I submit a proof for evolution, by which I mean the fact of and explanation for mutability of species.
We will proceed by observation.
1. Life forms have offspring.
2. When those offspring are the result of sexual reproduction, they vary amongst themselves and from their parents in some respects.
3. More offspring are germinated/spawned/hatched/born than survive to reproductive maturity.
4. Variations exhibited by offspring are in some respects heritable.
5. Some heritable variations will make a certain individual offspring marginally more likely to breed successfully.
6. Heritable variation is passed between generations by means of the deoxyribose nucleic acid molecules known as chromosomes.
The first five observations, which are not reasonably refutable, lead one inevitably to the conclusion commonly known as "the survival of the fittest", though note that it is breeding success rather than actual survival which is enjoyed by the fittest; barren survivors don't come into the calculation.
When observation 6 and our detailed understanding of genetic heritability is added, it becomes perfectly _inevitable_ that a breeding population will change its heritable characteristics (i.e. EVOLVE) to fit its environment.
When populations are divided, observation 2 means that subsequent changes cause the two populations to diverge in their heritable characteristics, particularly if the populations are subjected to different environmental challenges or opportunities.
Sufficient genetic divergence then results in the appearance of different species, by which we mean a population with sufficiently different characteristics that a good taxonomist *says* they're separate species, or perhaps (given point 6) that chromosomal differences make interbred offspring non-viable or infertile. Q.E.D.
I genuinely would like to know in what ways a creationist might argue against the above, if by creationism we mean immutability of all species created by $DEITY. If creationism is reduced only to special pleading for Homo sapiens, as being created in God's image, perhaps, then the debate is somewhat altered.