First, what we call a chance mutation. A tiny genetic change occurred in that one Notothenioid species.
Its DNA allowed for the production of a special protein, a protein that prevents the fish from freezing.
The way this...this anti-freeze protein works is: it binds to any ice crystals that form inside the fish.
This binding action prevents the ice crystals from growing larger. And this is what prevents Notothenioids from freezing.
Now, at that time, the waters the Notothenioids inhabited were still not freezing cold, so the protein didn't really make a difference as far as the fish's survival.
But this would change, because in the same period of geologic time there was a shift in the earth's continental plates.
Continental drift caused Antarctica to move apart from the landmass of South America
and to drift into the Southern Polar Region. This resulted in a powerful water current encircling Antarctica,
which prevented the Antarctic waters from mixing with warmer water.
So the southern ocean, isolated from that warm airflow from the north, cooled down drastically,
to the kinds of sub-freezing temperatures we associate with it today.
Now, most fish species couldn't survive in this frigid environment and they became extinct.
But that one Notothenioid species, with its unique ability to produce that anti-freeze protein, thrived.
It had virtually the entire southern ocean to itself! So? Well, there was little or no competition for food or space.
You might think of it as...um...as a...a kind of ecological vacuum. And the Notothenioids exploited fully.
The species migrated into different habitats throughout the southern ocean.
And its population increased dramatically, with various sub-populations migrating into different parts of the ocean.