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1 .<-NARRATOR:-> Listen to part of a lecture in a biology class.
1 .<-FEMALE PROFESSOR:-> [responding to a student’s comment]…Ways in which animals adapt to their environment are often quite ingenious, actually.
2 .And, as an example of this, lemme tell you about a fish … a-a group of fish known as the notothenioids.
1 .There's over 90 known species of notothenioids, and they inhabit both shallow and very deep waters-mostly around Antarctica.
2 .Many are fairly small, though the largest species can weigh up to 150 kilograms.
3 .Notothenioids can be identified by their large eyes, which are covered by a thick, insulating layer of clear tissue.
4 .This tissue protects their eyes from freezing.
5 .Remember, the freezing point of ocean water-saltwater-is lower than for freshwater-negative 1.9 [one-point-nine]degrees Celsius … so it can get a lot colder for fish in an ocean, say, than in a river or lake.
1 .So … this means that the ocean waters around Antarctica are cold enough to freeze most types of fish. But notothenioids don't freeze. In fact, they thrive.
2 .They account for some 95 percent of all fish in the Southern Ocean-the ocean that surrounds Antarctica.
1 .So, how unusual is that …to have a single family of fish dominating an entire ocean?
2 .I mean, think of, say, tropical or temperate marine environments,which have incredibly diverse fish populations.
3 .Coral reefs, for example, support over 4,000 types of fish,along with sponges … crustaceans … and many other organisms.
1 .So, exactly when-and how-did the notothenioids come to dominate the Southern Ocean?
2 .Well, around 30 million years ago, the waters around Antarctica were a lot warmer than they are today.
3 .Um … at that time, Antarctica was connected to South America, which means that warm air, from the north, could flow southward and heat up the Antarctic waters.
4 .Because the water around Antarctica then was relatively warm, it supported many types of fish, and we know this from fossil evidence.
5 .But the 90 or so species of notothenioids that exist today didn't exist at all back then ….
6 .In fact, only one ancestral notothenioid species existed.
1 .But somewhere between five million and fourteen million years ago,two major changes took place.
2 .First, what we call a chance mutation-a tiny genetic change-occurred in that one notothenioid species.
3 .Its DNA allowed for the production of a special protein,a protein that prevents the fish from freezing.
4 .The way this-this antifreeze protein-works is:it binds to any ice crystals that form inside the fish.
5 .This binding action prevents the ice crystals from growing larger … and this is what prevents notothenioids from freezing.
1 .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.
2 .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.
3 .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 subfreezing temperatures we associate with it today.
1 .Now, most fish species couldn't survive in this frigid environment,and they became extinct.
2 .But that one notothenioid species, with its unique ability to produce that antifreeze protein,thrived. It had virtually the entire Southern Ocean to itself- so … well, there was little or no competition for food or space.
3 .You might think of it as, um …[searching for a metaphor] as a—a kind of … an ecological vacuum; … and the notothenioids exploited it fully.
4 .The species migrated into different habitats throughout the Southern Ocean,and its population increased dramatically, with various subpopulations migrating into different parts of the ocean.
1 .Over time, these subpopulations, in all those different habitats … well, they developed very different physical traits; they adapted to survive in their particular ecological niche—[explaining the term] their … their position within a particular ecosystem.
2 .We call this type of species diversification within a species adaptive radiation.
3 .And what adaptive radiation is: is [slowly, stating a definition] an evolutionary process by which a parent species rapidly undergoes changes resulting in various new species, in order to fill multiple ecological niches.
4 .So, in the case of the notothenioids, that single species started colonizing empty habitats to such an extent that it evolved into a broad range of new species-the 90 or so notothenioid species that we have today.
5 .[slight pause] So, lemme switch to adaptive radiation with regard to another species that’s also been very successful ….