Surprisingly, minor mutations reduce fitness just as much as those with larger effects, because they rise to higher frequency in a mutation selection balance.

These will reduce fitness by,.

Low amounts of neuroticism may increase a person's fitness through various processes, but too much may reduce fitness by, for example, recurring depressions.

The effect of organisms upon their host can vary from being symbiotic commensals that are beneficial, to pathogens that reduce fitness.

Too much, however, may reduce fitness by producing, for example, recurring depressions.

Such insertions can be very mutagenic and thus reduce drastically individual fitness, so that there is strong selection against elements that are very active.

Meiotic-drive alleles have also been shown strongly to reduce individual fitness, clearly exemplifying the potential conflict between selection at different levels.

This slows growth rates and reduces stamina and fitness.

Under these cramped conditions, most mutations are likely to severely disrupt some aspect of viral function and thereby reduce fitness (Holmes 2003).