Final answer:
Evidence that mating with close relatives leads to lower fitness offspring can be found in the phenomenon of inbreeding depression, where increased genetic abnormalities and diseases are observed in inbreeding populations, due to the expression of deleterious mutations and reduced genetic diversity.
Step-by-step explanation:
One piece of evidence that mating with close relatives results in lower fitness offspring is the concept of inbreeding depression, which manifest as an increase in abnormalities and disease in inbreeding populations.
Inbreeding increases the likelihood of offspring inheriting rare, deleterious mutations that can lead to harmful phenotypes. Since close relatives share a greater proportion of their genes, there's a higher chance for recessive alleles, which could be harmful when paired, to be expressed. It also reduces genetic diversity within the population, which can lead to reduced ability to adapt to new environmental challenges as potentially beneficial new genes are less likely to be introduced.
Avoiding inbreeding is also practiced by breeders who understand that mating close relatives can inhibit gene flow and dilute the gene pool with desired traits. Thus, mate selection in breeding programs is often strategic to avoid these negative consequences and maintain or enhance the fitness of the offspring.