<u><em>Once a species starts dwindling in numbers, there's a smaller pool of available mates and often a corresponding lack of genetic diversity. This is the reason it's much healthier to marry a complete stranger than your first cousin, since, otherwise, you run the risk of "inbreeding" undesirable genetic traits, like susceptibility to fatal diseases.</em></u>
The element that is most likely to be featured in a work of realistic fiction is "well-defined character suggested by actual experience." This would provide the most realistic character.
