There are plenty of mutations that cause birth defects, but none that cause "birth improvements."
As Edward E. Max writes:
Does the fact that we know many human detrimental mutations but essentially no clear ones mean that there have been no beneficial mutations in human history? Not at all, since there is a clear bias in what medical scientists have studied. The human mutations we know most about are detrimental because medical scientists preferentially study illnesses that cause significant morbidity and mortality. Consider the theoretical possibility that a beneficial mutation has occurred in a particular human gene; even if this mutation were identified by a comparison of the mutated gene in a child versus the unmutated version of the same gene in both parents, there is no way that this mutation would ever be recognized as beneficial. If the mutation increased intelligence, strength, longevity or specific disease resistance, this would never be apparent without long-term breeding experiments that could obviously never be done on humans. Therefore, since such beneficial mutations in humans could never be recognized in humans, our ignorance of examples cannot be taken as evidence that they don't exist. However, the experiments necessary to demonstrate a beneficial mutation can be done with laboratory organisms that multiply rapidly, and indeed such experiments have shown that rare beneficial mutations can occur. (Max 1999)
The situation is in fact somewhat better than Max describes, as there are known beneficial mutations in humans. For instance, as pointed out here, one mutation in the gene for CCRS causes one to progress slower from HIV to AIDS, and two mutations in the same gene increases one's resistance to infection by HIV in the first place (Cohen 1996:1797).
Cohen J. 1996. Receptor mutations help slow disease progression. Science 273(5283):1797-1798.
Max EE. 1999. The evolution of improved fitness by random mutation plus selection. www.talkorigins.org/faqs/fitness.html.
Last updated: 21 Mar 2008
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