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Quantitative tests of primary homology Ingi Agnarsson1,2

Summary: Quantitative tests of primary homology
Ingi Agnarsson1,2
* and Jonathan A. Coddington1
Department of Entomology, NHB-105, Smithsonian Institution, PO Box 37012, Washington, DC 20013-7012, USA; 2
The University of British
Columbia, Departments of Botany and Zoology, 3549-6270 University Blvd., Vancouver, BC V6T 1Z4, Canada
Accepted 14 September 2006
In systematic biology homology hypotheses are typically based on points of similarity and tested using congruence, of which the
two stages have come to be distinguished as ``primary'' versus ``secondary'' homology. Primary homology is often regarded as prior
to logical test, being a kind of background assumption or prior knowledge. Similarity can, however, be tested by more detailed
studies that corroborate or weaken previous homology hypotheses before the test of congruence is applied. Indeed testing similarity
is the only way to test the homology of characters, as congruence only tests their states. Traditional homology criteria include
topology, special similarity, function, ontogeny and step-counting (for example, transformation in one step versus two via loss and
gain). Here we present a method to compare quantitatively the ability of such criteria, and competing homology schema, to explain
morphological observations. We apply the method to a classic and difficult problem in the homology of male spider genital sclerites.
For this test case topology performed better than special similarity or function. Primary homologies founded on topology resulted in
hypotheses that were globally more parsimonious than those based on other criteria, and therefore yielded a more coherent and
congruent nomenclature of palpal sclerites in theridiid spiders than prior attempts. Finally, we question whether primary homology


Source: Agnarsson, Ingi - Department of Biology, Universidad de Puerto Rico


Collections: Biology and Medicine