Answer: The traits of those who died before reproducing deemed "undisireable" (like a tale,scales,wings etc.) would have died out as they would not have had the opportunity to pass on these genes. Those of the same species who survived due to a lack of the "undisireable" trait would have the opportunity to reproduce and pass on the complete lack of the trait itself or a smaller version of the trait. This is what enables different species to adapt to live in different enviorments,you can observe this in different species that share a common ancestor.
Explanation: It's kind of in the answer
