Here are the FACTS:
Organisms are related to each other to a greater or lesser degree (this is obvious, a zebra is more closely related to a horse then to a dandylion).
Organisms demonstrate that they have common descent (morphology, genetics, fossil record).
Allele frequencies in populations can and do occur in response to envrinmental changes (the famous peppered moths).
Organisms reproduce, and have more offspring then can possible survive. THe offspring had inherited genes from their parents, so that they are very similar but also unique. The expression of some of these genes may improve certain individuals' chances of survival given current conditions.
Mutations can and do occur that are beneficial, or can even endow an organism with something totally new (like the bacteria that was observed to have developed an enzyme capable of digesting nylon due to a missing base pair in a DNA sequence for an existing gene).
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