Hypergamy refers to the tendency of the woman to only become romantically involved with men who are of higher value than she is. Hypergamy exists in two forms: genetic hypergamy and financial hypergamy.
Women practice genetic hypergamy during their prime years. This means the woman only has sex with men who are of the highest genetic quality she can get; that is, he is tallest and best looking man she can get.
When women practice genetic hypergamy, this often leads to pregnancy. The good looking guy who impregnated the woman leaves her, so the woman now needs a source of income to care for her child, so she resorts to financial hypergamy. In other words, she marries the wealthiest man she can get. She then uses this beta provider's money to care for her child she had from another man.
Typically, prime women practice genetic hypergamy, whereas post-prime single moms practice financial hypergamy.