At random, whether or not the change would be useful✓Bonne réponse
Only in organisms that are already well suited to their surroundings
Whenever an organism repeatedly uses one part of its body
Explication
Copying errors and damage to DNA arise by chance, so a change appears without any regard to whether the organism could make use of it. If need produced mutations, a population under pressure would reliably develop the feature it lacks, and instead most such populations simply decline.