As the generations go by, we would start to see, on average, an
"improvement" in natural inborn nut cracking ability.
Now, imagine what would happen if some of these animals had teeth/jaws
strong enough to crack the nuts, and a few others had CLAWS sharp/strong
enough to crack the nuts.
There would then be two groups that would survive into the next generation:
The jaw-crackers and the claw-crackers. As the generations went by, these
two groups would become better and better at opening nuts in thier particular
way, and would also become increasingly DIFFERENT from each other as a group.
If they become different enough, they become "incompatible", and only mate
with each other.
Hence from one species, two are formed.
The Origin of Species.
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