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. 2006 Jan 7;273(1582):11-7.
doi: 10.1098/rspb.2005.3260.

Multiple gene evidence for expansion of extant penguins out of Antarctica due to global cooling

Allan J Baker  1 , Sergio Luiz Pereira, Oliver P Haddrath, Kerri-Anne Edge
Affiliations

Multiple gene evidence for expansion of extant penguins out of Antarctica due to global cooling

Allan J Baker et al. Proc Biol Sci. .

Abstract

Classic problems in historical biogeography are where did penguins originate, and why are such mobile birds restricted to the Southern Hemisphere? Competing hypotheses posit they arose in tropical-warm temperate waters, species-diverse cool temperate regions, or in Gondwanaland approximately 100 mya when it was further north. To test these hypotheses we constructed a strongly supported phylogeny of extant penguins from 5851 bp of mitochondrial and nuclear DNA. Using Bayesian inference of ancestral areas we show that an Antarctic origin of extant taxa is highly likely, and that more derived taxa occur in lower latitudes. Molecular dating estimated penguins originated about 71 million years ago in Gondwanaland when it was further south and cooler. Moreover, extant taxa are inferred to have originated in the Eocene, coincident with the extinction of the larger-bodied fossil taxa as global climate cooled. We hypothesize that, as Antarctica became ice-encrusted, modern penguins expanded via the circumpolar current to oceanic islands within the Antarctic Convergence, and later to the southern continents. Thus, global cooling has had a major impact on penguin evolution, as it has on vertebrates generally. Penguins only reached cooler tropical waters in the Galapagos about 4 mya, and have not crossed the equatorial thermal barrier.

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Figures

Figure 1
Figure 1
Alternative phylogenetic hypothesis proposed for all extant genera of penguins. Hypothesis including all genera were based on (a) morphological (O'Hara 1989), (b) behavioural (Jouventin 1982), (c) myological (McKitrick 1991), (d) integumentary and breeding (Giannini & Bertelli 2004) and were compared to the topology we obtained with (e) nuclear and mitochondrial DNA sequences. Hypothesis based on (f) myology (Schreiweis 1982) and (g) DNA hybridization studies (Sibley & Ahlquist 1990) did not include all genera and, therefore, were not compared in the AU test.
Figure 2
Figure 2
Bayesian estimate of phylogenetic relationships of modern penguins. Phylogenetic reconstruction was based on 2802 bp of RAG-1 and 2889 bp of mitochondrial 12S and 16S rDNA, cyt b and COI, excluding gaps and ambiguously aligned positions. Numbers above branches are Bayesian posterior probabilities/ML bootstrap proportions/MP bootstrap proportion, which are represented as open star when (1.0/100/100). Branches for more distant outgroups were shortened for graphic purposes. A bar represents the expected number of DNA substitutions per site. Each genus is colour-coded. Penguin drawings were modified from del Hoyo et al. (1992), with permission, from Lynx Edicions, Barcelona, Spain.
Figure 3
Figure 3
Bayesian estimates of ancestral areas. Areas were defined as southernmost breeding regions for each species within Antarctica and the Antarctic Convergence (blue), outside Antarctic convergence and up to latitude 45 °S (green), between 45 °S and 30 °S (red), and north of 30 °S (grey). Posterior probabilities for each state are shown as a pie diagram in each internal node.
Figure 4
Figure 4
Chronogram of penguin diversification. Nodes A and B were fixed at 104 and 90 myr. Credibility intervals (95%) are indicated by grey bars at numbered internal nodes. Vertical dashed line indicates the K/T boundary. Periods that Antarctica was ice-covered (black continuous bars) are projected as shaded grey rectangles in the chronogram. Ocean temperature is based on high-resolution deep-sea oxygen isotope records. The MMCT is indicated by an arrow. Geological time scale is given as defined by the Geological Society of America.
Figure 5
Figure 5
Polar stereographic projection to 35 °S at 40, 25, 15 and 5 mya. Reconstructions have continents represented by present-day shorelines (ODSN 2004). Antarctica is indicated as partially (b–c) and fully covered in ice (d–f) (Shevell et al. 2004). Genera are represented by different coloured capital letters, following the coloured names indicated at the bottom. As they start to diversify, species are represented by small letters according to the first letter of common names given in figure 2, except royal penguin represented by r1. Oldest and biggest penguin fossils (Simpson 1976; Clarke et al. 2004) are numbered 1–6 and projected at (a) 40 and (b) 25 mya. The Antarctic circumpolar current, indicated by arrows in the reconstruction at 25 mya only, was formed at the end of the Oligocene.
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