There are some errors in what you are exposing here.
The essence of a fish does not change merely because it has flopped on the shore of the beach. A fish, by definition, needs water flowing past its gills to breathe. On land, no water would flow past the gills of the fish, meaning the fish would die. A dead fish is not a species of fish, "dead fish" is merely the closest label to cover the concept "that which looks like a fish, yet is neither animated nor a fish" that we have. It is technically a carcass.
It is not correct to call it the natural process of "adaptation" when what was formerly a fish flopping on land, getting ready to die, sprouts legs and breathes the air. That is what is called "a miracle." Only God, by extraordinary means, can change what was once a fish into what is now some sort of reptile. It is not within the nature of a being to change its essence.
Human beings have souls, just as animals have souls. A soul is the substantial form which informs the primal matter that comprises an animate being. In merely animate beings, it is not possible to have a substantial form without it informing the primal matter, and it is not possible to have the primal matter without it being informed by the substantial form. The two do not exist without one another. However, in human beings, there is a spiritual aspect to our souls: we have an intellect, will and intellective memory. This, by gift of God, allows our souls to continue on in the violent state of separation from our primal matter after death (though we will be united with it again after the General Resurrection).
Animals do not have what is called a merely vegetative soul/substantial form. A tree has a vegetative substantial form, because it can only nourish itself, grow and reproduce. However, an animal has an animate substantial form because it is sentient, though it does also have a vegetative aspect, since like the tree, an animal can nourish itself, grow and reproduce. An animal is a sentient being with a sentient soul. Sentience is what allows the animal to sense its surroundings, and animation is the logical result. Animation allows the animal to move away from what it senses as painful and toward what it senses as pleasant. Plants do not have this ability to move in such a fashion, because they are not sentient.
Animals do not have a capacity for intelligence, as they have no intellect (the intellect is the faculty of the spirit which allows an intelligent being to know what is true and what can be true). They do not have a capacity for choice, as they have no will (the will is the faculty of the spirit which allows an intelligent being to choose what it will make true). Unless the being has the capacity to ponder abstract concepts, it is not intelligent. Animals work off of passion primarily which is ordinarily governed by their nature. Animals have a capacity to learn by what is called "association," however. Due to the fact that sentient souls have a sentient memory, animals can remember associations based upon whatever passion is striking them at the time. Humans train or condition the association of animals by affecting the animals' passions. Animals do not gain any intelligence whatsoever from training or conditioning, as neither give them the capacity for abstract thought. The fact is, if animals can gain intelligence, then they would have to be baptized...since they would technically be human at that point.
What causes problems in observing this, is that there is a specific group of people who choose to live as little more than clever animals. Of course, humans have the capacity for abstract thought and choice, but nonetheless, these people live for the most part irrational lives governed by passion and association. Other human observers then take this as normal human behavior and begin calling animals people, too, since animals have these same traits (that is, being irrational and governed by passion and association).
In answer to your question: the only reason why a person would apostatize (totally repudiate the Christian faith, and embrace atheism) is because there is a sin he loves, and will find whatever reason he can in order to justify it...even if that means rejecting God.