Solving the Sex Problem in Evolution
Sex May be More Important for Stability than Change
July 15, 2011
Explaining the evolution of sex has been a problem for a very long time. Most ideas have focused on the advantages to populations. For example, sex causes a genetic mixing which is good for a species' survival in a changing environment.
Unfortunately evolution can't work on populations - it has to have advantages for the individual as well. This is especially true for sex given its high cost to the individual (the existence of males, sexually transmitted diseases, etc.).
A new review out (plus ones here and here) argues that sex is actually around to keep species intact. Sex makes sure that the next generation has the right number of chromosomes, helps fix any DNA damage that has built up during a lifetime, and reprograms cells so they can eventually become stem cells through fertilization. In this argument, the extra genetic diversity is just a lucky side effect.
If true, this is a big deal in the field of evolution. It involves a major shift in focus about how evolution works and what sex is for. And it may help to finally explain why the vast majority of eukaryotes have sex to produce offspring.
The High Cost of Sex
Sex is expensive in terms of competitiveness. This becomes especially clear when scientists run some population simulations.
The advantage of no sex is
obvious after just 2 generations.
Imagine two groups of similar animals. One uses sex to have babies and the other has them without sex. An obvious key difference is that the first group needs two individuals to have babies and the second needs just one. This might not seem like a big advantage for the second group, but it is.
Now imagine there are a million sexually reproducing individuals in a group. Through some random mutation, one individual gains the ability to reproduce asexually. Within 50 generations, the asexual individual will have replaced all of the sexual ones.
None of this is to say that random mutations would easily create an asexual individual. But what it does show is how costly sex is. For something to be so expensive, it needs to have benefits that outweigh these incredibly high costs. Otherwise sex would never have survived and thrived in the first place.
For over a hundred years, scientists have tried to make greater genetic diversity that benefit. To understand how sex leads to genetic diversity, we need to remember that humans and other eukaryotes have two stages of development diploid and haploid.
There are two copies of each chromosome in the diploid stage. For example, humans have 23 pairs of chromosomes when they are diploids (i.e. except when they are sperm and eggs).
So humans have two copies of chromosome 1, two copies of chromosome 2, etc. When sperm and eggs are made, only one chromosome in each pair is included. This is the haploid stage where cells have one copy of each chromosome.
Because the chromosome selection is random, this means there are 8.4 million or so possible combinations for each child of each human couple. And the chances that two children of the same parents would have the exact same DNA are mighty slim about 7 in 5 X 1013 or once every 70 trillion times. But even this isn't all the diversity that sex serves up.
DNA is swapped between chromosomes
to create a new unique chromosome.
Before a sperm or egg is made, chromosomes in the same pair swap DNA in a process called recombination. What this means is that each of these chromosomes is actually a random combination of the ones the organism got from mom and dad. So there really are an infinite variety of possible genetic variants.
So is this genetic variation enough to make up for the cost of sex? Probably not. Especially since asexual creatures have way more variation than scientists previously thought.
Scientists used to think of all members of an asexual species having nearly the same DNA. This has turned out not to be true. When they look at both kinds of beasts, scientists see as much genetic variation between asexual individuals as between sexual ones. Sex doesn't seem to actually give a whole lot more variation compared with no sex.
The variation for asexual beasts probably comes from a higher mutation rate and a less stable genome. And this points to what the real advantage of sex might be. A dampening of destabilizing changes in an individual's DNA.
Sex Has Benefits
What does sex offer the individual? To answer this, we need to step back and ask what happens when eggs and sperm are made.
Eggs and sperm are created through a process called meiosis. In meiosis, chromosomes come together to swap DNA and then one set of chromosomes is moved into each egg or sperm. Lots of things are going on here.
First off, the DNA swapping actually repairs a lot of DNA mistakes. This decreases the number of mutations that happen in each generation.
But this DNA swapping is only safe if the chromosomes in a pair are lined up properly. Otherwise recombination can sometimes happen in the wrong place between similar or identical DNA on the same or different chromosomes. And this can cause all kinds of problems.
Meiosis is a safe time
Cells can end up with chromosomes stuck together, duplicated, or even missing. This usually does not end well for the individual.
For example, cancer cells have lost their ability to control when recombination happens and they are usually a mess when it comes to their chromosomes. They are successful as cells in their environment but are obviously not good for the organism. This is just one example of why recombination needs to be controlled.
But a cell with lots of DNA and many chromosomes still needs recombination to heal DNA damage. In other words, complex creatures need some way to recombine safely. The answer that eukaryotes apparently stumbled upon was meiosis and sex.
In eukaryotes, most recombination happens during meiosis when chromosomes are lined up properly. A key benefit of sex, then, is to prevent a genome from becoming a jumbled mess over time. Meiosis does not eliminate problems, but it does bring them way down.
Another benefit of meiosis has to do with something called epigenetic reprogramming. Complicated beasts like humans have many different kinds of cells that all share the same DNA. Cell types end up different based on which genes are turned on and off.
Cells accomplish this through marking the DNA (and some other things attached to the DNA) with small chemical groups. When a beast is all grown up, it has lots of cells all with differently marked up DNA.
Making sperm and eggs gives cells the opportunity to erase those marks and to start over. Now the cell can restart the process of making a new individual.
What this all points to is sex evolving to keep DNA change from spinning out of control. So instead of increasing genetic diversity, it actually helps keep it in check and manageable.
It is important to keep in mind that at this point, this is just an intriguing idea with some solid facts to back it up. A whole lot more research will need to be done for it to unseat the genetic diversity theory.
Like any good scientific hypothesis, though, this one makes predictions that can be tested. And they will be. Then maybe we'll have a clearer picture about whether sex is here to increase diversity or to maintain our genomes.