And God created great sea creatures...Gen.1:21
When we wish to zero in on the pertinent aspects of a story but need some incidental information, notwithstanding its importance we will paint the digression with the broadest brush and the barest strokes.
In any case, the book of Genesis is not concerned with scientific inquiry at all. It hardly needs to involve itself in teaching us how to figure out fire, water, air, earth. Our faculties, sharp, sensitive, and adaptable to a great degree, can easily ferret out the mysteries of our physical world without much help. In fact, when the Bible touches on this aspect of experience, it does so with almost a playful poetic touch, a sort of inside joke we nod at to harmonize us, to give us bearing in order to focus on what really matters.
The Darwinian axiom, survival of the fittest, contrasts with the Biblical account of creation in key aspects, and one should recognize them as alternative frames for referencing life. For the purpose of Genesis in particular, and the Bible in general, the Spencerian-Darwinian model slips. It is Enlightenment-age Western in outlook, formulated by a mind shaped by ideals of objectivity, individuality, empiricism, some sense of self-determination (or acute anti-fatalism), and, most importantly, the ever-forward march of Progress. One can be seduced into thinking of life as an endlessly perfecting program going through levels of improvement culminating in us, humans, the as-yet final step on the ladder to heaven. One could be forgiven for thinking that we are to take control of the process from here. (I often wonder what scientific theory for evolution Darwin would have come up with if he was, say, a Gupta or a Wang. I can’t say Cohen because the term Judeo-Christian attests to a cross-cultural influence that makes isolating parent strains difficult)
The fittest survive. Fit for what purpose? Made fit by what factors? The scientific mind is purpose-adapted and the scientist fits out his models for tasks. He thinks of life with this bias and sees it fitting its subjects with tools that make them perform the service of life well. Darwin's conception of a species and evolution is such a purposefully conceived tool. With it we can better categorize organisms in order to make comparative studies, make predictions of behavior, expect certain results to certain treatments, you know, practical stuff.
A species is a definite thing; evolution is a state constantly in flux. There is a subtle indeterminacy about the species that is often missed in common, non-scientific dealing with this topic, an uncertainty relationship between the evolution of a species and the fact of the species. For example, can one draw the line between the last Australopithecus and the first Homo individual?
What does this mean? If the member of a type encounters a condition favorable for its continued existence, it learns to seek the continuation of said conditions. Its success ensures that more of its type come forth, especially more of the type that optimally benefits from this boon. If it encounters unfavorable conditions, it learns to avoid or adapt to these conditions, and its viability will hang on its ability to bring forth the type able to avoid or adapt to these conditions.
On the other hand, the set of conditions for life are constantly changing as soon as acted upon by life and other factors, and is not a static canvas upon which life is painted. A resource favorable to a species might become depleted for the simple fact that the species thrive so well that they exhaust it. Or the fortune is affected by another quite unrelated factor. (An interesting one is where indiscriminate poaching make small tusks a fitness factor for African elephants). A sickly monkey that dies very young and a colony of frisky monkeys that eat from a batch of poisoned crop and dies are equally fit for all practical considerations. A cliff collapses into a river, changing the course of the water or salting it with a greater concentration of a certain mineral, and this could change the water as an ecosystem. To be fair, these incidents cause ridiculously small ripples to apply in this discussion. However, ecological change is such that a very significant event is likely the compounding of a myriad small happenings. The Shabbat hymn, Nishmat, expresses this sentiment poetically, in that we exist as a combination of a billion billion graces of God.
A species, therefore, is an expression of life in a general form stable across time, against the dynamic external conditions that allow for the continued existence of that form. Fitness as a concept necessary for life hardly applies here. Evolution of life and environmental changes are locked in an eternal, endlessly vast, and very intricate mating-snakes' dance.
The point? The biblical account of creation is as valid for its task as Darwin's for scientific needs. What species are in reality they remain, outside our specific frames.
“Survival of the fittest” is appealing to the modern man, in that it dials down a deterministic idea of the universe to the degree that it no longer rankles. It calls forth images of fit bodybuilders and hardworking office people, reinventing themselves into apex creatures of not only their world, but the whole of existence. It suggests, and this man finds empowering, that by man’s understanding the workings of nature, he may pry himself out of nature's grip, as the fittest of all the types life and nature has ever seen, fully in control of the conditions of his (well-)being as well as his evolution.