Whether or not to use an article would depend on the context, but generally speaking, and without seeing further context, I would say the following: 1a would usually be used: several representatives of the main XYZ taxa were selected and analyzed. Then, in subsequent places where these representatives are mentioned, you'd typically use the article: The next construction, based on further analysis of the selected representatives, reveals... 2a would be used: these traits are shared by representatives of the genera X and Y.
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