<Tr> <Td_colspan="3">--Charles Darwin, On the Origin of Species, page 577 </Td> </Tr> <P> An evolutionary tree (of Amniota, for example, the last common ancestor of mammals and reptiles, and all its descendants) illustrates the initial conditions causing evolutionary patterns of similarity (e.g., all Amniotes produce an egg that possesses the amnios) and the patterns of divergence amongst lineages (e.g., mammals and reptiles branching from the common ancestry in Amniota). Evolutionary trees provide conceptual models of evolving systems once thought limited in the domain of making predictions out of the theory . However, the method of phylogenetic bracketing is used to infer predictions with far greater probability than raw speculation . For example, paleontologists use this technique to make predictions about nonpreservable traits in fossil organisms, such as feathered dinosaurs, and molecular biologists use the technique to posit predictions about RNA metabolism and protein functions . Thus evolutionary trees are evolutionary hypotheses that refer to specific facts, such as the characteristics of organisms (e.g., scales, feathers, fur), providing evidence for the patterns of descent, and a causal explanation for modification (i.e., natural selection or neutral drift) in any given lineage (e.g., Amniota). Evolutionary biologists test evolutionary theory using phylogenetic systematic methods that measure how much the hypothesis (a particular branching pattern in an evolutionary tree) increases the likelihood of the evidence (the distribution of characters among lineages). The severity of tests for a theory increases if the predictions "are the least probable of being observed if the causal event did not occur ." "Testability is a measure of how much the hypothesis increases the likelihood of the evidence ." </P> <P> Evidence for common descent comes from the existence of vestigial structures . These rudimentary structures are often homologous to structures that correspond in related or ancestral species . A wide range of structures exist such as mutated and non-functioning genes, parts of a flower, muscles, organs, and even behaviors . This variety can be found across many different groups of species . In many cases they are degenerated or underdeveloped . The existence of vestigial organs can be explained in terms of changes in the environment or modes of life of the species . Those organs are typically functional in the ancestral species but are now either semi-functional, nonfunctional, or re-purposed . </P> <P> Scientific literature concerning vestigial structures abounds . One study complied 64 examples of vestigial structures found in the literature across a wide range of disciplines within the 21st century . The following non-exhaustive list summarizes Senter et al. alongside various other examples: </P>

The presence of vestigial structures is explained by the common descent hypothesis. true false