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Unformatted text preview: s a set of taxa the share a common ancestor. It is important to remember that a cladogram is a hypothetical phylogeny; not having been around millions of years ago to observe these speciation events, we can only infer how organisms evolved by examining anatomy, physiology, developmental stages and events, or gene and protein structure. Reading Cladograms Cladograms convey quite a bit of information. To figure out what that information is, you have to be able to understand what the cladogram illustrates. A cladogram is comprised of branches and nodes. A node is the location where the cladogram branches. When a line branches at a node, the cladogram is representing a common ancestor diverging into two descendent taxa. As you read a cladogram, keep in mind that the most important information is conveyed by the relative positions of each taxon. Cladograms can be drawn in several ways that all represent the same pattern of a specific common ancestor leading to the evolution of two descendent taxa. Reading the taxa from left to right conveys no useful information regarding their hypothetical phylogeny. Reconstructing trees: Cladistics Cladistics is a method of hypothesizing relationships among organisms — in other words, a method of reconstructing evolutionary trees. The basis of a cladistic analysis is data on the characters, or traits, of the organisms in which we are interested. These characters could be anatomical and physiological characteristics, behaviors, or genetic sequences. The result of a cladistic analysis is a tree, which represents a supported hypothesis about the relationships among the organisms. However, it is important to keep in mind that the trees that come out of cladistic analyses are only as good as the data that go into them. New and better data could change the outcome of a cladistic analysis, supporting a different hypothesis about the way that the organisms are related. Assumptions There are three basic assumptions in cladistics: 1. Change in characteristics occurs in lineages over time. The assumption that characteristics of organisms change over time is the most important one in cladistics. It is only when characteristics change that we are able to recognize different lineages or groups. We call the "original" state of the characteristic plesiomorphic and the "changed" state apomorphic (see figure below). 2. Any group of organisms is related by descent from a common ancestor. This assumption is supported by m...
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