And God blessed them, saying, "Be fruitful and multiply and fill the waters in the seas, and let birds multiply on the earth." And there was evening and there was morning, the fifth day.
These verses are very interesting. The word that is translated "Be fruitful" could be translated as bear fruit. The next word is "multiply" and that word is translated well, it means to become great or increase in number. So, the fruitful, if only thought of as fruitful, as in having babies and laying eggs, seems to double up what God is commanding, unless you go with another possible translation of the word.
The other translation? branch out.
Take this into the context of the idea of "kind" mentioned in the previous verses. Here is the Strong's definition obtained from the Studylight site.
kind, sometimes a species (usually of animals) ++++ Groups of living organisms belong in the same created "kind" if they have descended from the same ancestral gene pool. This does not preclude new species because this represents a partitioning of the original gene pool. Information is lost or conserved not gained. A new species could arise when a population is isolated and inbreeding occurs. By this definition a new species is not a new "kind" but a further partitioning of an existing "kind".Can you see how these two ideas fit together? God created the "kinds" and then commands them to branch out. This is how we have the great "Showcase for Creation" found in places like the Galapagos Islands. Click here to read an article from the Institute for Creation Research that discusses this exact thing.