The arguments for evolution seem to take two or three stereotypical forms:
1. The Default Argument (a la Dawkins):
i) The spontaneous origin/evolution of life is wildly improbable.
ii) However, the existence of God is even more improbable since God would have to be more complex than anything he designed.
iii) Hence, naturalistic evolution still comes out on top by process of elimination.
iv) Given (i)-(iii), even if we had no physical evidence for evolution, the theory of evolution would still be warranted. Indeed, there is no justifiable alternative.
v) Given (iv), we don’t need any physical evidence for a stepwise, evolutionary pathway. It is sufficient to postulate a hypothetical pathway.
vi) Given (iv), we don’t even need to spell out a stepwise, working model. It is sufficient to postulate that B co-opted A on the way to C.
vii) Given (iv), we can substitute computer simulations for physical evidence.
2. The Metascientific Argument (a la Lewontin)
i) By definition, the scientific method is predicated on the uniformity of nature.
ii) Creationism, whether in the form of special creation or theistic evolution, would violate the uniformity of nature, and thereby undermine the scientific method.
iii) Given (i)-(ii), the only scientific explanation for the origin and/or development of life will be a naturalistic explanation.
iv) Hence, the scientific presumption will invariably favor naturalistic evolution, even if we had no physical evidence or feasible theories for naturalistic evolution.
3. The Cumulative Argument (a la Ernest Mayr)
i) The best evidence for evolution is the fossil record.
ii) Unfortunately, the fossil evidence for evolution is underdetermined by the fossil record because the fossil record is incomplete.
iii) However, we can supplement the fossil record with other, admittedly inferior, lines of evidence (e.g. morphology, microbiology, biogeography).