Balto-Slavic glottal stop

In the past, research focused on East Baltic, mainly because it's widely attested in Latvian and Lithuanian. East Baltic does seem to have been a distinct group. But more research into Prussian (West Baltic) and its relationship with the other Balto-Slavic languages has cause linguists to realise that there probably was no Proto-Baltic after all, but that West Baltic and Slavic split off at more or less the same time. It's only really the radical changes that later applied to Slavic that make it stand out and makes Prussian seem much closer to Latvian and Lithuanian, but the reconstructable history seems to suggest otherwise. So Baltic is probably a paraphyletic group, consisting of the non-Slavic Balto-Slavic languages.

The tension between similarity vs common history can be seen in the life sciences, too: sharks split off from the vertebrates before the ancestors of the tetrapods did, so we're more closely related to trout than sharks are- but they're both fish, and we aren't.

Right, fish, reptiles, dinosaurs, etc. comprise paraphyletic groups, which are occasionally convenient classifications.

And of course the biggest paraphyletic grouping of all, "animals", at least in common speech.

Estonian is closer to Finnish genetically than it is to Võro (south Estonian), Norwegian closer to Icelandic than to Danish, Catalan closer to French than to Spanish... the list goes on.

Wait, how is "animals" used in common speech?

To include all of animalia except for humans.

Ah, okay. I was confused because I thought you were referring to the use of the word which only includes zoo animals, farm animals, etc. while excluding some animals like insects and arachnids. (If you've ever heard a child make a distinction between "animals" and "bugs")