Archive for January, 2019

Speciations or a Sudden Origin?

Saturday, January 19th, 2019

Genomic analysis is yielding immense amounts of data about the evolution of all the species we’ve sampled. Some of the chronological results require some care when sharing with the general public, else they can be paraded as ‘proof’ of more mythological origin tales. A recent example comes from the work of David Thaler. Here’s a paper from 2014:

DNA Barcoding Works in Practice but Not in (Neutral) Theory

More recently there’s this particular 2018 paper:

Why should mitochondria define species?

Which gets this write up on the Intelligent Design web-site “Evolution News” of:

New Paper in Evolution Journal: Humans and Animals Are (Mostly) the Same Age?

And a somewhat less restrained write-up in the Murdoch Press:

Why did the overwhelming majority of species in existence today emerge at about the same time?

It seems both write-ups didn’t read much past the abstract. Of course, the pro-Science web-sites had to respond with more explanatory explanations of what the findings really meant…

No, Humans Are Probably Not All Descended From A Single Couple Who Lived 200,000 Years Ago
And this…
Did 90% of Animal Species Appear about the Same Time as Human Beings?

My initial reaction was to ignore the breathless reportage and contact David Thaler himself. He emphasised that the chief thesis of the paper – and the 2014 paper before it – was to argue that species are biologically real entities, defined well by their mitochondrial genomes. The suggestion is that humans – and other species – derive from small founder populations and, thus, singular mitochondrial lineages. But does that argue for a Universal Noah’s Flood or a singular Adam & Eve in a Garden of Eden?
Here’s Thaler’s eloquent response to deriving that conclusion from his work on DNA barcodes…

To say that our paper is evidence for Noah’s ark is like saying that it is evidence for the moon being made of cheese or the earth being flat since we do not provide evidence against these ideas. Scientific discourse is not free of rhetoric but we hold out the hope that it can be at a higher level.

He goes on to explain that the variance within species is quite ‘variable’ and the molecular clocks that give estimates for origin times for the individual mitochondrial species barcodes are hard to calibrate. Thus while species largely show the same pattern of ‘singular’ origin, their individual origin times likely vary across the past few hundred thousand years. What the genetic barcodes imply is that species are coherent entities at a genomic level, with selection suppressing intraspecific variation. Essentially that’s exactly what “Punctuated Equilibrium” described, and it was once a controversial viewpoint in the 1970s-80s, but was largely absorbed into the mainstream by the 1990s. Species are real units of selection, so to speak, and undergo their own patterns of either extinction or diversification. Evolution between the higher taxonomic levels is then best described by “mosaic evolution” rather than infinitesimal gradations in form.

Even in the human fossil/DNA record there’s ample signs of this – just 300,000 years ago at least five ‘species’ of ‘Homo’ (our biological genus) are known: Homo sapiens (appeared c. 300,000 years ago), Homo neanderthalis, Homo naledi, Homo floresiensis and the enigmatic ‘Denisovans’ who are only known from genomic fragments in ancient remains and modern humans. We know that Sapiens, Neanderthals and the Denisovans interbred, but what was our relationship with the smaller brained Naledi and Floresiensis? And, even after that mixing, one lineage came to dominate. Yet all showed a mix of features, a mosaic, so much so that focusing on one diagnostic feature to define a species could be utterly misleading.

In conclusion, from Thaler’s research, further molecular studies of just how mitochondrial mutation affects selective advantage will clarify the mechanism of how species appear and then remain distinct from their close kin. Noah’s Ark or Adam and Eve will just have to be demonstrated on their own merits.