Q. In your recent work, you've examined changes in genetic material - for the layperson, what does that mean?
A. We took samples of modern human DNA and examined the mutations that occurred over evolutionary history, and compared them across populations from around the world.
What are some of the examples of the evolutionary changes that have occurred to humans in modern times?
One example of natural selection that’s happened in modern humans is the ability for adults to digest lactose. Originally, lactose intolerance was diagnosed as a genetic disease, because it was first studied in Eastern European populations where most people evolved an allele, which is a variant of a gene, that allows them to digest lactose as an adult.
But it turns out that when we study the whole world, we find that most people can’t digest milk as adults. Ancestrally, there was a mutation that occurred in some European population that spread through Europe and independent African populations that allowed people to digest milk as adults and it spread through the population and that mutation got inherited with high frequency in both males and females.
In your recent research, you suggest that something other than natural selection has caused evolutionary changes. What was that?
So certain kinds of natural selections [such as the ability of adults to digest milk] are still happening. But we've found that, in addition, cultural changes may also be influencing our evolution.
It turns out that, that an accumulation of material goods may have allowed some males to more successfully produce offspring, their offspring may have in turn inherited their material wealth, as well as their genetic predispositions, and passed those on to modern day.
So for example, being able to digest milk as an adult is a beneficial advantage - it allows you to absorb more nutrients - [so is developing an immune response] to some disease. However our findings are a little surprising because there aren't many examples of cultural [values] affecting our DNA.
When did these changes begin to occur in our ancestry?
So there were two things. One, we know that modern humans originated in Africa. A subset, a small group, migrated out of Africa and started to populate the rest of the world, and that small group is a bottleneck, a small neck of genetic males and females. We observed that bottleneck about 50,000 years ago.
The surprising thing is that about 4,000 to 8,000 years ago we can observe a second bottleneck, a second reduction in the number of people who are passing on their DNA, but only in the genetic male lineage. Now, the whole world didn't undergo this reduction at the same time, but this is fascinating because, on average, [it all began] a few thousand years after agriculture spread across the globe.
So is survival of the wealthiest more important than "survival of the fittest"?
I’ll say that two things. First, we observed these changes slightly after the emergence of agriculture, so it was some time before people settled down and actually started to accumulate wealth. But, the second point to understand is that the public thinks about natural selection as "survival of the fittest," but the better way of thinking about natural selection is "survival of the good enough." We have huge populations and those that are very fit will pass on their DNA. But lots of other people who are perfectly reasonable will pass on their DNA as well. So we think of it as survival of the good enough—you were good enough to find a mate and pass on your DNA.
So we have to be very careful when we’re talking about this because what we’re saying is that we observed a significant reduction in the number of males who passed on their DNA and that DNA survived until today. And it was only in the male lineage, not the female lineage. [But this all] coincides very nicely with the onset of agriculture and alongside the accumulation of wealth at different times in different populations.
It's a challenge to say that it was more than all natural selection is [difficult] because we could only study DNA that was passed through the genetic male lineage, the Y chromosome and the genetic female lineage, the Mitochondrial DNA. But yeah it looks that something was really going on...and best as we can tell, the accumulation of wealth and power may have been a [contributor].
So is the next step getting more DNA samples?
Yes, part of the next step is getting more samples and maybe looking at some of the places [where we found samples] that may have been burial sites and so we could get ancient DNA from more people who lived at that time.
However, another avenue is to work more with anthropoligsts who are very familiar with the global pattern of wealth distribution and agriculture. The third part is we've only looked at the six [sex] biased regions of the genome—but there are all the non-sex chromosomes, the autosomes, and you inherit one copy from your genetic dad and one copy from your genetic mom. These pieces hold additional information that we can analyze as well.
Melissa Wilson Sayres
Melissa Wilson Sayres is a professor of computational biology at Arizona State University. Her main research focuses on sex-biased biology where she studies the evolution of sex chromosomes (X and Y in mammals), why mutation rates differ between males and females, and how changes in population history affect the sex chromosomes differently than the non-sex chromosomes. Generally she studies mammals, but is also curious about the sex-biased biology of flies, worms and plants.
Wilson Sayres is also active in public science engagement and outreach. She writes for the evolution blog, pandasthumb.org, routinely teaches in K-12 classrooms, and regularly engages the public in discussions about the difference between sex and gender, the importance (or not) of genetic inheritance, and understanding evolution.