Such as chimpanzees branching into common chimpanzees and bonobos, as explained in this video
Presumably it’s not that some chimpanzees just suddenly started giving birth to bonobos overnight…?
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If I have one population of monkeys, then some monkeys travel over to the other side of a mountain range where they are effectively isolated from the original population of moneys, and the environment is different in that new location like types of foods or predators or weather, then the new population of monkeys will have different survival pressures than the original population and their offspring will begin to exhibit different physical characteristics that reflect those different pressures. Over a long enough time frame they can become very unique and distinct from the other population back in the original location.
The mechanism is a mutation that proliferates in one population and not in another.
Chimpanzees wouldn’t have given birth to a different species, but one mutation proliferated in the first generation and the two groups never remerged.
Each population would then undergo subsequent mutations, embracing some and rejecting others, until the compound effect of them is such that they could no longer interbreed.
I’ll give you a simple example. Say there’s a peninsula. A long strip of land coming off of a larger body of land. And there’s a big group of monkeys. Some of the monkeys from the group split off and go to the end of the peninsula, because they prefer the fruit that grows there, and they don’t have to compete with the monkeys back on the mainland for food. Then there’s a big ass earthquake, and the strip of land that connects the tip of the peninsula to the mainland gets washed away. Well now the monkeys from the peninsula live on an island. They’re stuck there. The monkeys on the mainland will call the control group. They stay mostly the same. But the monkeys on the island, they now have a different environment, with different pressures. First they’re going to have to start adjusting their diets, certain things available on the mainland aren’t available here. The monkeys on the mainland get most of their protein from eating termites, but the island doesn’t have as many termite mounds so the monkeys learn to start eating fish instead. Those who can’t adapt to eating fish die from malnutrition. A couple of generations past like that, and then a disease hits the island. The monkey’s on the mainland don’t have that disease, but half the monkeys on the island die out, the only ones that survive are the ones that can resist the disease most successfully. And 10 or 20 more generations past like that. Every generation has different pressures, different environmental changes that they have to adapt to. After about 50 generations, if you took the monkeys on the island, and tried to get them to breed with the monkeys on the mainland, they wouldn’t be able to produce an offspring. At all. After about 15 generations you could maybe get a mule offspring, a sterile crossbreed between the two. But after a full 50? They are now two different species. They are related, they have many similarities, but they cannot successfully reproduce if their populations are mixed.
Usually for this example I use corn, but you wanted to talk monkeys so there you go. Monkeys.
We can, to a limited extent, demonstrate speciation in things like fruit flies that have very short generations. Speciation for complex mammals is a different story.
In fact, we argue about speciation constantly, DNA helps but we still need to figure when a species is really a different species or a sub-species.
There are a group of wolves that evolved to hunt by swimming on Vancouver. These are not their own species…yet. After many generations, they may evolve into a separate species provided that when they get into the same range as other wolves they don’t inter-mate as a regular part of their lives.
So the short answer is that an ecological pressure can cause a subset of a species to specialize, using genetic abilities that exist in all of the members. All wolves can swim, only one set swims to stay alive. They specialize to the point that other characteristics are no longer selected for, a new species begins to emerge.
This is a deep question, it is known that Neanderthals and man regularly mated, but we still consider ourselves a distinct species from one another.
Anything that separates the gene pools. Physical isolation, behavioral differences that prevent breeding, etc. As soon as the gene pools are separated, you have “branching”.