If I had to pick one thing, I guess it'd be the following line of reasoning.
Do genes determine to an extent the physical characteristics of an organism? yes.
Is the mechanism for producing more copies of genes perfect? No. Perfect copies are not always created, thus there will be physical variation between individuals across a population.
Are all individuals in a group equally likely to survive and pass on their genetic material to offspring? No. Those with charactaristics better suited to their environment are more likely to have more young.
With these three points in mind, I'm not sure how it could be disputed that over time, there will be changes in the genetic makeup of a species and therefore its physical characteristics.
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