UK scientists believe they have discovered why the spread of "good genes" throughout the population does not make everyone good-looking.
Greater DNA variation helps combat bacteria and diseases
If women select the most attractive men, the genes should quickly become commonplace, according to Darwin.
But the new research suggests that genetic mutations routinely affect the DNA repair kit, resulting in greater variations as damage goes unrepaired.
The findings, by Newcastle University, are published in the journal Heredity.
Critics of evolutionary theory claim that widespread physical differences among sexually selecting species such as humans are proof that Darwinism is flawed - the so-called "lek paradox".
However, the new research by Professor Marion Petrie and Dr Gilbert Roberts suggests that sexual selection can in fact cause greater genetic diversity by a mechanism not previously understood.
The scientists claim that since genetic mutations can occur anywhere in the genome, some will affect the DNA repair kit possessed by all cells.
As a result, some individuals have less efficient repair kits, resulting in greater variation in their DNA as damage goes unrepaired.
Professor Petrie said: "We started this research 10 years ago and our model has now produced a good fit with what we observe in terms of genetic variation, which leads us to believe that our theory is correct.
"We find that sexual selection can promote genetic diversity despite expectations to the contrary."
It has long been known that greater variation of DNA in the disease defending regions makes it more likely that an individual can resist attacks by bacteria and viruses.