for example elephant isn’t the same size as a sheep
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There’s a very important concept in evolutionary biology, called niche [“nish”].
A niche is a set of things that can serve as food, home and other needs of a living being. For example a an alga and an underwater cave can be a niche for an alga-eating cave fish. A rabbit can be a niche for a little bug that lives in the fur of the rabbit and eats the blood of the same rabbit.
There are some rules in evolution that we can observe. One is that if you have two species that need the same niche, they will compete and one will win.
The other rule is that of there is an empty niche, sooner or later some living being will evolve and take that niche. Actually an empty niche is one main driver for evolution.
But here’s the thing: different niches require different ways to live there. You cannot be a cave fish if you can’t fit in the cave.
As you see, if there’s an empty niche for cave fish, there will be evolution making that size of fish. But then there will be ant sized things living in the small cracks. There must be bacteria living in the guts of the ant.
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