Have you heard of the Great Lakes?

No, not the ones in the US and Canada! Though Lake Michigan does happen  to be one of my favorites.

I’m talking about the African Great lakes!  Lake Edward, Lake Kivu … Lake Malawi. Well, there’s something really  weird happening in that last one. This lake has more than 850  species of ONE type of fish.

That’s more than the total  number of species of fish in all of Europe's freshwater systems. So, what the heck is going on in Lake Malawi?! [♪ INTRO] Not long after Charles Darwin  made it to the Galapagos Islands and began studying the famously diverse  finches there, news arrived in Europe that a massive body of water had been  discovered in southeastern Africa. Of course, the people who  had been living there had known about it for centuries.

But  that’s a subject for another video. Anyway, it would come to be known as Lake Malawi. Today, the lake is a hot topic of  conversation among evolutionary biologists.

Why? Because it has more than  850 species of just cichlids! Cichlids make up around 90% of all  the fish that exist in the lake!

If you have an aquarium, you’re  probably familiar with cichlids. There’s diversity galore in the coloration,  shape, and the diet of these vibrant fish. Lake Malawi really has the original

DEI: Diversity, evolution, and  ichthyology, the study of fish. But why do we see so much  diversity in this one lake? The reason is a process called adaptive radiation. It’s the rapid increase in the number  of species that share a common ancestor.

Which is great. We have a definition. But what would provoke such an  extreme diversification of these fish?

One cause is competition between male cichlids. Metriaclima mbenji is a beautifully  colored speckled cichlid that has some aggressive tendencies,  specifically towards other male cichlids. When given their own territory, you can  bet they’ll make an effort to protect it!

When an mbenjii male is forced  to defend his territory, studies have shown that he'll do  so more aggressively against males of his own species and males of  other species that look like him. So there's some incentive to  evolve differences in coloration so that cichlids that look like  you aren’t constantly attacking. Another driver of cichlid diversity  in Lake Malawi is habitat.

Which seems weird considering  they live in the same lake! But this lake is a whopper—almost  30,000 square kilometers. Plenty of room for several  different types of habitats.

For example, one study focused  on the hunting preferences of two different benthic, or  bottom dwelling, cichlids: the flavescent peacock cichlid and a  species from the genus Tramitichromis. Tramitichromis cichlids tend to  use their sight for prey detection. The flavescent peacock, on the other  hand, often uses its lateral line system.

That’s an extra sense that fish have  along the sides of their body that helps them detect movement in the water. It’s also a sense I very much wish I had. When assessing the prey detection  abilities of these two species in varying levels of light, scientists found  that our flavescent friend was able to detect more prey at lower light  levels than Tramitichromis.

When you think about their  habitat, that makes perfect sense! Tramitichromis lives in shallower  waters, typically less than 15 m deep, whereas the flavescent peacock can  live as deep as 120 meters down, in an environment where light levels are much lower than what Tramitichromis is used to. So, the need to detect prey better  in lower light may have driven the evolution of a new species that  could operate at greater depths.

It seems as though this town is, in  fact, big enough for the two of them. Thanks to Incogni for supporting this  SciShow video! Incogni is here to help you stay away from cybercrime  and personal privacy breaches.

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So to take your personal data back  with Incogni, use the code SciShow at incogni.com/scishow or the link in the  description and get 60% off an annual plan. Competition and habitat aren’t the  only drivers of adaptive radiation. Diet is another huge one.

There are cichlids that specialize  in scraping algae off rocks, and others that prefer insects. There are even cichlids that feast  on the scales of larger fish! One kind of scale eating  cichlid, Docimodus evelynae, goes through a diet shift as it gets older.

It starts out more like a cleaner  fish, eating parasites and fungus. As it grows larger and its teeth  start to change, so does its diet. It begins eating plankton when it’s  available during its adolescence.

Finally, once it reaches its larger sizes,  it has lost all its baby teeth and has gained bigger, pointer ones,  perfect for plucking chunks of skin and scales right off of larger fish. Picking on someone bigger than you may  seem like a precarious way to live, but it seems to have worked  for a few cichlid species! This specialization seems  to have led to the evolution of a distinct set of teeth to match that diet.

It’s the kind of thing that, over  time, can lead to a new species! These drivers of diversity can  work especially fast in cichlids, since their generation times are so short. Think of it this way: lots of organisms,  like us, have long generation times.

Passing down traits to your offspring won’t  lead to the formation of a new species very quickly if you’re only passing  them down every 20-30 years. But some cichlids reach reproductive  age at 6 months to 3 years old. And with so many aspects of their lives,  like habitat, behavior, ecology, etc., being so different, there’s a lot of  diversity to pass down very quickly.

Some scientists suggest we may get  a new species or two of cichlids in this lifetime, meaning  we’ll be able to watch the formation of a new species  occur right before our eyes! And adaptive radiation is happening  in more than just cichlids. One of the most well known  examples is Darwin’s famous Galapagos finches I mentioned earlier.

They show incredible diversity between islands, namely in their beak size and shape. This allows the different  species to specialize on whatever food is available on their island. This was one of the observations that  inspired Darwin’s theory of evolution.

Anolis lizards in the Caribbean  are another great example! The lizards took over different  niches all over the Caribbean islands, from the ground, to the bushes, to  the trunks and tops of the trees. And now there are about 150 species of them.

So it has happened in other  places… but why Lake Malawi? Well, lakes and islands are the  perfect setting for rapid speciation. Notice how the two other  examples I named were on islands?

In such places, competition with  other types of organisms is limited. Once the ancestral species  arrives, there are usually tons of niches available for them to fill. They’d have a field day with  all those opportunities!

And like I said before, Lake Malawi is massive. With all that water, there’s  a lot of space for a lot of different niches and a lot of diversity! But if another type of fish got to  Lake Malawi before the cichlids, would they have radiated the same way?

Cichlids are wildly diverse in Lake Malawi. But they’re also diverse  across the world in the Amazon. Clearly they’ve got this  whole speciation thing down.

With fish that seem to diversify  quickly and a lake with enough space for the fish to do their  thing, Lake Malawi and cichlids seem to be the dream team in  the game of adaptive radiation. [♪ OUTRO]