Quantitative criteria for species delimitation

To: Birding Aus <>
Subject: Quantitative criteria for species delimitation
From: Laurie Knight <>
Date: Thu, 30 Sep 2010 09:29:27 +1000
The following item published in the Journal Ibis may be of interest

Quantitative criteria for species delimitation
        • JOSEPH A. TOBIAS1,*,
        • JOHN D. PILGRIM3,
        • LINCOLN D. C. FISHPOOL3,
        • NIGEL J. COLLAR3
Article first published online: 17 AUG 2010
DOI: 10.1111/j.1474-919X.2010.01051.x
Ibis Volume 152, Issue 4, pages 724–746, October 2010

Species are the fundamental units of biology, ecology and
conservation, and progress in these fields is therefore hampered by
widespread taxonomic bias and uncertainty. Numerous operational
techniques based on molecular or phenotypic data have been designed to overcome this problem, yet existing procedures remain subjective or
inconsistent, particularly when applying the biological species
concept. We address this issue by developing quantitative methods for a classic technique in systematic zoology, namely the use of
divergence between undisputed sympatric species as a yardstick for
assessing the taxonomic status of allopatric forms. We calculated mean levels of differentiation in multiple phenotypic characters –
including biometrics, plumage and voice – for 58 sympatric or
parapatric species-pairs from 29 avian families. We then used
estimates of mean divergence to develop criteria for species
delimitation based on data-driven thresholds. Preliminary tests show
that these criteria result in relatively few changes to avian taxonomy in Europe, yet are capable of extensive reassignment of species limits in poorly known tropical regions. While we recognize that species
limits are in many cases inherently arbitrary, we argue that our
system can be applied to the global avifauna to deliver taxonomic
decisions with a high level of objectivity, consistency and
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