Hydrobiologia

, Volume 480, Issue 1–3, pp 111–128

Morphological stasis in the Eurytemora affinis species complex (Copepoda: Temoridae)

  • Carol Eunmi Lee
  • Bruce W. Frost
Article

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 (allozyme) 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 morphometric 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. In contrast, nuclear and mtDNA phylogenies were mostly congruent. Reproductive isolation proved to be the most sensitive indicator of speciation, given that two genetically and morphologically proximate populations showed evidence of hybrid breakdown. Quantitative genetic (morphological) subdivision (QST = 0.162) was lower than nuclear genetic subdivision (GST = 0.617) for four laboratory-reared North American populations, indicating retarded evolution of morphological characters. This result contrasts with most other species, where QST typically exceeds GST as a result of directional selection. Thus, in all but the European populations, evolution of the secondary sex characters was marked by morphological stasis, even between reproductively-isolated populations.

cryptic speciation species concepts hybrid breakdown copepod systematics phylogenetics morphometrics allozyme genetic architecture 

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Copyright information

© Kluwer Academic Publishers 2002

Authors and Affiliations

  • Carol Eunmi Lee
    • 1
  • Bruce W. Frost
    • 2
  1. 1.Department of ZoologyUniversity of WisconsinMadisonU.S.A.
  2. 2.School of OceanographyUniversity of WashingtonSeattleU.S.A.

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