This Newsweek article talks about how hunters screw up natural selection by killing the biggest and most attractive members of a species. This allows the weakest animals to continue propagating, effectively creating a weak species.

Scientists like Charles Darwin suggested that the fittest of a species survive to reproduce and pass along their traits to succeeding generations, while the traits of the unfit gradually disappear. New evidence shows that hunters screw up the entire process. Hunters kill individuals with the best horns or antlers or the largest piece of hide rather than the one that is deformed or diseased. Studies are showing that species such as the bighorn sheep, red kangaroo, and even salmon are growing smaller because hunters are killing so many of the larger animals.

The authors give some great evidence as to why this type of 'unnatural selection' is so dangerous:

"Tusks used to make elephants fitter, as a weapon or a tool in foraging—until ivory became a precious commodity and having tusks got you killed. Then tuskless elephants, products of a genetic fluke, became the more consistent breeders and grew from around 2 percent among African elephants to more than 38 percent in one Zambian population, and 98 percent in a South African one." Tusked elephants, like the old dominant males on Ram Mountain, were "genetically 'better' individuals," says Festa-Bianchet. 'When you take them systematically out of the population for several years, you end up leaving essentially a bunch of losers doing the breeding.'"