Choosing the most parsimonious tree The geneticists studying the alien organisms have deciphered their genetic code. For familiarity, they have used A, C, G, and T to indicate the bases in this code. A comparable genetic sequence in each alien has been identified in the genes that code for eye growth. This genetic sequence has allowed the aliens to be divided into six groups, with the single-tail, three-eye group split in half. The geneticists also studied several outgroup species (not shown) to compile a likely ancestral DNA sequence. From this information, they have constructed four possible phylogenetic trees.

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