Tuesday, June 9, 2015

[Herpetology • 2015] Hybridization masks Speciation in the Evolutionary History of the Galápagos Marine Iguana Amblyrhynchus cristatus


Figure 1. Temporal framework of iguana evolution on the Galápagos Islands.
(
a) Partial timetree based on four nuclear genes (3000 bp) time-calibrated using multiple time constraints applied to a total dataset of 78 squamates (full tree in electronic supplementary material, figure S1). Numbers at nodes indicate support from partitioned BI analyses (posterior probability values; PP) and maximum-parsimony (MP) bootstrapping (bootstrapping values; BS); asterisks indicate maximum support. Bars are 95% credibility intervals of time estimates.
 (
b) Maximum credibility tree from a partitioned BI analysis of 5557 bp of mtDNA. Black asterisks indicate concordant maximum support from a partitioned BI analysis, timetree analysis and MP bootstrap analysis. Small grey asterisks indicate high support (PP > 0.94; BS > 70%) from at least two of these analyses. Time estimates and 95% credibility intervals from a timetree analysis are given at selected nodes.

ABSTRACT 
The effects of the direct interaction between hybridization and speciation-two major contrasting evolutionary processes-are poorly understood. We present here the evolutionary history of the Galápagos marine iguana (Amblyrhynchus cristatus) and reveal a case of incipient within-island speciation, which is paralleled by between-island hybridization. In-depth genome-wide analyses suggest that Amblyrhynchus diverged from its sister group, the Galápagos land iguanas, around 4.5 million years ago (Ma), but divergence among extant populations is exceedingly young (less than 50 000 years). Despite Amblyrhynchus appearing as a single long-branch species phylogenetically, we find strong population structure between islands, and one case of incipient speciation of sister lineages within the same island-ostensibly initiated by volcanic events. Hybridization between both lineages is exceedingly rare, yet frequent hybridization with migrants from nearby islands is evident. The contemporary snapshot provided by highly variable markers indicates that speciation events may have occurred throughout the evolutionary history of marine iguanas, though these events are not visible in the deeper phylogenetic trees. We hypothesize that the observed interplay of speciation and hybridization might be a mechanism by which local adaptations, generated by incipient speciation, can be absorbed into a common gene pool, thereby enhancing the evolutionary potential of the species as a whole.

KEYWORDS: restriction site-associated DNA (RAD) sequencing, single-nucleotide polymorphisms, El Niño, volcanism, introgressive hybridization, morphometrics


Figure 1. Temporal framework of iguana evolution on the Galápagos Islands. (a) Partial timetree based on four nuclear genes (3000 bp) time-calibrated using multiple time constraints applied to a total dataset of 78 squamates (full tree in electronic supplementary material, figure S1). Numbers at nodes indicate support from partitioned BI analyses (posterior probability values; PP) and maximum-parsimony (MP) bootstrapping (bootstrapping values; BS); asterisks indicate maximum support. Bars are 95% credibility intervals of time estimates. (b) Maximum credibility tree from a partitioned BI analysis of 5557 bp of mtDNA. Black asterisks indicate concordant maximum support from a partitioned BI analysis, timetree analysis and MP bootstrap analysis. Small grey asterisks indicate high support (PP > 0.94; BS > 70%) from at least two of these analyses. Time estimates and 95% credibility intervals from a timetree analysis are given at selected nodes. (c) Timetree based on a complete matrix of 1 793 845 nucDNA sequences obtained by RADSeq of three Galápagos marine iguanas (selected to represent the deepest splits within the species) and one land iguana (Conolophus pallidus), showing the extremely shallow divergences within Amblyrhynchus.

An adult male marine iguana (Amblyrhynchus cristatus) of the Loberia type, at the ‘La Loberia’ colony on San Cristóbal Island.
photo: Amy MacLeod 

No ugly at all - marine iguanas are fascinating inhabitants of the Galápagos islands and the only sea going lizard world-wide
photo: Alejandro Ibáñez Ricomá

Coming a long way - a red migrant marine iguana from Española on San Cristóbal island
photo: Amy MacLeod


Amy MacLeod, Ariel Rodríguez, Miguel Vences, Pablo Orozco-terWengel, Carolina García, Fritz Trillmich, Gabriele Gentile, Adalgisa Caccone, Galo Quezada and Sebastian Steinfartz. 2015. Hybridization masks Speciation in the Evolutionary History of the Galápagos Marine Iguana. Proceedings of the Royal Society B: Biological Sciences. 282(1809). DOI: 10.1098/rspb.2015.0425


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Hybridization (the merging of species) and speciation (the diversification of populations into species) are classically seen as opposing evolutionary processes. However, this study on Galapagos marine iguanas, the only seagoing lizard worldwide, offers novel insights into how these processes can act in concert. On San Cristóbal Island it was found that two resident populations of marine iguanas behave as distinct species from one another; they do not interbreed and they have morphological differences. This is the smallest island in which a speciation process in a highly mobile animal has been found. However these two populations also seem to breed freely with migrant iguanas from other Islands.

The research team believe that the combination of speciation between populations generates new adaptations and then hybridization between these populations absorbs those adaptations into a common genepool. This may enhance the evolutionary success of marine iguanas and help to explain their persistence on the archipelago (they are one of the oldest vertebrates of the Islands). However, the divergence between marine and land iguanas is far more recent than was previously thought, and it seems that they diverged around 4.5 million years ago which is about the same age as the oldest of the present Islands.

The Evolutionary History of the Galapagos Marine Iguana - Galapagos Conservation Trust Blog [@galapagossip] https://shar.es/12Yoew 
Darwin’s ugly duckling surprises evolutionary biologists http://www.idw-online.de/-CaVMAA

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