So, me and some lads were discussing earlier in a discord about the deep and emotionally investing lore that Basculin totally has and I've come to a realisation about and ability that I previously never really understood what the big thought process behind it was
Now, some cases like

Eevee being literally the most adaptable mon, capable of evolving into 8 different species; or

Porygon-Z being based on an evolutionary line that adapts with technology over time, those I could understand why they had the ability, but most other Adaptability mons confused me a little as to why they have the ability. Today, when looking at Basculin's dex entries is when I realised
Adaptability is often tied to Invasive Species. (For those who are unfamiliar with the term, they're species that inhabit an ecosystem they're not usually native to, and cause a general imbalance in said ecosystem. Think of it how some Hippos escaped from captivity into the Amazon and nothing there can take down a Hippo)
Out of the 10 Evolutionary Lines that get Adaptability (2 of which I've mentioned already and 2 of which are Megas), the following are Invasive/implied to be Invasive by the following Dex Entries:
They look ragged, so no one catches them. They look like they'd taste bad, so predators won't eat them. And their numbers continue to grow.
It came from overseas. It is a very hardy creature that will quickly proliferate, even in polluted streams. No matter how dirty the water in the river, it will adapt and thrive. It has a strong will to survive. | Individuals that have been set free by Trainers who could no longer raise them have become common, and they can now be found in Alola.
Crawdaunt has an extremely violent nature that compels it to challenge other living things to battle. Other life-forms refuse to live in ponds inhabited by this Pokémon, making them desolate places.

Anglers love the fight this Pokémon puts up on the hook. And there are always more to catch— many people release them into lakes illicitly. When a school of Basculin appears in a lake, everything else disappears, except for Corphish and Crawdaunt. That's how violent Basculin are.

While not a direct dex entry, its entire lore is based on being brought to hunt Rattata but then failing at that and becoming found all of the region themselves instead, which in on itself is a reference to irl invasive mongoose species in Hawaii
This relationship between Ability and Ecological Niche is genuinely one of my favourites things to have learned from these Pokémon, and it leaves me wondering is there's any more abilities out there based on ecology such as this.
Still no idea why Skrelp, Dragalge, Mega Luke and Mega Bee have it though