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.