By far the greatest weakness, indeed not merely a weakness but a vast and decisive flaw, in the explanatory power of the theory of natural selection is the inability satisfactorily to explain the evolution of complexity.
This is either something you see, or you don't see. The advantages of complexity are clear enough - complexity enables efficiency but as complexity increases so does the interdependence of parts and the probability that things will go wrong.
People who think for any length of time about natural selection will sooner or later gravitate to this problem, and will realize that the big problem is not the origin of life, or the origin of complexity - but the sustaining of life and of increased complexity in face of the tendency of natural selection top subvert it by enhancement of the short-termism and selfishness of component parts.
Pure natural selection would dismantle complexity as fast as it evolved complexity - to maintain complexity, to maintain the cohesion of complexity - cooperation, organization, interdependence etc; there must be something else at work.
In a nutshell - teleology, purposiveness, cannot be eliminated from biology. It may be hidden, it may be denied - but it is really there.
But what is that something else generates that purposiveness - what kind of a thing is it?
A one level, this is the problem for which group selection is posited as the answer; because that is the most obvious aspect.
The 'group' may be the cell - which is a group of specialized component organelles (mitochondria, centrioles, chloroplasts etc) and other structures such as the cell membrane, vacuoles, endoplasmic reticulum... - each with a degree of autonomy and the potential for 'selfish' exploitation of the cell as an environment. Another group is the group of cells in a multicellular organism which must be made to cooperate - and not simply evolving by natural selection to become cancers and other neoplasm which turn parasite and live-off the organism by exploiting it. And the same applies in relation to individuals compared with the group in the society of social animals - what keeps the individual fulfilling its cooperative function, and maintains the coordination of individuals, instead of becoming a free rider.
The abstract reality of group selection is easy enough to accept -but where and what is this thing which has a purpose and imposes coordination? If it is just a part of the lower level entity, then the problem is not solved - somehow, that which imposes group selection must be situated outwith that which is group selected...
Most biologists who have thought through to this point have reached to physics or mathematics to explain the elaboration and sustaining of complexity. This idea comes from Aristotle via Thomas Aquinas.
For example, Rupert Sheldrake explains it in terms of fields - morphic resonance fields, which organize matter into certain forms (which can themselves evolve) in a fashion analogous to magnetic fields. This could be regarded as a geometrical theory of complexity. Or there are those who focus on archetypal forms, into which biology will spontaneously fall. D'Arcy Thomson seemed to be of this school, and CH Waddington was the most powerful advocate of the twentieth century. More recently, the mathematics of chaos have been used to generate these explanations - as in the work of Stuart Kauffman or Brian Goodwin.
Another (much less formal, more philosophical) version of this is the Life Force - which drives biology upward, towards higher forms, perhaps towards greater consciousness of itself - this had currency from various literary authors such as Bernard Shaw - who wrote popular plays about it, Teilhard de Chardin; and there were similar ideas in mainstream Germanic philosophy.
My point is that there is one class of physical science explanations of complexity, which have as the bottom line either a geometrical concept of forms, or else some equations.
These teleological explanations are regarded as fringe science by mainstream biologists - and are in practice completely ignored because they cannot be integrated with natural selection theory.
However, they do have some status among non-biologists, and as popular scientists - for example, Waddington was the King-maker of mid-twentieth century biology; and Kauffman used to edit the prestigious Journal of Theoretical Biology.
But the other type of teleological explanation is much older and currently has zero/ negative status as being not merely not-biology but regarded as pseudo-scientific nonsense: that is to regard the agents sustaining of higher complexity as personifications - in other words as gods and angels.
By this kind of explanation, there is some kind of very large and functionally-specialized hierarchy of spiritual beings that are responsible for the maintaining the fragile complexity of biological phenomena.
Yet, such explanations are, in principle, more biological than those dependent on physics and mathematics. In essence, the personified model of complex organization is explicitly cognitive and purposive: complexity increases because that is what the ruling entities want.
So, whereas the teleology of fields and forces is an 'as if' phenomenon (the idea being that things just gravitate towards already existent forms or evolve in-line-with fundamental law-like equations). In other words, fields and forces are abstract, and carry no transcendental imperative - they 'just are' and whether you or I fit in with them or not makes no difference.
Fields and forces are nothing to do with the meaning or purpose of your life or my life; the concept is that they just happen to be the way that things are - overall and in the long term.
But if the evolution of complexity really is driven by entities (such as gods or angels) - and if, further, these entities have some kind of personal relationship with us - then there is a possible basis for using the word 'ought' in relation to evolution. It is no just something that 'happens'; instead evolution is something that is being done by personal agency - and is (to some significant extent) being done to us, and/ or on our behalf.
If we take Owen Barfield's description of the evolution of human consciousness - from an Original Participation when we were immersed in nature, via a detached and objective consciousness of the Self - when we perceive our-selves as separate from nature, and aiming at a Final Participation in which we are in a relationship with nature - then we can see a possible parallel, and a possible implication that we will at some point return to a personalized conceptualization of forces acting within biology.
On this basis, I would predict that there will be a return of the gods and angels to displace physics-mathematical models as an explanation for the teleological aspects of biology.
There may be scientific advantages to this - there certainly appear to be disadvantages, at least from the mainstream biological perspective!
But, in the end, it is a question of reality. Are there, or are there not, personal entities at work in such matters as the evolution of complexity or consciousness?
If we decide (on whatever grounds) that there are entities at work in driving evolution in particular directions and sustaining certain forms; then that is how we ought to understand such matters - and we should think this way whether it is, or even if it is not, biologically-fruitful to do so.