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Did a lineage with six antennal segments evolve into

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Unformatted text preview: xamining insects, these characteristics might be number of segments in the antennae or the presence of an upper vein on wing, etc. You would next examine your organisms and describe each of the characteristics, or character states, in the suite of anatomical traits for each taxon. Again, for the insects you would describe the character states such as whether the insect has six antennal segments or has five antennal segments, has an upper vein or does not have an upper vein. If instead of anatomical traits, you were examining gene sequences, you might select the 362 bases in a particular gene as your characters, and your character states would then be A, T, G, or C for each of the 362 characters. Note that it is important to select characters that seem to be homologies, that is, characters that are similar because they were inherited from a common ancestor. 3. Determine the polarity of characters Figure out the order of evolution for each character. For example, did the beetle species under consideration all evolve from an ancestor with five antennal segments — and only later did six evolve, or was it the other way around? Did a lineage with six antennal segments evolve into a lineage with five? Figuring out the polarity of a character can take some work. In some situations, it is reasonable to assume that the character states in the outgroup are the ancestral states for the taxa of interest. In other situations, paleontologists may have fossil evidence that indicates the probable ancestral state of the character. Many different methods may be used to reason about character polarity. (Note that for some types of cladistic analysis, determination of character polarity is not absolutely necessary.) 4. Group taxa by synapomorphies Group taxa by derived or "changed" character states shared by two taxa, not by the original character states shared by two taxa. So for example, imagine that we have determined that the common ancestor of our beetle clade had five antennal segments and passed that character state onto its immediate descendents: seven of the modern beetle species still have that character state. However, one lineage within the clade evolved six antennal segments and passed that character state onto its descendents — 14 of our beetle species. According to this ru...
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