Full random. Something like 50% of human babies have random mutations which make them unable to develop past the stage of being invisible to the naked eye. Such miscarriages seem like normal menstruation.

There's a knack to optimizing the mutation rate. If you mutate too much, 99.9999% of offspring can't develop and in the end there's too little reproduction. If you don't mutate enough, you don't evolve. Evolution has already optimized this hyperparameter.