Biological<unk>  is the study of the diversity of life on the planet Earth, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees (synonyms: cladograms, phylogenetic trees, phylogenies).  Phylogenies have two components, branching order (showing group relationships) and branch length (showing amount of evolution).  Phylogenetic trees of species and higher taxa are used to study the evolution of traits (e.g., anatomical or molecular characteristics) and the distribution of organisms (biogeography).<unk> , in other words, is used to understand the evolutionary history of life on Earth.
The term<unk> " is sometimes used synonymously with "taxonomy" and may be confused with "scientific classification."  However, taxonomy is properly the describing, identifying, classifying, and naming of organisms, while "classification" is focused on placing organisms within groups that show their relationships to other organisms.  All of these biological disciplines can be involved with extinct and extant organisms.  However,<unk>  alone deals specifically with relationships through time, requiring recognition of the fossil record when dealing with the<unk>  of organisms.<unk>  uses taxonomy as a primary tool in understanding organisms, as nothing about an organism's relationships with other living things can be understood without it first being properly studied and described in sufficient detail to identify and classify it correctly.  Scientific classifications are aids in recording and reporting information to other scientists and to laymen.  The<unk> t, a scientist who specializes in<unk> , must, therefore, be able to use existing classification systems, or at least know them well enough to skillfully justify not using them.
Phenetic<unk>  was an attempt to determine the relationships of organisms through a measure of similarity, considering plesiomorphies (ancestral traits) and apomorphies (derived traits) to be equally informative. From the 20th century onwards, it was superseded by cladistics, which considers plesiomorphies to be uninformative for an attempt to resolve the phylogeny of Earth's various organisms through time.  Today's<unk> ts generally make extensive use of molecular biology and computer programs to study organisms.<unk>  is fundamental to biology because it is the foundation for all studies of organisms, by showing how any organism relates to other living things.<unk>  is also of major importance in understanding conservation issues because it attempts to explain the Earth's biodiversity and could be used to assist in allocating limited means to preserve and protect endangered species, by looking at, for example, the genetic diversity among various taxa of plants or animals and deciding how much of that it is necessary to preserve.
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