Head binding now and the past

[brewer]

Elephants that happen to be tuskless won't be killed by poachers, and elephants with shorter tusks are less likely to be killed by poachers, and thus have a better chance to survive to mate. Rats aren't going to develop shorter tails just because you chop them off, as brewer points out.

There is another higher order effect.   If you keep cutting off the tail of every mouse, that means whatever survival benefit tails confer is lost to every mouse.   But every mouse still expanded energy prior to the amputation to develop the tail.  

Since this investment now garner much less return,  evolution will subtlety favor those mouse whose genes cause them to invest less energy in growing and maintaining a tail that will be cut off any way.

So, yes, even cutting off the tail of every mouse, regardless of whether the tail is long or short, will still subtlety cause the mouse to grow smaller tails over many generations.

The beauty of evolution is its principles are so simple and straight forward, and it’s actions and affects are infinitely complex and multilayered.

How many mice will this 'entity' pursue to eliminate tails from the breeding pool? In a limited/confined environment it may make a difference, but not across the genome. Not having a tails needs to have a larger-higher ecological/biological/reproductive advantage. If it has an advantage to begin with then there wouldn't be tails. No need for chopping.