pair Q and O, while 4 marks a further point of divergence. If you have the DNA sequences from different species you can align them and then look for the number of differences between sequences. In either case the current distribution of lichenized and non-lichenized taxa calls for one or more losses or gains of lichenization to account for the current mix of lichenized and non-lichenized taxa. A very large proportion of lichens belong to just one class and, on current evidence, that class arose from one gain of lichenization. Intermediate types include leprose and squamulose lichen, among others. There are rules governing the creation of taxonomic names and one is that when a biologist proposes the creation of a new taxon at some taxonomic rank, he or she must ensure that there is at least one new taxon at each lower taxonomic rank. In particular events, these organisms are also observed on tiny branchlets, flat leaf-like structures, a powder-like appearance and in various other forms. If 1 had not been RED, then we could explain the current distribution of RED by positing a gain of RED by 6 and a loss by U - just two changes. The situation would have been quite different if the lichenized fungi were spread widely through the fungal kingdom. Another fact is that over the past twenty years there has been much new evidence relevant to fungal phylogeny as a whole, considerable changes in classificatory ideas and during those twenty years there has been integration of lichenized and non-lichenized fungi. 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As you can see there are three other groups of ascomycetes, all non-lichenized, and they have been labelled X, Y and Z. I have emphasized that there is some notion of similarity behind any classification scheme but I have not yet said how similarity is defined. Once you have determined the degrees of closeness between different species you can display this information in a diagram such as the one alongside this paragraph, similar to a human family tree. It does not distinguish between the hypothesis of two gains of lichenization (early in each of the Verrucariales and Pyrenulales) from a non-lichenized common ancestor and the hypothesis of a gain of lichenization by a common ancestor of the Verrucariales and the Pyrenulales, followed by a loss of lichenization in the Chaetothyriales. THE CLASSIFICATION OF LICHENS THE CLASSIFICATION OF LICHENS WATSON, W. 1929-05-01 00:00:00 PART II HE genera which have been previously arranged into families contain the most highly developed of all the lichens in regard to thaUine characters.
2020 classification of lichens