Melissa Crisp, of Rangi Ruru Girls's School, asks :-

If things on Earth just evolved, why aren't there any mistakes or things that evolved wrong?

Neil Gemmell, an evolutionary geneticist at the Department of Zoology, University of Canterbury, responded

Many mistakes (mutations) occur during evolution, but unless a mutation has strong advantages to the organism's ability to survive and reproduce, it is unlikely to persist for any great length of evolutionary time due to the action of natural selection - evolution could be compared to the Olympics where many may attempt to qualify, but only the best proceed to the games themselves.

Evolution is driven by countless random mistakes (mutations) that occur within the DNA of individual organisms. The vast majority of new mutations will not affect the physical characteristics (phenotype) of that organism. However, those mutations that do result in phenotypic changes are more often than not disadvantageous to the organism's ability to survive and reproduce, which collectively constitute an individual's evolutionary fitness.

Charles Darwin and Alfred Wallace proposed that in the competition for survival, individuals that possess advantageous phenotypic variants would succeed by surviving and reproducing more than others, a concept that has become known as 'survival of the fittest'. However, for our purposes it is probably more useful to view the action of natural selection as a sieve that eliminates mutations (mistakes) which result in phenotypic variants that lower the evolutionary fitness of an organism, while maintaining those that increase evolutionary fitness.

If we view natural selection as a mechanism that eliminates variants that reduce the evolutionary fitness of an organism, it is probably now obvious why we don't see any 'mistakes or things that evolved wrong' that persist over evolutionary time scales. Individuals who do not survive long enough to reproduce or who produce fewer offspring than their competitors will leave few if any descendants and their lineage will consequently be rapidly eliminated from the evolutionary history of life on earth.

The following website may be of interest. http://www.pbs.org/wgbh/evolution/