We have spent considerable time discussing various forms of evidence for evolution. Plus a lot of discussion about mechanisms and looking ofr it taking place currently.
I want to suggest that it is possible to start with a very small set of facts about the real world and arrive at evolution HAVING to happen.
Facts:
1) Our phenotypes (bodyily physical forms ) are very greatly determined by our genotype( the detailed sequences of our DNA)
2) The genotype changes with each individual born (perhaps not for each bacteria or others but almost all). These changes include sexual (and other) mixing up of the genome to produce a new unique one and a variety of mutations which produce completely novel sequences.
3) Not all individuals born will produce offspring and different individuals will produce different numbers of surviving progeny.
4) The reasons for 3 will vary a great deal. Much of it will be random accident. This will apply to almost all individuals equally (that is the nature of "random"). However, some portion of the differences are a result of genetic differences between individuals.
5) At some point in a populations duration there will be changes in environmental conditions that change the "rules" for who will reproduce more successfully and who will reproduce less successfully.
6) The genetic changes
can affect all aspects of the resulting individuals.
7) There are cases where populations will be split or separated by a variety of circumstance (new river, desert, mountain, swamp, lake or what have you)
8) There is a rather continuous range of variation in DNA across ALL species. That is we can make a small (1, 2 3 or so) % change in the DNA of one species and get the DNA of another. From this new one we can find another few % and get to a third. Such changes allow us to connect most (perhaps not all) forms of life with each other.
Reasoning:
Micro-Evolution
Given the first 5 above:
Evolution HAS to happen!
If not all individuals reproduce equally successfully and if the environment allows those with some particular characteristics to have a non-random increase in success then the population MUST change it's make up to be different that it was.
I think that everyone will agree with this. This is adaptation or "micro-evolution".
Macro-Evolution
Now given 6 behaviours that lead to choice of mates or the finding of mates can change or that the nature of the geneome changes can produce individuals that can NOT interbreed Given 7 that allows populations to change INDEPENDANTLY of each other there is nothing to stop the split populations from losing the ability to interbreed. This is the definition of new species. It is also macro-evolution in the biological sense of the word. Many creationists seem to agree that this too is possible although they are rather fuzzy about actually saying so. Some clearly do not agree. For those that don't I would like to see they idea of what stops it.
Full Diversity of Life
Given that we have separated populations that are still undergoing change (and may be split again and again since the mechanisms of 7 keep operating) and given 8 (mostly continuous DNA ) then there is nothing to stop what was one population from diverging into a large number of more and more different populations. The differences will grow large enough to be at the Genus, Family, order and higher levels of taxonomy. There is nothing to stop this and like water will run down hill and follow the low spots and channels life will evolve to fill many different niches producing vary different forms.
This is, of course, what creationists (in the common use of the term) disagree with. But why? What do they have to show that there is an error in fact or reasoning.
This belongs in biological evolution.
™ Version 4.2