What are some ways biologists define a species, and the cut-off point between two different species?
The common definition is that two groups of animals belong into different species if they cannot interbreed succesfully. So because life evolves, the distinction can be quite blurred. For example, there are fossils that are classified into one species by some experts and into another one by others. That’s exactly what one would expect, evolution is fully gradual, and like you cannot say when exactly a child turns into an adult, you cannot say when exactly a species turns into another one. A parent never gave birth to a child of a different species, only with hindsight and lots of dead intermediates in between, we can classifiy two sorts of related fossils into different species. In case of child-adult, the cut-off point is arbitrarily set at the age of 18, yet nothing magical happens in the night of one’s 18th birthday. It’s the same with species. There are also so-called ring species, some gulls or salamander are an example. They live in huge ‘rings’ spanning a lot of geographical spa
