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英['lɪs(ə)n] 美['lɪsn]
vi. 听,倾听;听从,听信
n. 听,倾听

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草莓小菇凉:说的非常好,十分有道理,棒棒棒!

06-08 15:44:55

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TPO 33-L3 .Notothenioids 点击收藏

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    Listen to part of a lecture in a biology class.

    Ways in which animals adapt to their environment are often quite ingenious actually.

    And as an example of this, let me tell you about a fish, a group of fish known as the Notothenioids.

    There's over 90 known species of Notothenioids and they inhabit both shallow and very deep waters, mostly around Antarctica.

    Many are fairly small, though the largest species can weigh up to 150 kilograms.

    Notothenioids can be identified by their large eyes, which are covered by a thick insulating layer of clear tissue.

    This tissue protects their eyes from freezing.

    Remember, the freezing point of ocean water, salt water, is lower than for fresh water, negative 1.9 degree Celsius.

    So it can get a lot colder for fish in an ocean, say, than in a river or lake.

    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.

    They account for some 95% of all fish in the southern ocean, the ocean that surrounds Antarctica.

    So, how unusual is that, to have a single family of fish dominating an entire ocean?

    I mean, think of... say, tropical or temperate marine environments, which have incredibly diverse fish populations.

    Coral reefs, for example, support over 4000 types of fish, along with sponges, crustaceans, and many other organisms.

    So, exactly when and how did the Notothenioids come to dominate the southern ocean?

    Well, around 30 million years ago, the waters around Antarctica were a lot warmer than they are today.

    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.

    Because the water around Antarctica then was relatively warm, it supported many types of fish.

    And we know this from fossil evidence.

    But the 90 or so species of Notothenioids that exist today didn't exist at all back then.

    In fact, only one ancestral Notothenioid species existed.

    But somewhere between 5 million and 14 million years ago, two major changes took place.

    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.

    Over time these sub-populations in all those different habitats... well, they developed very different physical traits.

    They adapted to survive in their particular ecological niche, their... their position within a particular ecosystem.

    We call this type of species diversification within a species "adaptive radiation".

    And what adaptive radiation is, is an evolutionary process by which a parent species rapidly undergoes changes resulting in various new species in order to fill multiple ecological niches.

    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.

    So let me switch to adaptive radiation with regard to another species that's also been very successful.

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