<em>It makes the species more susceptible to diseases.</em>
Explanation:
High genetic variations are crucial for any species to survive in a changing environment. Scientifically speaking, these variations help the species to evolve against changing environmental conditions. Thus, if the genetic variations (or genetic pool) is reduced due to any reason (either man-made or natural), it will result in diseases and possibly their elimination from nature.
For example, first-cousin marriages are biologically not recommended because the genetic pool will reduce with the time (i.e. members of the same family have a similar genetic pool). Thus, the new recombinations during meiosis are not sufficient to make future generations more genetically versatile. Therefore, their offsprings will have more genetic disorders and they will be less fit compared to the other individuals whose parents belong to different families.
