Sex may have evolved to cut down on genetic variation.

No, I don’t mean in the red light district of Amsterdam or at Mustang Ranch. What I am talking about is the high biological cost of sex. In fact, it is so expensive it can be hard to imagine how it ever evolved in the first place.

The main reason sex is so costly is it takes two parents to have a kid. Asexual creatures can do it on their own.

This doesn’t sound like much of an advantage, but it is. Some computer simulations show that a single asexual individual can overtake a population of one million sexual creatures in about 50 generations. That is about 1000 years for people and only 8 years for mice.

So sex needs to have some pretty big advantages to have ever evolved in the first place. Otherwise the first sexual creature would have been quickly swamped out by all of its asexual brethren as soon as it appeared.

In the past, scientists have pointed to variation as one of sex’s big advantage. This probably isn’t the whole story though. Or even most of it.

Sex does create additional variety through the mixing of genes but it probably isn’t enough to explain the rise of sex. You’d have to live in some pretty chaotic times for this variation to offer enough an advantage to an individual to overcome its cost. Eight or a thousand years just isn’t that long in an evolutionary time scale.

A new review out is bringing up an old idea that Muller came up with back in 1932. The main advantage of sex is to provide a safe time to recombine our DNA.

Recombination is simply the swapping of DNA between two identical (or nearly identical) pieces of DNA. For us that means swapping between the chromosomes we got from mom and dad. So DNA is swapped between our two chromosome 1’s our two chromosome 2’s and so on.

This is where part of that variation we were talking about earlier comes from. But more importantly, recombination actually helps repair DNA damage. You can see the effects of no recombination by looking at our sad little Y chromosome which is slowly disappearing because it has no one to recombine with except itself.

But recombination is a double edged sword. Cells need it to repair their DNA but it can cause lots of DNA damage if it isn’t controlled. For example, even with all of our controls in place, 1 in 1000 humans still ends up with one chromosome stuck to another.

You can see what happens with uncontrolled recombination by looking at cancer cells. These cells end up with extra chromosomes, chromosomes stuck together, and lots of other chromosomal problems because they recombine willy-nilly. They do well for themselves but are definitely bad for the individual.

So it makes sense to contain recombination to some easily controlled time. Sex may have arisen and took over the world because it provides a safer way to keep harmful DNA damage in check. The variation that everyone goes on about may simply have been a beneficial side effect.

After just nine generations, the sexual beasts on the left are already being swamped out by the asexual ones on the right.

See the following for more information:

Muller’s Ratchet
Extra chromosomes and new species

Are they related to me? I still don't know…

In my last blog post, I showed how the two most powerful ancestry tests, mitochondrial DNA (mtDNA) and Y chromosome, were useless to me in my hunt. Now I want look at the rest of my DNA.  So here we go!

The Y chromosome and mtDNA are a small fraction of my DNA—something like 0.8% of the total DNA in one of my cells.  But they are incredibly useful because they change very little from generation to generation.  The mtDNA I got from my mom is probably exactly like hers.  Same with most of the Y I got from my dad.

The other 99.2% of my DNA is a lot trickier to look at from an ancestry perspective because it has changed a lot from generation to generation over time.  For example, the chromosomes I inherited from my parents are not the same as the ones they have.  I got a mix of their chromosomes

For example, my mom had two copies of chromosome 1 (and two copies of her other 22 chromosomes too).   As you know, she passed one chromosome 1 to me (my dad gave me my other one).  But, through a process called recombination, her two copies of chromosome 1 swapped DNA so that I got a hybrid of her two copies.  I inherited a unique chromosome never before seen.

This is all well and good from a survival of the species point of view, but it is a problem for ancestry testing.  Imagine that instead of my mom, we look at my Cherokee great-great grandmother.  She has just had a child who inherited a mix of her chromosome 1’s.  This chromosome will look Native American and the child would appear half Native American.

Actually, the test isn’t perfect yet and so there isn’t yet a “Native American” set per se.  Instead, here is how 23andMe describes Native American DNA in their tests:

“…people who identify themselves as Native American exhibit fairly consistent Ancestry Painting proportions of about 75% Asian and 25% European, plus or minus 10%.”

This means the chromosomes the child got from his or her mom won’t look Native American but instead will look 75% Asian and 25% European.  (See a realted post of mine elsewhere for why it looks like this.) Now imagine that this half Native American child grows up and has my grandfather as his or her son.

My grandpa will inherit a mix of his parents’ DNA too.  In this case the Native American DNA will mix with the European DNA to create a hybrid.  On average, you would now see something along the lines of 37.5% Asian (this is a simplification but it gets us into the ballpark of the number we might expect).

Each generation would see, on average, a continued dilution of this Asian part.  My dad would have 18% Asian, I would have 9%, etc.  Here are my ancestry results (click the image to enlarge):

Not a hint of Asian.  Looks like my great-great grandma wasn't Cherokee.  Or was she?

There are lots of ways she could still be Cherokee.  First off, I don’t know how solid the 75% number is for all Native Americans.  I don’t know how many Native Americans are in their database.  I also don’t know how much variation there will be tribe to tribe.

Secondly, you may have noticed that I was very careful to always say, “on average.”  This is because the recombinations don’t have to be a 50-50 swap.  It is true that if you look at a large number of recombination events, the average will be 50%.  But individual recombination events can be biased towards one or more chromosomes.  Occasionally you’ll get mostly one chromosome and sometimes mostly the other.

Sort of like flipping a coin—do it enough and you’ll get pretty close to half heads and half tails.  But if you flip a coin twice, you might get one head and one tail.  And you might not.  Half the time you’ll get two heads or two tails.

This is less a problem than you might think with our chromosomes since the recombination is spread over 23 pairs with each pair being independent of the others.  But it can still throw a monkey wrench into the works.  23andMe actually has a nice chart that hints at this by giving the most likely range of possibilities.  Unfortunately, this chart didn’t come up with my results and I had to stumble on it while I was playing around.

Using the chart, I can see that the bottom end of my expected results in 0.24% “Native American” (if I am reading the chart correctly).  That is pretty low and it seems like a pretty minor mistaken assumption at the beginning might knock this down to zero.

So where am I after this?  Still in the dark.  This is actually how many genetic tests end up.

The positive result tells you a lot.  Had there been Native American DNA, that would have been a slam dunk.  (This isn’t always the case with genetic tests but it would be here.)  But there wasn’t.  Which means, given that I was on the edge of detection, that she may or may not have been Cherokee.

Now, this isn’t 23andMe’s fault.  The test itself couldn’t be conclusive given how far back we need to go and the DNA tests that 23andMe offers.  In fact, 23andMe does an excellent job of presenting the data.  There are pretty chromosome paintings, graphs superimposed on world maps, etc.  All very nice.

I am still worried that the explanations that go along with these images assume an awful lot of knowledge that most people might not have.  Without that knowledge, it can be hard to assess the significance of a certain result.  Next blog that’ll become even more important as I tackle health conditions.

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