Note, however, that we do not have enough information to determine whether hair originated before or after mammary glands, despite their order of depiction on the tree.
The platypus, kangaroo, and elephant all have hair and mammary glands, but also the derived features that appeared earlier in the history of the ingroup (i.e., four legs and amniotic eggs). This is indicated on the phylogenetic tree by the hashmark labeled "four legs" just below the node representing the common ancestor of these five species (this common ancestor possessed four legs). For example, the frog, crocodile, platypus, kangaroo, and elephant all have four legs. Synapomorphies are hierarchically nested on phylogenetic trees. For example, "four legs" can be read in this case as "four legs present" and "hair" can be read as "hair present." The character state name to the right of each hashmark indicates the derived condition of the character, or derived character state. Synapomorphies are mapped below each node and are indicated by blue hashmarks in fact, the synapomorphy identified below each node provides the basis (and computational support) for that node. The phylogenetic tree below summarizes relationships among the five ingroup species we considered earlier ( Sec. Thus, synapomorphies represent shared evolutionary novelty, distinguishing clades from their ancestors. Feathers are a synapomorphy that unites the two bird species, separating them from the crocodile. Suppose your ingroup included two birds and a crocodile. The system of phylogenetic reconstruction developed by Willi Hennig (parsimony) groups organisms according to their possession of shared derived character states, which he called synapomorphies. In other words, a feature is considered to be useful for phylogenetically grouping two or more species if that same feature was also present in the shared common ancestor of the species, but not present in more distantly related taxa. It is also possible to use character mapping in combination with analytical approaches to reconstruct the individual characters present in common ancestors, which are represented by nodes on phylogenetic trees.Īpomorphies, Synapomorphies, and PlesiomorphiesĪn apomorphy is a derived (sometimes called "advanced") feature. 4.2.4), which features are ancestral, which provide no grouping information at all, and which arose multiple times. Viewed in the context of a phylogenetic tree, mapped characters can be used to evaluate which features define different clades (i.e., monophyletic groups see Sec. The order of the characters mapped on a branch does not indicate the order in which they changed from one state to another.
Characters that change along the same branch can be mapped in any order. Sometimes, character state changes may be indicated in other ways (for example, a change in the color of a branch). Typically, a character state change is indicated by a hash mark (horizontal line) or another sort of mark (like a dot) placed on a brach each mark may then be labelled with a number corresponding to a character, a description of the character, or a description of the new character state. A character state change is mapped between the ancestor (node) the has one state, and the descendent (node or terminal taxon) that has another state. Some or all of these changes may be mapped on a tree, a procedure called character mapping. Phylogenetic trees may be used to investigate the sequence and timing of origination of particular features of organisms, as well as the distributions of those features among different groups of species.
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