Unnatural Selection

 is a gameplay achievement/trophy in Red Dead Redemption

Description
"Kill one of every animal species in the game in any game mode."

List of Required Species
To unlock this achievement/trophy, the player must kill each of the following animals at least once:
 * 1) Armadillo - Nuevo Paraiso and the Cholla Springs, Gaptooth Ridge, and Río Bravo areas of New Austin
 * 2) Bear - The Tall Trees area of West Elizabeth
 * 3) Beaver - The Tall Trees area of West Elizabeth
 * 4) Bighorn - The mountains of New Austin and Nuevo Paraiso and the Tall Trees area of West Elizabeth
 * 5) Boar - North of MacFarlane's Ranch, the marsh near Thieves' Landing, and the Tall Trees area
 * 6) Bobcat - Nuevo Paraiso and the Tall Trees area of West Elizabeth
 * 7) Buffalo
 * 8) Cougar - The mountains and hills of New Austin and Nuevo Paraiso
 * 9) Coyote - New Austin and Nuevo Paraiso (listen for their whiny howls)
 * 10) Crow - All three areas (listen for their caws)
 * 11) Deer - All three areas
 * 12) Duck
 * 13) Eagle - The Tall Trees and Nekoti Rock areas of West Elizabeth as well as in Mexico.
 * 14) Elk - The Tall Trees area of West Elizabeth
 * 15) Fox - The Riley's Charge, Rio del Lobo, and Del Lobo Rock areas of New Austin and the Tall Trees area of West Elizabeth
 * 16) Hawk - New Austin and Nuevo Paraiso
 * 17) Horse - Wild
 * 18) Horse - Domesticated
 * 19) Owl - All three areas (only at night)
 * 20) Rabbit - All three areas
 * 21) Raccoon - All three areas
 * 22) Snake - All three areas (listen for snake hissing or rattling while riding and check around on the ground)
 * 23) Seagull - The docks in Blackwater (Sometimes not needed to unlock achievement)
 * 24) Skunk - All three areas
 * 25) Songbird - All three areas
 * 26) Vulture - All three areas (usually near death, i.e. completed gang hideouts or signs of trouble in missions)
 * 27) Wolf - All three areas (in abundance in Nuevo Paraiso and the Tall Trees area)

List of Unnecessary Species
For reference, and to address any questions about animals missing from the above list, the following species are not required for this achievement/trophy. For tips on hunting or locating these animals, please refer to the individual animal pages. Note that since this is a trophy/achievement for the main game, new animals from Undead Nightmare are not included.
 * 1) Bat
 * 2) Bull
 * 3) Chicken
 * 4) Cow
 * 5) Dog
 * 6) Goat
 * 7) Mule
 * 8) Jackalope
 * 9) Sheep (domestic, not Bighorn as listed above)
 * 10) Pig

Tips

 * Animal kills do not cross over from Single Player to Multiplayer. The player must kill the full list of animals in one or the other, and not a combination, in order to acquire this achievement/trophy.
 * It is not necessary to skin the animals.
 * Track what kind of animals have been killed in the "stats->misc" menu. Animals that haven't yet been killed won't be listed at all (there are no "zeros" displayed).
 * Though the trophy is worded as "every animal species" in reality several are not included (see the "List of Unnecessary Species" above).
 * The animals must be shot, not run over with a horse/wagon..

Trivia

 * The name of the achievement is a play on Charles 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 meat cleaver and a pair of small meat hooks used when butchering. This is a reference to the equipment used in dressing animals, such as in meat factories.