This line switched me off too. 'Survival' means genes being present in future generations. If all the females suddenly preferred males with a certain weird bea

This line switched me off too. 'Survival' means genes being present in future generations. If all the females suddenly preferred males with a certain weird beak shape - that beak shape would propagate. The opposing pressure would be how much other aspects of life would reduce the opportunities weird beak males had to breed.