Crocodile (Crocodylus acutus) from a population living on the coast of
Panama.Image: José Avila-Cervantes.
Once in a while we get a piece of scientific research that shows how science, and the answers science provides, all mesh together to give a much larger picture, showing how several different branches of science all work to support one another and confirm our conclusions about what happened in the past to produce what we see today.
In this case, evidence from genetics, geology and climatology all work to explain what can be observed in the evolution of the American crocodiles either side of the Panamanian isthmus. It was provided by researchers from McGill University, Quebec, Canada who initially set out to see how far the Caribbean and Pacific populations of crocodiles had diverged from one another in the 3 million years since the Isthmus of Panama rose up and joined North and South America but separated the Atlantic and Pacific Oceans.
The news release from McGill University explains:
Crocodiles are resilient animals from a lineage that has survived for over 200 million years. Skilled swimmers, crocodiles can travel long distances and live in freshwater to marine environments. But they can’t roam far overland. American crocodiles (Crocodylus acutus) are found in the Caribbean and Pacific coasts of the Neotropics but they arrived in the Pacific before Panama existed, according to researchers from McGill University.
Over 3 million years ago, the formation of the Isthmus of Panama altered global ocean circulation, connecting North and South America and establishing the Caribbean Sea. This resulted in widespread mixing of species on the continent and separation in the seas. On land, mammals from North America such as mammoths, sabre-toothed cats, horses, and camels invaded South America, and strange mammals like giant ground sloths, armadillos, and opossums from South America invaded North America. This event is known as the Great American Interchange, and the opposite happened in the seas, where new species of corals, clams, and fishes evolved in the separated Pacific and Caribbean waters.
The question a group of McGill and Panamanian researchers asked was: how distant are the Pacific and Caribbean populations from each other and does it match the geological record? Researchers have long suspected that American crocodiles living on the Pacific coast should have diverged genetically enough from Caribbean populations to become unique species.
“We assumed we would detect significant genetic differences between Pacific and Caribbean crocodile populations that were isolated for the past 3 million years,” thought José Avila-Cervantes, a recent PhD graduate of McGill University under the supervision of Professor Hans Larsson.
Capturing crocodiles
To test this, Avila-Cervantes captured and took blood samples of crocodiles from several populations living on both coasts of Panama. Back at McGill University, he sequenced their genomes to look for small variations in their DNA. He used the genetic differences to estimate how much evolutionary divergence and gene flow existed between populations. With this information, the team found that Pacific and Caribbean crocodile populations have been separated for only about 100,000 years.
This time of separation is a far cry from the 3 million years we were
expecting, but it did match the last interglacial period of the Ice Age.
It surprised us to see that during the warm inter-glacial periods, most of Panama was underwater with the coasts separated by brackish lagoons, small rivers, and thin stretches of land. These are the reasons why we think crocodiles were able to pass from coast to coast freely and explain why their oldest genetic signature of separation coincides with this time.
This is one of the first studies to implicate Ice Age glaciation-interglaciation cycles with the evolution of a tropical organism.
Our study not only highlights the resilience of crocodiles to ancient climate changes and their great capacity to survive large geological events, but also their vulnerability to our voracious need to modify their environments.
The result was surprising. It showed that the two populations had only been
separated for about 100,000 years - far too short to comply with the known date
for the formation of the Isthmus! However, this date did comply with what
is known from about the timing of the last interglacial period and consequent
changes in sea levels.
Professor Hans Larsson, Co-author
Director of the Redpath Museum
McGill University.
Director of the Redpath Museum
McGill University.
It surprised us to see that during the warm inter-glacial periods, most of Panama was underwater with the coasts separated by brackish lagoons, small rivers, and thin stretches of land. These are the reasons why we think crocodiles were able to pass from coast to coast freely and explain why their oldest genetic signature of separation coincides with this time.
This is one of the first studies to implicate Ice Age glaciation-interglaciation cycles with the evolution of a tropical organism.
Dr Jose Avila‐Cervantes, co-author
McGill University
McGill University
Our study not only highlights the resilience of crocodiles to ancient climate changes and their great capacity to survive large geological events, but also their vulnerability to our voracious need to modify their environments.
