LWT,
"It is not necessary for both partners to share the same mutation for it to get passed on. The advantageous allele only needs to be dominant."
That would mean then that animal A who had the mutation of, let us say, the light-sensitive cell with the added concave feature (some believe may have occurred) would have mated with animal B who had inherited the light-sensitive cell (without the new concave feature) and thus each shared a corresponding gene in the DNA but the allele variant of the new gene possibly being dominant would pass itself down to the offspring. Okay that for now, and for me, makes sense if I'm understanding it correctly.
The question that could be asked of course is: are most mutations dominant? And also, with the complexities of survival in the natural world being what they are it would seem that hundreds of animals with the new variant would have to survive in order to ensure the survival of the concave feature, let us say, which in itself may not have been necessary for survival at that early stage.
I'm not a creationist who believes the Bible's creation story should be understood literally. It was written at a time when discussions such as this one could not be had and so the simplicity. Much like the simple terms a young father and mother may use to answer their small child's questions on how they came to be. Most parents don't get out the biology books and discuss reproduction and genetics, no, simple terms, truthful, but simple terms, suffice until the child is a little older to understand better. I believe the creation story is just that.
I truly appreciate your responses and through them I hope to learn more and move forward toward understanding. You have opened a door and I will walk through it knowing that truth as Jesus taught makes us free. Thank you.