The shape of evolution: systematic tree topology

Three hypotheses that predict probabilities associated with various tree shapes, or topologies, are compared with observed topology frequencies for a large number of 4, 5, 6 and 7-member trees. The united data on these n-member trees demonstrate that both the equiprobable and proportional-to-distinguishable-types hypotheses poorly predict tree topologies, while all observed topology frequencies are similar to predictions of a simple Markovian dichotomous branching hypothesis. Differences in topology frequencies between phenetic and non-phenetic trees are observed, but their statistical significance is uncertain. Relative frequencies of highly asymmetrical topologies are larger, and those of symmetrical topologies are smaller, in phenetic than in non-phenetic trees. The fact that a simple Markovian branching process, which assumes that each species has an equal probability of speciating in each time period, can predict tree topologies offers promise. Refinement of Markovian branching hypotheses to include the possibility of multiple furcations, differential speciation and extinction rates for different groups of organisms as well as for a single group through geological time, hybrid speciation, introgression, and lineage fusion will be necessary to produce realistic models of lineage diversification.