Tuesday, 9 January 2018

Changing Our Mind When the Evidence Changes


European brown bear (Ursus arctos)
Photo credit: Ivan PC
The Iberian brown bears do not descend from those fled from the north during the Ice Age / News / SINC - Servicio de Información y Noticias Científicas

To a creationist, with their simplistic black vs white, right vs wrong mentality, the fact that science can make mistakes or that the understanding is provisional - and all scientific understanding is provisional, even the best-supported - tells them that science, unlike their firmly held and unchangeable dogmas, is unreliable and therefore wrong. Little better than a guess, in fact.

To a scientist, and to anyone interested in truth rather than simple certainty, however, discovering that you have been wrong is actually welcome news and news that was well worth doing the research to discover because it means that what you now think you know is closer to the truth than it was before. Science works because scientists can and do change their minds when the evidence changes, unlike fundamentalist religions, where not changing your mind and not even questioning the dogmas is a prerequisite.

We have investigated the mitochondrial DNA [the cellular energy 'factories'] of a significant number of samples of current bears, from the Holocene the and Pleistocene, in the European context, and we have seen that the glacier refugee theory, commonly accepted, does not work for this species.

Take, for example, this little item by a Spanish team led by Ana García-Vázquez of Coruña University.

It had been assumed that the population of European brown bears was descended from a population that survived the last ice age in the Iberian Peninsula having been driven south as the ice advanced into this southern refuge south of the Pyrenees Mountains and then radiated out from there as the ice retreated.

However, by studying mitochondrial DNA and so identifying maternal lineages, this new study has shown that not only are the modern brown bears not all descended from the Iberian population, but that the modern Iberian population post-dates much of the rest of Europe, including the British Isles. It seems to have been a relative new-comer, entering Iberia about 10,000 years ago - 5,000 years after they repopulated the British Isles. This is just another example of how improved DNA extraction and analysis is revolutionising our interpretation and understanding of the palaeontological evidence.

Abstract
The European brown bear (Ursus arctos) shows a particular phylogeography that has been used to illustrate the model for contraction-expansion dynamics related to glacial refugia in Southern European peninsulas. Recent studies, however, have nuanced the once generally accepted paradigm, indicating the existence of cryptic refugia for some species further north. In this paper we collected available data on chronology and mitochondrial haplotypes from Western European brown bears, adding new sequences from present day individuals from the Cantabrian (North Iberia) area, in order to reconstruct the dynamics of the species in the region. Both genetics and chronology show that the Iberian Pleistocene lineages were not the direct ancestors of the Holocene ones, the latter entering the Peninsula belatedly (around 10,000 years BP) with respect to other areas such as the British Isles. We therefore propose the existence of a cryptic refugium in continental Atlantic Europe, from where the bears would expand as the ice receded. The delay in the recolonization of the Iberian Peninsula could be due to the orographic characteristics of the Pyrenean-Cantabrian region and to the abundant presence of humans in the natural entrance to the Peninsula.


This implies that the Pleistocene lineage was lost, and the Holocene bears, after the last glacial maximum, entered the peninsula from some unidentified area -probably France- and they did it 5,000 years after having colonized the British Isles.
So now we have two possibilities:
  1. The idea of re-population from an Iberian refugia is basically correct but the population that survived the Ice Age died out completely after re-populating Europe and was replaced by new-comers later.
  2. There was a refugia some where else and the Iberian population did not survive the Ice Age.

On balance, it seems that the latter is the more likely. The authors suggest that the abundant presence of humans in the Pyrenean-Cantabrian region could have delayed the re-population of Iberia. They also suggest that a likely location of this Ice Age refugia would be on the Atlantic slopes of Western Europe.

So, now we know that science was wrong in its original idea but our understanding of the truth in respect of survival of European brown bears through the Ice Age and their subsequent re-population of Europe has improved, albeit that we don't now know where they did survive.


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