Abstract
BackgroundMolecular studies have revealed that many putative 'species' are actually complexes of multiple morphologically conservative, but genetically divergent 'cryptic species'. In extreme cases processes such as non-adaptive diversification (speciation without divergent selection) could mask the existence of ancient lineages as divergent as ecologically and morphologically diverse radiations recognised as genera or even families in related groups. The identification of such ancient, but cryptic, lineages has important ramifications for conservation, biogeography and evolutionary biology. Herein, we use an integrated multilocus genetic dataset (allozymes, mtDNA and nuclear DNA) to test whether disjunct populations of the widespread nominal Australian gecko species Crenadactylus ocellatus include distinct evolutionary lineages (species), and to examine the timing of diversification among these populations.ResultsWe identify at least 10 deeply divergent lineages within the single recognised species Crenadactylus ocellatus, including a radiation of five endemic to the Kimberley region of north-west Australia, and at least four known from areas of less than 100 km2. Lineages restricted to geographically isolated ranges and semi-arid areas across central and western Australia are estimated to have began to diversify in the late Oligocene/early Miocence (~20-30 mya), concurrent with, or even pre-dating, radiations of many iconic, broadly sympatric and much more species-rich Australian vertebrate families (e.g. venomous snakes, dragon lizards and kangaroos).ConclusionsInstead of a single species, Crenadactylus is a surprisingly speciose and ancient vertebrate radiation. Based on their deep divergence and no evidence of recent gene flow, we recognise each of the 10 main lineages as candidate species. Molecular dating indicates that the genus includes some of the oldest vertebrate lineages confounded within a single species yet identified by molecular assessments of diversity. Highly divergent allopatric lineages are restricted to putative refugia across arid and semi-arid Australia, and provide important evidence towards understanding the history and spread of the Australian arid zone, suggesting at a minimum that semi-arid conditions were present by the early Miocene, and that severe aridity was widespread by the mid to late Miocene. In addition to documenting a remarkable instance of underestimation of vertebrate species diversity in a developed country, these results suggest that increasing integration of molecular dating techniques into cryptic species delimitation will reveal further instances where taxonomic conservatism has led to profound underestimation of not only species numbers, but also highly significant phylogenetic diversity and evolutionary history.
Highlights
Molecular studies have revealed that many putative ‘species’ are complexes of multiple morphologically conservative, but genetically divergent ‘cryptic species’
Whereas traditional field and morphological studies continue to discover new species [1], complexes of phenotypically similar unrecognised taxa are increasingly identified through molecular systematic examination of ‘known’ taxa [2,3,4]
Many clades are characterised by relative morphological stasis over very long time periods [9]; within such groups, ‘cryptic species’ might be divergent lineages as ancient as ecologically diverse nominal ‘genera’ or even ‘families’ of more morphologically variable clades [9,10]
Summary
Molecular studies have revealed that many putative ‘species’ are complexes of multiple morphologically conservative, but genetically divergent ‘cryptic species’. In extreme cases processes such as nonadaptive diversification (speciation without divergent selection) could mask the existence of ancient lineages as divergent as ecologically and morphologically diverse radiations recognised as genera or even families in related groups The identification of such ancient, but cryptic, lineages has important ramifications for conservation, biogeography and evolutionary biology. Many clades are characterised by relative morphological stasis over very long time periods [9]; within such groups, ‘cryptic species’ might be divergent lineages as ancient as ecologically diverse nominal ‘genera’ or even ‘families’ of more morphologically variable clades [9,10] Identifying such ancient cryptic diversity is likely to provide important insights into biogeographic history and processes of morphological stasis, and is essential for the effective allocation of conservation resources to preserve the maximal breadth of evolutionary diversity [5]. Even though the techniques are readily available, cryptic species assessments have not systematically integrated techniques such as internally calibrated molecular dating to assess the phylogenetic diversity [6,7] of newly identified taxa
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