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Title: A PRIORI AND A POSTERIORI METHODS IN COMPARATIVE EVOLUTIONARY STUDIES OF HOST-PARASITE ASSOCIATIONS

Author
item DOWLING, ASHLEY - UNIVERSITY OF MICHIGAN
item VAN VELLER, MARCO - WAGENINGEN UNIVERSITY
item Hoberg, Eric
item BROOKS, DANIEL - UNIVERSITY OF TORONTO

Submitted to: Cladistics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/14/2003
Publication Date: 4/20/2003
Citation: DOWLING, A.P., VAN VELLER, M.G., HOBERG, E.P., BROOKS, D.R. A PRIORI AND A POSTERIORI METHODS IN COMPARATIVE EVOLUTIONARY STUDIES OF HOST-PARASITE ASSOCIATIONS. CLADISTICS. 2003.

Interpretive Summary: The structure and history of host-parasite associations are intricate, although most biologists have assumed a paramount role for cospeciation to explain the history and distribution of hosts and parasites. Only in the past 20 years, however, have robust methods been developed to examine the roles of coevolution (including cospeciation and coadapation) and other processes including host switching and extinction in the history of host-parasite assemblages. We provide a direct comparison of 2 current methods that differ substantially in their ontology or conceptual frameworks. Despite using the same null hypothesis, Brooks Parsimony Analysis and reconciliation methods in studies of host-parasite associations differ fundamentally. Reconciliation methods may eliminate or modify input data in order to maximize fit of single parasite clades to a null hypothesis of co-speciation, by invoking different a priori assumptions, including a known host phylogeny. By examining the degree of phylogenetic congruence among multiple parasite clades, using hosts as analogs of taxa but not presuming a host phylogeny or any degree of cospeciation a priori, Brooks Parsimony Analysis (BPA) modifies the null hypothesis of co-speciation if necessary to maintain the integrity of the input data. Host-switching events are essential for explaining complex histories of host-parasite associations. The conceptual framework for BPA assumes coevolutionary complexity (historical contingency), relying on parsimony as an a posteriori explanatory tool to summarize complex results, whereas reconciliation methods, that embody formalized assumptions of maximum co-speciation, are based on a priori conceptual parsimony. If the coevolution of hosts and parasites is predominantly a matter of cospeciation, rather than of historical contingencies a priori methods are to be preferred. As Hennig (1966) noted, however, we will never know if cospeciation is common enough to be considered to act in a law-like manner unless we test the hypothesis using methods that do not pre-suppose any degree of cospeciation. BPA is such a method and virtually all coevolutionary studies to date using BPA have discovered significant departures from cospeciation, strongly validating the application of a posteriori approaches.

Technical Abstract: Despite using the same null hypothesis, Brooks Parsimony Analysis and reconciliation methods in studies of host-parasite associations differ fundamentally. Reconciliation methods may eliminate or modify input data in order to maximize fit of single parasite clades to a null hypothesis of co-speciation, by invoking different a priori assumptions, including a known host phylogeny. By examining the degree of phylogenetic congruence among multiple parasite clades, using hosts as analogs of taxa but not presuming a host phylogeny or any degree of cospeciation a priori, Brooks Parsimony Analysis (BPA) modifies the null hypothesis of co-speciation if necessary to maintain the integrity of the input data. Two exemplars illustrate critical empirical differences between reconciliation methods and BPA: (1) reconciliation methods rather than BPA may select the incorrect general host cladogram for a set of data from different clades of parasites, (2) BPA, not reconciliation methods, provides the most parsimonious interpretation of all available data, and (3) secondary BPA, proposed in 1990, applied to data sets for which host-switching producing hosts with reticulate histories and provides most parsimonious and biologically realistic interpretations of general host cladograms. The extent to which these general host cladograms, based on cospeciation among different parasite clades inhabiting the same hosts, correspond to host phylogeny can be tested, a posteriori, by comparison with a host phylogeny generated from non-parasite data. These observations lead to the conclusion that BPA and reconciliation methods are designed to implement different research programs based on different conceptual frameworks. BPA is an a posteriori method that is designed to assess the host context of parasite speciation events, whereas reconciliation methods are a priori methods that are designed to fit parasite phylogenies to a host phylogeny. Host-switching events are essential for explaining complex histories of host-parasite associations. The conceptual framework for BPA assumes coevolutionary complexity (historical contingency), relying on parsimony as an a posteriori explanatory tool to summarize complex results, whereas reconciliation methods, that embody formalized assumptions of maximum co-speciation, are based on a priori conceptual parsimony. Modifications of basic reconciliation methods, embodied in TreeMap 1.0 and TreeMap 2.02, represent the addition of weighting schemes in which the researcher specifies allowed departures from cospeciation a priori, with the result that TreeMap results more closely agree with BPA results than do RTA results.