What is at the heart of new species formation? It all starts with a single population of a species which for whatever reason (off-spec: genetic bottlenecks, founder effect, etc.) ends up being split geographically to the point where no interbreeding occurs for a certain length of time.
Given that the two habitats are different, the individuals in each population will adapt differently to counteract different selection pressures. Say for example the ants in the forest experience a warmer and more nutrient-rich surrounding compared to the emigrated ants on a nearby, although disconnected, beach.
The adaptations acquired by both populations over a long time will get increasingly disparate. When these pass a threshold, the two populations can no longer interbreed, even if the opportunity were given (due to excessive genetic difference). They have now become separate species! This process is called speciation.
Speciation due to an established barrier such as geographical separation is termed allopatric.
Speciation can also occur in absence of a barrier. The individuals of a starting species can share the same physical space and be able to come into contact with each other, yet for other reasons subspecies can still separate within that population in what is termed sympatric speciation.
Sympatric speciation may occur as a result of different members of the former species occupying different niches within the same habitat. Perhaps they start feeding on different sources, behaving differently, having different mating signals, etc.
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