Species-genus ratios reflect a global history of diversification and range expansion in marine bivalves

Andrew Z. Krug, David Jablonski, James W. Valentine

Research output: Contribution to journalArticlepeer-review

78 Scopus citations


The distribution of marine bivalve species among genera and higher taxa takes the form of the classic hollow curve, wherein few lineages are species rich and many are species poor. The distribution of species among genera (S/G ratio) varies with latitude, with temperate S/G's falling within the null expectation, and tropical and polar S/G's exceeding it. Here, we test several hypotheses for this polar overdominance in the species richness of small numbers of genera. We find a significant positive correlation between the latitudinal range of a genus and its species richness, both globally and within regions. Genus age and species richness are also positively related, but this relationship breaks down when the analysis is limited to genera endemic to climate zones or with narrow latitudinal ranges. The data suggest a link between speciation and range-expansion, with genera expanding out of the tropical latitudinal bins tending to speciate more prolifically, both globally and regionally. These genera contain more species within climate zones than taxa endemic to that zone. Range expansion thus appears to be fundamentally coupled with speciation, producing the skewed distribution of species among genera, both globally and regionally, whereas clade longevity is achieved through extinction-resistance conferred by broad geographical ranges.

Original languageEnglish (US)
Pages (from-to)1117-1123
Number of pages7
JournalProceedings of the Royal Society B: Biological Sciences
Issue number1639
StatePublished - May 22 2008

All Science Journal Classification (ASJC) codes

  • General Biochemistry, Genetics and Molecular Biology
  • General Immunology and Microbiology
  • General Environmental Science
  • General Agricultural and Biological Sciences


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