Unnatural Selection

 is a gameplay achievement and trophy in Red Dead Redemption in which the player must "kill one of every animal species in the game in any game mode."

Guide
It is not necessary to skin the animals. Track what kind of animals killed in "stats->misc", just compare this list with the animals in the stats.

List of Species
To unlock this achievement/trophy, the player must kill each of the following animals at least once:
 * 1) Armadillo - You can find these in the Frontier and in Mexico.
 * 2) Bear - You can find these in the Tall Trees area in the North.
 * 3) Beaver - These are in the Tall Trees area in the North.
 * 4) Bighorn - You can find this in the mountains in the Frontier and Mexico, and in the Tall Trees area in the North.
 * 5) Boar - You can find these in Pike's Basin and the Tall Trees area.
 * 6) Bobcat - You can find these in Mexico and the Tall Trees area.
 * 7) Buffalo - As you first enter the Great Plains from Thieves' Landing.
 * 8) Cougar - In the mountains and hills on the Frontier and Mexico.
 * 9) Coyote - On the Frontier and Mexico, listen for their whiny howls.
 * 10) Crow - All three areas, listen for their caws.
 * 11) Deer - All three areas
 * 12) Duck - On the Great Plains near the edges where the water meets the land. And the South East area of Lake Don Julio. Also found around The Old Bacchus Place. They can also be easily found at the end of the last mission.
 * 13) Eagle - Tall Trees and Nikoti Rock areas of the North.
 * 14) Elk - Tall Trees area.
 * 15) Fox - Near Riley's Charge on the Frontier and in the Tall Trees area in the North.
 * 16) Hawk - The Frontier and Mexico.
 * 17) Horse - Both wild and domesticated horses are needed.
 * 18) Owl - All three areas, only at night.
 * 19) Rabbit - All three areas.
 * 20) Raccoon - All three areas.
 * 21) Seagull - Blackwater near the water.
 * 22) Skunk - Around MacFarlane Ranch on the Frontier.
 * 23) Snake - All three maps, listen for snake hissing or rattling while you are riding then check around on the ground.
 * 24) Songbird - Near the towns on all three maps.
 * 25) Vulture - All three areas, usually near death. I.e: Completed gang hideouts.
 * 26) Wolf - All three areas, but in abundance in Mexico and the Tall Trees area.

Trivia

 * The name of the achievement is a parody of Darwin's theory of natural selection. Players must seek out and eliminate animals, intervening in the natural process.
 * While shooting an animal Marston will occasionally shout "I love natural selection!" or "Blame Darwin!"
 * The picture seems to be a meatcleaver and a pair of small meat hooks used when butchering. This is because it is resembling a butchering of the animals, which means a unnatural killing of something or doing it in a horrific way.