Today’s concept of the day is assortative mating.

Assortative mating (also called assortative pairing) takes place when sexually reproducing organisms tend to mate with individuals that are like themselves in some respect (positive assortative mating) or dissimilar (negative assortative mating). In evolution, these two types of assortative mating have the effect of reducing and expanding the range of variation, respectively, when the assorting is cued on heritable traits.

Wikipedia

The more you know.