Antagonistic pleiotropy

Pleiotropy occurs when a single gene influences multiple phenotypic traits. Consequently, a new mutation in the gene will have an effect on all traits simultaneously. This can become a problem when selection on one trait favours one specific mutant, while the selection at the other trait favours another mutant.
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The effects of a gene which are beneficial early in life (i.e., increasing fitness) but deleterious later in life (no change in fitness after the reproductive age). Such genes will be maintained by selection, because by the time the gene exerts its damage, its bearers will already have had more offspring than other individuals.