Genus (pi- genera) is the next higher taxon and comprises different species that have several important features in common but differ sufficiently to still maintain their status as individual species. All bacterial species belong to a genus, and relegation of a species to a particular genus is based on various genetic and phe-notypic characteristics shared among the species. However, microorganisms do not possess the multitude of physical features exhibited by higher organisms such as plants and animals. For instance, they rarely leave any fossil record and they exhibit a tremendous capacity to intermix genetic material among supposedly unrelated species and genera. For these reasons the ability to confidently establish microorganism relatedness so they may be classified in higher taxa beyond the genus level is difficult. Therefore, although grouping similar genera into common families and similar families into common orders and so on is used for classification of plants and animals, these higher taxa designations (i.e., division, class, order, family) are not usually useful for classifying bacteria.
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