OFFICIAL33 What is the lecture mainly about?

[00:00.00]NARRATOR: Listen to part of a lecture in a biology class.[00:07.10]FEMALE PROFESSOR: ...Ways in which animals adapt to their environment are often quite ingenious actually. [00:13.68]And as an example of this, let me tell you about a fish, a group of fish known as the Notothenioids.[00:20.87]There's over 90 known species of Notothenioids and they inhabit both shallow and very deep waters— mostly around Antarctica. [00:29.70]Many are fairly small, though the largest species can weigh up to 150 kilograms.
[00:35.53]Notothenioids can be identified by their large eyes, which are covered by a thick insulating layer of clear tissue. [00:43.40]This tissue protects their eyes from freezing.[00:46.16] Remember, the freezing point of ocean water—salt water—is lower than for fresh water—negative 1.9 degree Celsius ... [00:54.33]so it can get a lot colder for fish in an ocean, say, than in a river or lake.[00:59.26]So... this means that the ocean waters around Antarctica are cold enough to freeze most types of fish, but Notothenioids don't freeze.[01:09.59] In fact, they thrive. [01:11.46]They account for some 95% of all fish in the southern ocean, the ocean that surrounds Antarctica.[01:18.10]So, how unusual is that? [01:20.54]To have a single family of fish dominating an entire ocean. [01:24.42]I mean, think of, say, tropical or temperate marine environments, which have incredibly diverse fish populations. [01:32.53]Coral reefs, for example, support over 4000 types of fish, along with sponges, crustaceans, and many other organisms.[01:40.74]So, exactly when and how did the Notothenioids come to dominate the southern ocean? [01:46.95]Well, around 30 million years ago, the waters around Antarctica were a lot warmer than they are today. [01:53.48]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 Antarctica waters. [02:03.20]Because the water around Antarctica then was relatively warm, it supported many types of fish.[02:09.78] And we know this from fossil evidence.[02:11.94]But the 90 or so species of Notothenioids that exist today didn't exist at all back then. [02:18.30]In fact, only one ancestral Notothenioid species existed. [02:22.73]But somewhere between 5 million and 14 million years ago, two major changes took place.[02:29.45]First, what we call a chance mutation. [02:32.78]A tiny genetic change occurred in that one Notothenioid species. [02:37.95]Its DNA allowed for the production of a special protein, a protein that prevents the fish from freezing. [02:44.99]The way this—this anti-freeze protein works is: it binds to any ice crystals that form inside the fish. [02:53.50]This binding action prevents the ice crystals from growing larger. [02:57.77]And this is what prevents Notothenioids from freezing.[03:01.22]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. [03:12.78]But this would change, because in the same period of geologic time there was a shift in the earth's continental plates. [03:20.15]Continental drift caused Antarctica to move apart from the landmass of South America and to drift into the Southern Polar Region.
[03:29.22]This resulted in a powerful water current encircling Antarctica, which prevented the Antarctic waters from mixing with warmer water. [03:38.62]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.[03:49.53]Now, most fish species couldn't survive in this frigid environment and they became extinct. [03:56.46]But that one Notothenioid species, with its unique ability to produce that anti-freeze protein, thrived. [04:03.82]It had virtually the entire southern ocean to itself!
[04:07.67]So? Well, there was little or no competition for food or space. [04:12.20]You might think of it as, um...as a—a kind of ecological vacuum. [04:17.74]And the Notothenioids exploited fully.[04:20.28] The species migrated into different habitats throughout the southern ocean.[04:25.11]And its population increased dramatically, with various sub-populations migrating into different parts of the ocean. [04:32.23]Over time these sub-populations in all those different habitats...well, they developed very different physical traits. [04:40.34]They adapted to survive in their particular ecological niche, their...their position within a particular ecosystem.
[04:48.05]We call this type of species diversification within a species adaptive radiation. [04:54.25]And what adaptive radiation is: an evolutionary process by which a parent species rapidly undergoes changes resulting in various new species in order to fill multiple ecological niches. [05:10.31]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. [05:25.25]So let me switch to adaptive radiation with regard to another species that's also been very successful....