Two competing hypotheses to account for the increase in the number of hox genes from the last common ancestor of bilaterians to the last common ancestor of insects and vertebrates are: (1) a single duplication of the entire four-gene cluster, followed by the loss of one gene, and (2) three independent duplications of individual hox genes. to prefer the first hypothesis on the basis of parsimony requires the assumption that

The duplication of a cluster of four Hox genes is equally likely as the duplication of a single Hox gene.

Explanation:

This comes from the definition of the parsimony principle, which is the action of choosing the simplest scientific explanation fitting the given evidence, meaning that all other elements are equal when evaluating the hypothesis. In biology the best hypothesis is the one that requires the fewest evolutionary changes.

Since three independent duplications of an individual Hox gene is in  evolutionary terms, more complex than a single duplication or a cluster, then that hypothesis would be the most parsimonious.

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