I already know that crossing over causes genetic variation. My question is that since DNA is a stable molecule, why would it undergo process like crossing over during which it becomes so unstable and develop a chance of mutation?
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1$\begingroup$ Cross over often result in mutation, this is true. But of course, not all mutations are caused by cross-over. Note that cross overs do not lead to higher genetic variance (or only as an indirect effect of causing mutations). I am unclear whether your question is "Is there selection for increased mutation rate?" or "Is there selection for increased recombination rate?" $\endgroup$– Remi.bJun 2, 2017 at 14:32
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2$\begingroup$ Of course it leads to higher genetic variation: Without crossover, genes must mutate for the chromosomes used to vary at all. Splitting up previously-coupled genes creates potentially new chromosomes out of existing genes. That is increased variation in the population: it provides sexual reproduction a new pattern of genes to mix things up with. If the permutation isn't new, a specific case might not, but that process certainly increases diversity in the population. $\endgroup$– The NateJun 2, 2017 at 15:46
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$\begingroup$ It exists for the same reason sex exists at all -- for recombination of alleles of genes. $\endgroup$– mgkrebbsJun 2, 2017 at 17:19
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$\begingroup$ @TheNate It really is not the point of the question but recombination actually reduces genetic variance (unless some funny type of epistasis) both 1) by allowing selection to be more effective and 2) by reducing genetic differences between haplotype lineages. The idea that recombination increases genetic variance is a common misconception (hence the two upvotes you received). You can typically look at the work from Barton and and/or Otto. You might also want to have a look at this presentation by S. Otto starting at 5'30'' $\endgroup$– Remi.bJun 2, 2017 at 22:06
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$\begingroup$ @AKV I am voting to close as unclear until you have addressed my comment from yesterday. $\endgroup$– Remi.bJun 3, 2017 at 23:47
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Crossing over is believed to be beneficial for evolution. It does not cause only mutations, it allows for the switching and mixing of genetic information. Thanks to crossing over entire protein families evolved, like the hemoglobins. Indeed it is one of the mechanisms that shaped the chromosomes. Some in-silico simulation proved that, in general, adaptation take place faster if crossing over is applied.
Here some extra references for further readings.
https://www.ncbi.nlm.nih.gov/pubmed/28565671
https://www.ncbi.nlm.nih.gov/pubmed/28564047
https://www.ncbi.nlm.nih.gov/pubmed/28563027
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1$\begingroup$ The evolution of recombination rate is a complex subject (and is typically closely linked to the evolution of sex). You might want to go further in your answer. $\endgroup$– Remi.bJun 2, 2017 at 22:10