Hydrobiologia 480: 111–128, 2002. 111 C.E. Lee, S. Strom & J. Yen (eds), Progress in Zooplankton Biology: Ecology, Systematics, and Behavior. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. Morphological stasis in the Eurytemora affinis species complex (Copepoda: Temoridae) Carol Eunmi Lee1 &BruceW.Frost2 1Department of Zoology, 430 Lincoln Drive, University of Wisconsin, Madison, WI 53706, U.S.A. E-mail:
[email protected] 2School of Oceanography, Box 357940, University of Washington, Seattle, WA 98195-7940, U.S.A. E-mail:
[email protected] Key words: cryptic speciation, species concepts, hybrid breakdown, copepod systematics, phylogenetics, morphometrics, allozyme, genetic architecture Abstract Morphological stasis has long been regarded as one of the most challenging problems in evolutionary biology. This study focused on the copepod species complex, Eurytemora affinis, as a model system to determine pattern and degree of morphological stasis. This study revealed discordant rates of morphological differentiation, molecular evolution, and reproductive isolation, where speciation was accompanied by lack of morphological differentiation in secondary sex characters. Comparisons were made among phylogenies based on morphometrics, nuclear (al- lozyme) loci, and mitochondrial DNA (mtDNA) sequences from cytochrome oxidase I, for a total of 43 populations within the complex. These systematic relationships were also compared to patterns of reproductive isolation. In addition, genetic subdivision of nuclear molecular (allozyme) markers (GST) and quantitative (morphological) characters (QST) were determined to infer evolutionary forces driving morphological differentiation. The morpho- metric phylogeny revealed that all clades, excluding the European clade, were morphologically undifferentiated and formed a polytomy (multifurcation). Morphometric distances were not correlated with mtDNA distances, or with patterns of reproductive isolation.