Within metaphysics there is the Necessitarian Theory which puts forward the idea that the “Laws of Nature are the principles which govern the natural phenomena of the world”. But this was more of a nomic debate between must and is , and a questioning of free will which is certainly not where we want to go. Necessity is also implied, if not stated, in much of the debate on final causes and purposiveness in biology, and, again, championed by theologians, but that is not where we want to go either.
A surprising upholder of final causes and purposiveness as a driving force in biological design was Kant. For him, mechanistic materialism was not enough to explain biology. Nature needs more than just an understanding of its laws to explain it.
But to avoid the philosophical pitfall of tripping over into theology when unravelling metaphysical ideas, perhaps the best way to tackle this would be through an examination of the information used by systems – mechanical or biological – in order to drive themselves. Traditionally we have called much of this information laws: But how do these laws happen? Why must systems act in a certain way? Doesn’t this obligation imply necessity?
If atoms in water must solidify at 0ºC, and must agitate and become gaseous at 100ºC there is a certain force of necessity involved. Once we have established that all natural-laws contain the element of necessity, we see that necessity is everywhere, running throughout the very fabric of the entire universe, on the cosmic and the sub-atomic levels. Things happen because they must; because necessity demands it.
Science thinks it understands phenomena when it has been able to understand its laws, or, in our terminology, when it has understood its necessity – although knowing that something happens necessarily is not the same as knowing why it happens. It is in this field of trying to understand why, that those who ask the question are drawn into the problematic area of teleology and final causes.
Or perhaps not … The answer to the question of why organised systems come about may have nothing to do with final causes – the finality in each law may be just the singularity of each separate law. Initially, the necessary function of each law may simply be that it works. Nevertheless, once a system becomes complex, it needs the individual elements that make it up to all work in a necessary way.
Could we say that physical laws, and therefore needs, evolve as the system evolves? Structurally there is really nothing static in the Universe, everything is changing or inter-changing. Everything is dynamic, evolving into systems that seem in macro-cosmological terms to be stable.
It is this dynamism that eventually produces, in at least one tiny speck of the Universe, conditions for life. With the emergence of organisms, complexity takes on a whole new form. Really there are three stages of complexity in the Universe: (i) the mechanical stage of organising matter; (ii) the organic stage of evolving life forms; and (iii) the perceptive and idealistic stages in the evolving of minds.
In each stage there are laws or needs which deal with: a) lack; b) the problems of the maintenance and preservation of the system; and c) the needs for adapting, changing and progressing through creativity. The interesting thing here is that at all three stages the systems are liable to fall into internecine relationships with themselves. There seems to be an effort to regulate itself and find equilibriums and conditions which are self-regulatory, but the overall rule seems to be that this is impossible, at least in an absolute sense. While systems seem to strive for regularity and permanence, any absolute permanence seems to be impossible. All systems must eventually collapse, even though this seems to be the opposite of the intentions of the needs.
But, if the intentions are real, then the Universe is imbued with purpose; with a struggle; with the need to overcome its own internecine tendencies and evolve into regularity and permanence or at least to keep moving in that direction through the progressive evolutionary forces of adapting, changing and creating.
Looked at in this way, the final cause is in the process and is embedded in progress and becoming geared towards permanence.
 See Stanley N. Salthe, DEVELOPMENT AND EVOLUTION; COMPLEXITY AND CHANGE IN BIOLOGY, p. 270