OTU stability and factors determining taxonomic stability: examples from the caminalcules and the leptopodomorpha

-Tests of OTU stability were carried out by random selection of subsets of OTUs from the Recent Caminalcules. From these samples we computed UPGMA phenograms and estimated cladograms based on Wagner parsimony and compatibility analysis. These were compared with pruned versions of the trees based on 29 OTUs representing: (1) their standards; and (2) the true cladogram known for this artificial data set. A similar analysis was carried out for a complete sampling of sets of OTUs from a data set based on the Leptopodomorpha (Hemiptera). Only the first type of comparison could be carried out for this data set. In the Caminalcules, when classifications based on samples of OTUs are compared with their own standards (classifications based on all OTUs)-which would be the typical situation in real organisms where true cladogenies are not known-stability seems to be a function of OTU sample size. For low OTU numbers, stability is higher in classifications based on estimated cladograms, but for high OTU numbers it is greater in classifications based on phenograms. While there is a trend towards increasing stability in the phenograms as OTU sample size increases, no such trend is evident in estimated cladograms of the standards or of the true cladogram. By contrast, estimated cladograms have a higher consensus with the true cladogram than do UPGMA phenograms regardless of number of OTUs employed. In the Leptopodomorpha, classifications based on UPGMA and Wagner parsimony methods are about equally stable with a suggestion of slightly better performance for Wagner parsimony at low OTU numbers and a reversal of this trend for higher OTU numbers. There is no evidence of marked superiority of Wagner parsimony as had been claimed in an earlier study of this group. The outcome of studies of both character and OTU stability is shown to be approximately related to the ratio n/(2t 3), where n is the number of characters and t is the number of OTUs. When this ratio is low, phenograms are more stable than estimated cladograms; when it is high the reverse relation holds. Also, when the ratio is low, phenograms are better estimates of the true cladogeny than are estimated cladograms. The reverse relation holds for high values of the ratio. To confirm these conclusions, we carried out combined studies of character and of OTU stability applied to both data sets. As character number decreased, or OTU number increased, classifications based on phenograms became more stable than those based on estimated cladograms, and became better for reconstructing the true cladogeny than estimated cladograms in the Caminalcules. Character stability and OTU stability may in fact be two separate aspects of the same phenomenon-taxonomic stability. The merits of various designs suitable for testing OTU stability in numerical taxonomy are discussed. All are based on the results of sampling subsets of OTUs for the entire taxon. [Phenetic classifications; cladistic classifications; taxonomic stability; Caminalcules; Leptopodomorpha.] In this study, we compare the stability of classifications based on phenograms and estimated cladograms to the removal of varying numbers of OTUs using two data sets, the Caminalcules and the Leptopodomorpha, as examples. As our analysis progressed, we discovered that the relative stability of these classifications was a function of the ratio of characters to OTUs in the data matrix, and this phenomenon is also explored in this paper. Stability of classifications can be examined from several perspectives (Rohlf and Sokal, 1981). One kind of stability is the robustness of a classification to the addition of new characters or to different selections of characters. If different sets of characters represent different classes of characters, such as external versus internal characters, male versus female characters, biochemical versus morphological characters, and the like, the test of stability is