different success in reproduction by different phenotypes resulting from interactions with the environment.
In other words, the environment naturally selects those who are more adapted to the environent, and these species will pass on their genes while those not adapted well enough will not.
The environemtn will naturally select, not the species...dont get this mixed up. Species are out there, "looking to get better" it happens naturally.
If the environment changes to where those with long necks will survive, the species will naturally go that route. cold, warm, skinny, heavy, big beaks, small claws; it doesnt matter.