 If the data are observed nucleotide sequences from different groups of terminal taxa, then the calculation we are after is the probability that those observed sequences of nucleotides would arise on the tips of competing phylogenetic hypotheses. The simplest way to calculate that is to use the Jukes-Cantor model of evolution (Jukes &amp; Cantor 1969). This model assumes an equal frequency of the four nucleotides, and a uniform probability that any one nucleotide might change to another. (These assumptions are typically violated, but reflecting that in a model of evolution introduces additional parameters, which increases computing time—a valuable and limited resource in 1969.) The Jukes-Cantor model provides a model of evolution on which the relevant probability that any given sequence might have evolved from another more primitive sequence may be calculated. Different tree topologies will generate different probabilities that the observed sequences evolved from a hypothetical ancestral state. This conditional probability, \(P(\text{Data} | \text{Tree})\), is called the likelihood of the tree.