To explore sensitivity of the results to changes in parameter
costs, alignment construction, and gap treatment
was explored by taking as a reference the topology
obtained from optimization alignment analysis under
equal parameter costs (gapผtransversionผ transition)
as a reference because we prefer this method in theory
(see Section 4). We assessed similarity between analyses
in two ways. Topology similarity is the ratio of the
number of shared clades between the trees found in each
analysis to the total number of clades obtained in the
reference analysis. Second, we calculated the increase in
length of the topologies found by each analysis under the
assumptions of the equal costs optimization alignment
method (i.e., diagnosed length of a particular tree under
equal cost optimization alignment minus the minimum
length of the equal cost optimization alignment analysis).
The different values obtained were rescaled by dividing
them by the difference between the maximum and the
minimum possible lengths of the equal cost optimization
alignment analysis. This index, which we call Rescaled
Length Increase (RLI), is reminiscent of the Retention
Index (Farris, 1989) and measures the overall decrease in
fit of characters to a non-optimal topology under a
particular set of assumptions.