What is the ultimate source of, and reason for, teleology in biology?

Teleology (purpose) in biology is based on, requires, that reality be coherent, cooperative and complementary. This, in turn, requires that there is a single and unifying governing entity. In sum, teleology requires deism.

Deism is the assumption that the universe has an organizing principle (or entity) - which may be a personal supreme creator god among other lower gods, or one God, or may be something impersonal - a 'god of the philosophers' much like the way Einstein referred to when he stated that 'God' does no play dice, or the Platonic idea that there is a coherent primary reality to which the reality we observe is only an approximate, partial and corrupt approximation.

But for life, for reality, to have purpose, it must hang-together - and for reality to hang together requires some unified conception of deity.

Biology needs a teleology, and indeed the more specific that teleology the more can be inferred from it. However, if biology is to be a coherent and general science, then that teleology cannot be more specific than what can be agreed-on by deism.

Therefore, scientists can, and indeed must, agree on a general concept of deity - but beyond that there will be disagreement because the attributes of deity, including the nature and specific purposes of deity, are the product of divine revelation (deity revealing itself to Men) - and the reality and content of such revelations has been in chronic dispute for all of recorded history.

Therefore, deism supplies teleology, but only to a limited degree. Therefore we need to distinguish between the implications of:

1. The fact of teleology
This includes the consequences of their being an ultimate unity - and these implications include an expectation of a primary and significant degree of coherence, cooperativeness and complementarity.

2. The specific direction of teleology
There will be disagreement over the specific nature of overall teleology - i.e. disagreement over what exactly is the specific purpose of 'everything' and of specific entities at lower levels.

3. The mechanism/s of teleology
There will be disagreement over the specific ways in which purpose is applied or arises at the lower levels of reality; including disagreement over whether there is a single mechanism or many such mechanisms.

So, all biologists ought to be able to agree that there is an ultimate teleology - and that this is the primary source of cohesion both in the universe generally and among the 'living-things' which are the subject of biology. Beyond that - there will be disagreement concerning the specific direction of teleology and how it operates on specific entities, in specific situations.