In biology there's a process called "speciation", which is the moving apart of two groups of individuals of the same species, usually geographically separated, until they can be counted as separate species. Something similar happens with languages, although as history has tended to create larger units and faster communication across longer distances as time went on, with languages we also see the process in reverse. At some stage the northern and southern Celtic languages diverged and later the two languages themselves subdivided, encouraged by being separated by non-Celtic speakers, so Cornish, for example, is neither Breton nor Welsh. Distinct dialects spoken by small numbers of people in limited areas, though, have been disappearing quite fast since the 19th century. Languages can also be created by what in biology would be called crossing: cross old Low German with Norman French and you get English. Fortunately the people who speak these languages are fertile, unlike most products of cross-species intercourse.
What on earth is all this about?
It's about British English and American English. Of course there are other Englishes with their own developments - Australian and Indian English, for example - but I fancy looking at these two.
Some characteristics of American English are down to recent crossing or gene-swapping. In parts of the U.S., for example, it seems to be acceptable to say "If I would have done more revision I would have passed the exam" instead of "If I'd done..." (had done). That would be grammatically correct in some other nordic languages. "Fresh" meaning forward (of a girl) comes from German "frisch" (joyful, bubbly). The use of "hopefully" to mean "I hope this" instead of "full of hope" comes from German "hoffentlich". It is better to travel, hopefully, than to arrive.
In other cases American English preserves versions less changed by time. "Dove" instead of "dived" is archaic British English. Pronouncing Lieutenant (a French word meaning place-holder) as "Lootenant" is much closer to the French pronunciation than the British "Leftenant". In fact I really have no idea how we managed to arrive at that pronunciation.
Now for some interesting differences.
The only British English term for the season including October is "autumn". Americans recognise this word, but the common term is "fall". OK until you say something like "What a wonderful fall!" or "She phoned me in mid-fall".
The space for storage at the back of a car in British English is the boot (odd, though you could put your foot in it). Americans say "trunk", presumably because at one time trunks (not the elephant sort) were strapped to the back of the car for storage. So I can put my boot in the trunk or my trunk in the boot. Elephants - please don't try the latter. For that matter, don't submit to the former.
A living-space within a larger building is a flat to the British. The invitation to look at my lovely new flat would not appeal to Americans who, extravagantly, can have up to four flats.
In Britain a lift can take you somewhere in a car, but can also take you up or down inside a building. American lifts are not capable of taking you directly up or down.
"Republican" carries some drastically different meanings depending on whether you're in the U.S., Britain or Northern Ireland. So I may be a Republican in Britain but lose that characteristic by travelling west (but not east unless I go far enough to cross the Pacific).
The two Englishes are, though, coming together now faster than they diverge. "On the weekend" is now common in Britain where once we would only have said "At the weekend" - and we now use "hopefully" in two quite different meanings. Still, there is hope for diversity as Brits may acquire bits of Polish and Americans bits of Spanish.
We may yet be two different species.
Once we're different species, of course, it's hard for us to interbreed.