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Phylogenetics+Lab+exercise+03

The other objection raised against this assumption is

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Unformatted text preview: any lines of evidence and essentially means that all life on Earth today is related and shares a common ancestor. Because of this, we can take any collection of organisms and hypothesize a meaningful pattern of relationships, provided we have the right kind of information. 3. There is a dichotomous branching pattern of lineage ­splitting. This assumption suggests that when a lineage splits, it does not radiate into several groups but divides into exactly two groups. There are some situations that violate this assumption. For example, many biologists accept the idea that multiple new lineages have arisen from a single originating population at the same time, or near enough in time to be indistinguishable from such an event; one example of this is with cichlid fish. The other objection raised against this assumption is the possibility of interbreeding between distinct groups. Hybridization or cross ­breeding between different species is rare, but is known to occur at least occasionally in some plants or bird groups (like ducks). While such exceptions may exist, for many groups they are relatively rare and so this assumption often holds true. Reconstructing trees: A step by step method A cladistic analysis typically requires the following steps. Though they may seem simple, each step actually requires a great deal of background knowledge and work. 1. Choose the taxa Choose a group whose evolutionary relationships interest you. These taxa will be the tips of your tree and must themselves be clades. For example, you might choose 20 species of beetle that all fall within the same genus. Alternately, you might choose to reconstruct the relationships among the major clades of insects (beetles, flies, moths and butterflies, true bugs, dragonflies, etc.), where each taxon includes many species. You will also need to choose an outgroup. The outgroup is a taxon from an evolutionary lineage that is known to have diverged from the group that you chose to study, a taxon outside the clade of the other organisms. If you were studying insects, you might choose a crustacean, like a lobster, or an arachnid, like the common garden spider. 2. Determine the characters and character states At this step, you would be describing a set of characteristics for the group of organisms you have chosen. For example, you might select a suite of anatomical traits as your characters. If we were e...
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