Natural vs Artificial Selection
In evolution, something has to peruse the current population, and decide which tiny fraction of them will live and go on to spawn the next generation. Anything that makes an animal unable to breed e.g. sterilisation, an injury, something that makes it repulsive to the opposite sex is for all practical purposes, the same as death. It takes an animal out of the gene pool.
This something can be a human breeder who makes his decisions based on beauty, meat produced per kg of feed, cuteness… This is called artificial selection.
You could let your cat make the decisions. It swats at the fish in and aquarium and eats the ones it kills. The ones too fast for it, are its fit selections. This is called natural selection. This will select for faster, more agile fish.
You could let nature in general make the decisions. You put your fish outside in a big pond. Various predators will kill some. Sunlight will kill some. Freezing will kill some. Starvation will kill some. Diseases will kill some. Nature has a quite multifaceted notion of what is fit. This too is called natural selection.
What’s the difference? It is just a matter of who is doing the selecting/killing. The pressure on the fish to conform to the arbitrary selection criteria is the same.
Professional fish breeders can get spectacular results in just a very short time, but that is because they cheat. They will do things like breed daughter to father, then granddaughter to grandfather etc. to freeze in a trait. They will also spawn an entire generation from just one individual. They are fanatically focussed on one trait, so they can do it quickly. Nature is more interested in general fitness, optimising all manner of traits simultaneously.
Since both natural and artificial selection work by the same underlying mechanism, if you accept one, you have to accept the other.~ Roedy (1948-02-04 age:70)