Comment Re:Wasn't there a book about this? (Score 1) 138
The conditions species live in aren't constant. Advantages of A and/or B fluctuate over time. If an animal has A, and the environment suddenly favors B, those closer to B win. For a while some animals will have both.
However, every feature comes at an energy cost, so animals quickly let what they don't need atrophy. If in the current environment B beats out A+B minus extra energy to generate both, then they will settle at B only.
At any rate, every organism is a mixture of thousands of features, from A0 to Z99999, many of which get added and deleted all the time, so your whole argument is bogus to begin with.