DNA and Morphology Unite Two Species and 10 Million Year Old Fossils

Simon F K Hills,Mary Morgan-Richards,Steven A Trewick,James S Crampton,Michael Knapp

doi:10.1371/journal.pone.0052083

Simon F K Hills, Mary Morgan-Richards + Show 3 more

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https://doi.org/10.1371/journal.pone.0052083

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Journal: PloS one	Publication Date: Dec 20, 2012
Citations: 46	License type: CC BY 4.0

Affiliation: Massey University, GNS Science

Abstract

Species definition and delimitation is a non-trivial problem in evolutionary biology that is particularly problematic for fossil organisms. This is especially true when considering the continuity of past and present species, because species defined in the fossil record are not necessarily equivalent to species defined in the living fauna. Correctly assigned fossil species are critical for sensitive downstream analysis (e.g., diversification studies and molecular-clock calibration). The marine snail genus Alcithoe exemplifies many of the problems with species identification. The paucity of objective diagnostic characters, prevalence of morphological convergence between species and considerable variability within species that are observed in Alcithoe are typical of a broad range of fossilised organisms. Using a synthesis of molecular and morphometric approaches we show that two taxa currently recognised as distinct are morphological variants of a single species. Furthermore, we validate the fossil record for one of these morphotypes by finding a concordance between the palaeontological record and divergence time of the lineage inferred using molecular-clock analysis. This work demonstrates the utility of living species represented in the fossil record as candidates for molecular-clock calibration, as the veracity of fossil species assignment can be more rigorously tested.

Highlights

Defining species is an age-old problem for evolutionary biologists and reflects, in part at least, the continuous nature of species formation
Deformation implied by the transition from mean A. wilsonae to mean A. knoxi corresponds to relative movement of landmark 1 towards the posterior and landmark 10 towards the anterior (Fig. 3)
There are indications that morphology may be related to water depth at which the animal lived, given the differences noted above and the fact that A. knoxi specimens tend to be taken from deeper water than A. wilsonae

Summary

Introduction

Defining species is an age-old problem for evolutionary biologists and reflects, in part at least, the continuous nature of species formation (see [1,2,3,4,5]). Whereas neo-taxonomists have successfully used morphological traits to identify distinct units that reflect biological species [7], genetic data have revealed many examples of cryptic or polymorphic species and morphological convergence that demonstrate the shortcomings of morphological species-delimitation [8]. Species as they are understood in the fossil record, do not necessarily equate to species defined in the modern biota [6]. The costs of misclassification are high when fossil taxa are inferred to be representatives of ancestral clades and used to calibrate nodes above the species level using molecular clocks, leading to erroneous inferences about the age of crown groups and the tempo of species radiation (see e.g. [10,11,12,13,14,15])

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