Final answer:
Inbreeding, the positive assortative mating for relatedness, leads to inbreeding depression by increasing the occurrence of harmful genetic traits within a population.
Step-by-step explanation:
Positive assortative mating for relatedness, when individuals prefer to mate with partners who are genetically similar to themselves, is also known as inbreeding. This practice can lead to inbreeding depression, which is an increase in abnormalities and diseases in inbred populations. Inbreeding depression occurs because the mating of closely related individuals can bring together rare, deleterious mutations that lead to harmful phenotypes. This is different from Hardy-Weinberg equilibrium, which assumes random mating in a population, and from outcrossing, which is mating with unrelated individuals. Inbreeding can significantly reduce a population's genetic diversity and evolutionary fitness by increasing the proportion of homozygous individuals, thereby potentially reducing the population's ability to adapt to environmental changes.