Contrary to the statements of most evolutionary writers, few (if any) creationists have ever advocated the idea of absolute fixity of species. The Biblical unit of biological taxonomy, of course, is the Genesis "kind" (Hebrew, min).

It may be worth mentioning that this fact was stressed in my first book on creationism some 55 years ago:

“Therefore being justified by faith, we have peace with God through our Lord Jesus Christ” (Romans 5:1).

“And the flood was forty days upon the earth; and the waters increased, and bare up the ark, and it was lift up above the earth” (Genesis 7:17).

“And so it is written, The first man Adam was made a living soul; the last Adam was made a quickening spirit” (I Corinthians 15:45).

“Because that Abraham obeyed my voice, and kept my charge, my commandments, my statutes, and my laws” (Genesis 26:5).