Monogamy

As well as humans, many birds, fishes and mammals are broadly monogamous. ( examples here )

However, monogamy seems to contradict traditional evolutionary theory because it appears to severely limit the gene-pool available to breeding animals.

There are several theories to explain how it might have evolved. (See Wikipedia) but all have obvious inconsistencies, and none has so far emerged as a definitive explanation.

The evolution of monogamy and paternal care in humans is often argued to have resulted from the needs of our expensive offspring. Recent research challenges this claim, however, contending that promiscuous male competitors and the risk of cuckoldry limit the scope for the evolution of male investment. So how did monogamy first evolve?"

Source : The evolution of monogamy in response to partner scarcity Nature, Scientific Reports, 2016.