eli5 : how did different animals come from common ancestors
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In general, if you take one population, split it up into two different populations that are separated from one another and not cobreeding, they can drift apart from each other and evolve independently.
The common ancestor animals initially live in the same habitat and continue to interbreed with each other so they remain genetically compatible. At some point something causes the group to split into two or more sun groups which become separated from each other by enough distance that there is less and less interbreeding between the two groups.
If that happens for long enough the random mutations that occur and are selected for by evolution add up and the species genetics become more and more different. If they remain separated for a long enough time the two groups will become so different they no longer can interbreed and are considered two different animals now.
Imagine a small mammal like a rat. If you took one group of rats and placed them in an environment with lots of rivers and lakes for a long time they might adapt to that kind of environment by say, developing webbed feet which make it easier to catch prey in the water.
Meanwhile if you took a different group of that same animal and put it in a forested area they might develop long sharp claws which makes climbing easier.
Over time more and more differences occur due to natural random mutations and add up.
Just look at domestic dogs as a perfect recent example.
Not that long ago at all, all dogs were wolves. But after we domesticated dogs we applied artificial selection to them, and a group of animals that used to be all pretty much the same became radically different, with examples as extreme as chihuahuas and great danes. And this was just because different selective pressures were put on different subpopulations.
Natural selection can do the same thing. Take elephants and strand them on an island with insufficient food for large size and what happens? You get a species of small elephant. Meanwhile on the mainland the elephants continue to be large, and might get furry as they move further North.
Imagine a sea full of one kind of fish. They all eat the same thing and live the same way. One day, a fish is born with a mutation which gives it the ability to survive in shallower water and hence it can hunt in places the others cant. It passes this ability on to its offsprings, and one day an offspring gets the ability to live at even shallower water or even breath above water. This fish is now luch different to its ancestors in the sea, but the sea is stilk full lf the original kind of fish, because it was super good at being a sea fish. Now you have two species.
Next thing the shallow water fish has an offspring with front legs, which can crawl ashore. Now we have three species. This thing happens very gradually over millions of years with very small changes each generation.
For example, you might be a bit taller, smarter, or have better eyesight than you parents, and hence you are changing from the original humans.
Separation.
We and chimpanzees share a common ancestor a couple of million years back. The descendants who became chimps and those that became us lived in different environments and were therefore exposed to different selective pressures, environmental conditions making certain traits favorable and others less favorable.
this is pretty much the ground floor of how speciation works. If you separate a population for long enough, the separated groups will each adapt to their new environment and eventually become incapable of interbreeding due to accumulated genetic differences.