Professor Hans Larsson
“This time of separation is a far cry from the 3 million years we were expecting,” said Professor Larsson, Director of the Redpath Museum at McGill. “But it did match the last interglacial period of the Ice Age.”The team's findings were published in Evolution on 14 December, 2020:
Glacial and interglacial cycles in the Ice Age mark periods of peak polar glaciations separated by relatively warm times. These warm times caused sea levels to rise over 100 meters globally compared to present-day levels. Using the record of Ice Age sea levels, Avila-Cervantes was able to reconstruct what Panama would have looked like during these peak cold and warm periods of the Ice Age.
Coastal movements explained
“It surprised us to see that during the warm inter-glacial periods, most of Panama was underwater with the coasts separated by brackish lagoons, small rivers, and thin stretches of land,” said Avila-Cervantes. “These are the reasons why we think crocodiles were able to pass from coast to coast freely and explain why their oldest genetic signature of separation coincides with this time.” A second younger signature of genetic separation is timed to about 20,000 years ago and coincides with the last glaciation cycle that they found made Panama about twice as wide as it is today, and probably a good barrier for these crocodiles. “This is one of the first studies to implicate Ice Age glaciation-interglaciation cycles with the evolution of a tropical organism.”
Yet the researchers discovered there is some genetic divergence between the populations on each coast despite the frequent inter-glaciations, and this diversity is at risk due to habitat destruction from human development. “It was difficult to find any population living on the Pacific coast near the Panama Canal,” said Avila-Cervantes.
One of the best-preserved populations is in the middle of the Panama Canal on the Barro Colorado Island Nature Monument. “Preserving the population around this island may be our best chance to preserve the unique genetic signatures of Panamanian American crocodiles,” said Professor Larsson. “Our study not only highlights the resilience of crocodiles to ancient climate changes and their great capacity to survive large geological events, but also their vulnerability to our voracious need to modify their environments.”
Here then we have a date of the last genetic intermixing of the two populations that exactly coincides with the date of the last inter-glacial maximum at 20,000 years ago, when intermixing of the two populations would have been possible, following separation by the rising of theAbstract
The final formation of the Central American Isthmus (CAI) about 3.5 million years ago altered global ocean circulation, connected North and South America terrestrial biotas, and established the Caribbean Sea. The nature of this event creates a natural scenario to test vicariance, divergence, and speciation by allopatry. Studies have shown the effect of the CAI on marine and terrestrial species, but none have examined a large‐bodied amphibious taxon. We used RAD sequencing on populations of the American crocodile (Crocodylus acutus) to study the genomic variation of C. acutus on both sides of the CAI, infer its demographic history, and measure the effect of the opening of the Panama Canal. Our results showed three genomic clusters: (1) Caribbean and the Panama Canal, (2) Pacific coast, and (3) Coiba island. The estimated divergence times between the Caribbean and Pacific populations are about 20,000 years ago, which is younger than the formation of the CAI, coinciding with the Last Glacial Maximum. We hypothesize the glacial/interglacial cycles facilitated gene flow between the Caribbean and Pacific crocodile populations after the formation of the CAI, masking any genomic divergence the CAI may have caused. There is no evidence of gene flow associated with the opening of the Panama Canal.
Avila‐Cervantes, J., Arias, C., Venegas‐Anaya, M., Vargas, M., Larsson, H.C.E. and McMillan, W.O. (2021),
Effect of the Central American Isthmus on gene flow and divergence of the American crocodile (Crocodylus acutus).
Evolution. DOI: 10.1111/evo.14139
© 2020 The Authors. Evolution © 2020 The Society for the Study of Evolution.
Published with kind permission under license #5011610666466
The genetic, geological and climatological evidence all converge on these same dates because all three methods are accurate ways to measure the same event. It goes without saying that this would not be so if the science was wrong in respect of any one of those and especially if the genetic divergence we would expect to see as two separated populations, starting from a common ancestor 3.5 million years ago and evolving according to the Theory of Evolution, was wrong.
This is how and why science tells a consistent story of how and when things happened.
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