Thursday, 19 March 2015

A Christadelphian evolution denialist completely misunderstands a paper on genetic bottlenecks

A few days ago, I commented on the misuse of a recent paper which found evidence for a bottleneck in the Y chromosome data by an evolution denialist in our community who declared that:
Some say that a "population bottleneck" never happened in man's history. Well, this is the latest in a growing line of academic papers on the subject:
Over the years that I have been monitoring evolution denialism in our community, it has been my experience that whenever evolution denialist refer to a scientific paper, either they have never read it, or have completely misunderstood it. Unsurprisingly, this appears to be the case.

One of the authors of the paper is Melissa A. Wilson Sayres, an evolutionary biologist and Assistant Professor in the school of Life Sciences at Arizona State University. She's posted at Panda's Thumb a number of times, and accepts evolution, so it is unlikely that she would see her paper as evidence for monogenism. She's now commented on this paper at her blog, and provided a helpful infographic which explains what is going on. Unsurprisingly, Wilson Sayres' post shows that the paper provides zero support for the special creationist belief that Adam and Eve were the sole ancestors of the human race.  Here's the executive summary: 

Let's take a look at this. What we have here is an extreme reduction only in male numbers. The lack of a bottleneck in female numbers alone blows the dogma that every human descended exclusively from two people clean out of the water. Anyone even thinking of appealing to this paper to support monogenism is merely demonstrating their complete ignorance of what it actually says.

Furthermore, the term 'bottleneck' is a relative one, as we are not talking about a bottleneck of one male, as the special creationists think. In addition, a genetic bottleneck does not necessarily refer to a reduction of the total population, given that not every person leaves behind children. This point is often overlooked, so needs to be emphasized:
A genetic bottleneck in this case does not necessarily refer to the total number of males but rather the number of breeding males.
In this case, the data is consistent with the possibility that around 4000 to 8000 years ago, only a small number of men had wives, with the vast majority of men remaining without partners and without children. This is completely inconsistent with the belief that every single human descends exclusively from Adam and Eve.

Closely related to this is the vexed question of “Mitochondrial Eve” and “Y Chromosome Adam”, both of which have been misunderstood and misused by special creationists. Wilson Sayres, writing in the Panda’s Thumb blog notes:
One of the misleading aspects of the “Adam and Eve” analogy, is the implication that there were only two humans alive at that time. In the video below I explain what the mtDNA is, how it can be used to trace back to find a common mtDNA ancestor, and why this genetic female was not alone. The same logic applies to the Y chromosome ancestor. Scientists estimate there were approximately 5,000 genetic females and 5,000 genetic males in the ancestral population of anatomically modern humans.

Talking with people and making this video brought up a couple other important points that are difficult to summarize in a sentence, so I’ll expand upon later: 

1. One person (or two people) did not have the ancestral state of all of our DNA. 

The person whose cells housed the common mtDNA ancestor (or Y ancestor) also had all of the other chromosomes (1-22 and X), but did not house the common ancestor of each of these chromosomes. These non-sex chromosomes are a lot more complicated. This touches on why it is also misleading to refer to the common ancestor of genetic “males” versus “females.” Genetic females are not only their mtDNA - we also have 22 non-sex chromosomes, and two X chromosomes! Genetic males are not only their Y (and mtDNA), they also have 22 non-sex chromosomes and one X chromosome! Because the non-sex chromosomes (autosomes) can swap DNA, and are inherited through both the sperm and the egg, they much more complicated history than the Y and mtDNA. 

2. A lower bound, not a point estimate. 

Tracing back to the common ancestral mtDNA or the common ancestral Y chromosome does not tell us when anatomically modern humans arose. We can estimate the TMRCA, or the Time to the Most Recent Common Ancestor, but this mtDNA surely existed much further back in time.
Returning to Wilson Sayre’s first blog post, she includes a second infographic that outlines in more detail the salient points of the research paper on bottlenecks and the Y chromosome data, with the main point being the bottleneck occurring only in the number of males:

Furthermore, it takes the time to differentiate between the effective male population, that is, those who had children, and the actual male population at the time, that is, those without children, with the actual population being larger than the effective population, This makes sense given that even today, not every man has children. What this study shows is that 4-8 thousand years ago, only a relatively small number of men had children, implying that these men had many wives, while most men at that time had none.

That a YEC would even think of appealing to this paper to prove that there was a bottleneck of two people 4-8 thousand years ago shows that they simply don't understand population genetics and do not understand this paper.

In passing, any doubts as to Wilson Sayres’ views on monogenism are laid to rest with this pithy comment in her You Tube video included earlier:
Thanks Heather! The answer is, we didn't evolve from two people!
When the co-author of a paper to which an evolution denialist refers explicitly states both her professional belief that monogenism is false, and in blogging on her paper shows that the bottleneck to which the evolution denialist refers was (1) male-only and (2) considerably larger than one male, it becomes clear that anyone citing this paper either has not read the paper, or has completely failed to understand it. Both options demonstrate that the evolution denialist simply has no idea what they are talking about, and can be safely dismissed as an informed, credible commentator on evolution.