Lumie your claim that mudskippers will only give birth to mudskippers is a strawmanning of evolution. Populations evolve not individuals. Do you think a Latin speaking mother ever gave birth to a Spanish speaking child?
Yet we know Spanish as a language evolved from Latin.
In the same way populations gradually evolve over periods of time into other morphological forms.
Lots of examples of this. If protestants came from Catholics, why are there still Catholics? If Americans came from Europe why are there still Europeans, etc.
Your ignorance of how evolution actually works isn't an argument against evolutionary theory. It is, however, a strong argument for staying in school.