Tuesday, 26 April 2016

How Science Works - Evolving Mammoths

Frontiers | Mammuthus Population Dynamics in Late Pleistocene North America: Divergence, Phylogeography and Introgression | Paleontology

To the delight of creationists no doubt, evolutionary paleontologists are changing their minds about North American mammoth evolution.

To most black-and-white-thinking creationists and other science deniers, science is either right or wrong and, since scientists frequently update their knowledge and understanding, to most of them this shows science is always wrong. It isn't of course; it's just science being honest and revising it opinion in view of new information. Science isn't so much a process of proving what's right and what's wrong as a process of gaining more understanding through new or better knowledge.

This recent paper on mammoth evolution in North America is a good example of how this works.

Previously, palaeobiologists thought that the North American mammoths had evolved into several different species following a founder population crossing Beringia from Asia. Based on mostly dental evidence, there were thought to be at least three and maybe as many as five separate species, including a dwarf species on islands off the coast of California. These species each occupied their own geographical area from Alaska in the far north to Mexico and Columbia in the south.

The molars are usually the best-preserved mammoth remains and consist of ridges of enamel which grind up the coarse vegetation on which mammoths, like elephants today, lived. The pattern of these ridges seemed to divide the mammoths into two distinct lines, further subdivided to give the three, four of five different species. The dwarf species was assumed to be a different species because of its stature.

But, this paper has thrown the whole neat pattern of migration, distribution and diversification into question by looking at the genetic evidence, which appears to tell a different story. In particular, the team looked at the mitochondrial DNA:

After evolving in Africa at the close of the Miocene, mammoths (Mammuthus sp.) spread through much of the northern hemisphere, diversifying morphologically as they entered various habitats. Paleontologically, these morphs are conventionally recognized as species. In Pleistocene North America alone, several mammoth species have been recognized, inhabiting environments as different as cold tundra-steppe in the north and the arid grasslands or temperate savanna-parklands of the south. Yet mammoth phylogeographic studies have overwhelmingly focused on permafrost-preserved remains of only one of these species, Mammuthus primigenius (woolly mammoth). Here we challenge this bias by performing a geographically and taxonomically wide survey of mammoth genetic diversity across North America. Using a targeted enrichment technique, we sequenced 67 complete mitochondrial genomes from non-primigenius specimens representing M. columbi (Columbian mammoth), M. jeffersonii (Jeffersonian mammoth), and M. exilis (pygmy mammoth), including specimens from contexts not generally associated with good DNA preservation. While we uncovered clear phylogeographic structure in mammoth matrilines, their phylogeny as recovered from mitochondrial DNA is not compatible with existing systematic interpretations of their paleontological record. Instead, our results strongly suggest that various nominal mammoth species interbred, perhaps extensively. We hypothesize that at least two distinct stages of interbreeding between conventional paleontological species are likely responsible for this pattern – one between Siberian woolly mammoths and resident American populations that introduced woolly mammoth phenotypes to the continent, and another between ecomorphologically distinct populations of woolly and Columbian mammoths in North America south of the ice.

Copyright: © 2016 The authors. Reprinted under terms of Creative Commons Attribution License (CC BY 4.0)

The authors interpret this new data as showing that there was extensive interbreeding between the different populations and the the 'wooly' characteristic may have been due to interbreeding between the Siberian wooly mammoth and the North American population. In effect, it looks more likely that rather than having fully diverged into different species, the North American mammoths behaved more like a ring species with geographical variations evolved to adapt to local conditions still able to interbreed with adjacent populations and including interbreeding with the Siberian population via the Beringia land bridge between Siberia and Alaska.

To a simplistic creationist this will mean scientists got it all wrong, and will be shown to have got it all wrong in the not-to-distant future when they have to 'change their minds' yet again. To an evolutionary biologist of course, this makes perfect sense given the rather hazy definition of species and the problems of trying to fit dynamic, evolving populations into fixed taxonomic groupings. What we have here is nothing more than evolution in progress and incomplete speciation. Populations were isolated enough to evolve according to local conditions but not isolated enough to evolve barriers to hybridization.

Creationists frauds will doubtless try to present this to the scientifically illiterate as evidence of a theory in crisis. In fact, it shows how palaeobiology only makes any sense in terms of an evolutionary process. Far from being evidence of a theory in crisis, this paper represent yet another confirmation of evolution in progress.

But perhaps a creationist with enough confidence in their faith could explain how this finding fits into a notion based on magic and a magic creator, starting with how that notion can be tested and falsified scientifically, and if it can't be, why this isn't evidence that creationism is a notion in crisis.

Or maybe not...

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