Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:1998), to provide great £exibility in testing evolutionary hypotheses (Huelsenbeck & Rannala 1997), and simply to give excellent results in the inference of evolutionary relationships (Kuhner & Felsenstein 1994; Huelsenbeck 1995). Constraints imposed by computational complexity are becoming less restrictive as computers improve. Given the recognition of phylogenetic inference as being inherently statistical in nature, it is surprising that so little attention has been paid to experimental design. A number of topics of relevance to experimental design in phylogenetics have been discussed previously. There is something of a folklore surrounding the choice of genes or other genomic regions for investigating particular evolutionary questions, but almost no published quantitative results. It is widely accepted that sequences that have undergone very little evolutionary change since their divergence from a common ancestor, through low substitution rates or short evolutionary times, will exhibit too few di¡erences to contain useful evolutionary information. Equally, sequences that have undergone very large amounts of change (high rates or long times) become `saturated' with changes and no evolutionary signal is detectable amid the noise. Consequently, a happy medium is expected at some intermediate level of sequence divergence, but the àsymptotic' results (for extreme high and low levels of divergence) give no clue as to where this lies. Although there is considerable experience with particular genes and organisms (see Hillis et al. (1996, pp. 336^339) for an extensive list of studies), where comparisons among genes have been made they tend to be evaluated empirically by the congruence of the results obtained among themselves and with researchers'a priori expectations. In addition, a large laboratory e¡ort is needed before even these qualitative conclusions can be reached. The methods introduced in this paper can quantify the e¡ects of varying levels of divergence. They con¢rm the belief that intermediate levels of sequence divergence are most useful, and are able to give estimates of optimal levels of divergence. The necessary analyses can be done before any data are collected. The only other method for assessing which genomic regions are likely to be most useful in phylogenetic questions is that of Yang (1998). This uses simulation to estimate probabilities of successful tree-topology inference. The approach may be extremely time-consuming for realistic problems. The choice of taxa to include in phylogenetic studies has also rarely been discussed. Li et al. (1987) considered the e¡ects of adding outgroup taxa, and Ritland & Clegg (1990) and Maddison et al. (1992) are agreed that if outgroups are to be added, they should not be too distantly related to the ingroup taxa (a conclusion that is con¢rmed in this paper).
Category: