Chapter 26: Phylogeny and the Tree of Life

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Welcome back to the Deep Dive.

Today, we are, well, we're opening a file that, and I don't say this lightly, might actually break your brain a little bit in the best way possible, of course.

Oh, it's definitely a perspective shift.

We're so used to looking at the natural world as the static picture, you know, but today we're going to see it as a moving family history that spans literally billions of years.

Right.

And to get us into that mindset, I want to start with a little mental test for you, the listener.

I want you to visualize something with me.

We are looking at the material from chapter 26 of Campbell Biology, 12th edition, specifically figure 26 .1.

So imagine you are walking through the tall, grass in a forest, say in Eastern Europe.

You hear a rustle.

You look down.

Set the scene for us.

You see a creature.

It's long.

It's slender.

It's covered in scales.

It is slithering through the grass in that very distinctive S -curve motion.

And most importantly, it has zero legs.

None at all.

Okay.

You're looking right at it.

What is your immediate gut instinct reaction?

What is that animal?

I mean, if I'm being totally honest, I'm jumping back three feet and screaming snake.

It's a snake.

It has to be a snake.

And that is exactly what most people would say.

It looks like a snake.

It moves like a snake.

It acts like a snake.

But if you're a biologist, specifically a systematist, you would look at that creature and say, actually, that is not a snake at all.

Wait, hold on.

It has no legs.

It is slithering.

How is it not a snake?

It's actually a lizard.

Specifically, it's Ophysaurus apotus, which is commonly known as the European glass lizard.

A lizard.

So nature is basically just gaslighting us here.

In a way.

Yeah.

But this brings us to the core problem of biology, which is that looks can be very deceiving.

If you get down on your hands and knees, which, by the way, I do not recommend if your brain is screaming that it's a venomous snake.

Yeah, bad idea.

Right.

But if you did, and you looked very closely at its head, you would see movable eyelids.

Wait, snakes can't blink?

Nope.

Snakes have a fixed transparent scale over their eyes.

They just stare.

But this glass lizard blinks.

It also has external ear openings, which snakes absolutely do not have.

Wow.

But the real smoking gun is inside.

If you looked at its skeleton, its jaw structure isn't detached like a snake's highly mobile jaw, and its vertebrae count and structure are totally different.

So despite the whole no legs thing, its internal machinery and its history basically shout lizard.

Exactly.

And that distinction between what something looks like on the surface and what it actually is based on its deep evolutionary history, that is the entire theme.

We are talking about phylogeny.

Phylogeny.

It sounds, I don't know, it sounds a bit like a painful dental procedure.

I promise it's much cooler than that.

It is the evolutionary history of a species or a group of species.

It is the master family tree of life on Earth.

So our mission today is to break down this chapter of Campbell biology for you.

We're going to figure out how scientists actually build these massive trees.

Like, how do they actually know who is related to whom?

We're going to look at the tools they use.

We'll talk about how we name things, which is a surprisingly heated battleground.

We're going to look at the tools they use.

We're going to look at the tools they use.

We're going to look at the tools they use.

And we'll look at how to read these trees of life without falling into some really common mental traps.

We're also going to get into some actual forensic files, literally using this exact science to catch poachers who were selling illegal whale meat.

That is such a fascinating case study in the chapter.

And then we'll get into the high tech stuff, right?

Molecular clocks using typos in DNA to actually tell time.

And finally, we are going to talk about how the entire tree of life

might actually be wrong, or at least way too simple.

The ring of life.

It's a massive shakeup in how we view the microbial world.

Okay, let's not get too far ahead of ourselves.

Let's start with the absolute basics.

If we're going to talk about family histories, we need to know who we are talking about.

We need names.

Taxonomy, naming the natural world.

This is section 26 .1.

I grew up calling things by their common names, you know, dog, cat, jellyfish, silverfish.

But the text implies that this is, well, bad practice.

It's imprecise.

And precision is basically everything in science.

Common names are a disaster because they're so often misleading.

Take jellyfish, for example.

Squishy.

Stings you at the beach.

Ruins your vacation.

Right.

But it is not a fish.

It's a clinarian.

It actually has more in common with a coral reef than it does with a salmon.

Or think about silverfish.

Those creepy little alien looking bugs in the bathroom sink.

Exactly.

Again, not a fish.

It's an insect.

And that's just the confusion in English.

Imagine you're a researcher in Germany talking to a researcher in Brazil and you say cat.

You might mean a little house cat and they might think you mean a massive jaguar.

It's pure chaos.

So back in the 18th century, a guy named Carolus Linnaeus steps in to clean up the mess.

Linnaeus.

He is the father of modern taxonomy.

He looked at this chaos and basically said, we need a universal system.

And he gave us binomial nomenclature.

Which is just fancy Latin for a two -name naming system, right?

It's brilliant.

And it's simple.

Simplicity, really.

Every single species gets a two -part name.

The first part is the genus.

That's the broader group that the organism belongs to.

The second part is the specific epithet, which identifies the exact species within that broader group.

So let's use the example right out of the text.

The leopard.

Panthera pardus.

Panthera is the genus.

And that includes other big cats too, right?

Right.

Lions are Panthera leo.

Tigers are Panthera tigris.

Jaguars are Panthera onca.

They are.

All live in the Panthera house, so to speak.

But pardu is completely specific to the leopard.

And there are strict formatting rules here.

You can't just scribble it on a napkin however you want.

No, definitely not.

The genus is always capitalized.

The specific epithet is always lowercase.

And the whole thing must be italicized in print.

And yes, it is heavily Latinized.

I actually always wondered about the Latin thing.

Is it just to make scientists sound smart?

I mean, partially, maybe.

But mostly it was because Latin was the universal language of scholars in the 18th century.

It meant the name wouldn't change whether you spoke French, English, or Swedish.

It was a dead language, so it was stable.

It wasn't evolving with slang.

That makes a lot of sense.

So Linnaeus gives everyone a standardized name tag.

But he went further than that.

He didn't just name them.

He organized them.

He basically built a massive biological filing cabinet.

The hierarchical classification system.

This is crucial for students to understand.

He grouped species into increasingly inclusive categories.

The text uses a postal...

The postal address analogy here.

And I want to walk through this with you because it really helps visualize how we all fit into this system.

It's a great analogy.

Think about mailing a letter to a specific person.

Let's use our friend, the leopard.

Okay.

So the specific person getting the letter is the species.

Panthera pardus.

Right.

Now that person lives in a specific apartment building.

That building is the genus, Panthera.

Who else is in the building?

The lion, the tiger, the jaguar.

They're all close neighbors.

Got it.

But that building is located on a street.

The street is the family.

In this case, Felidae.

The cat family.

Now the neighborhood has gotten a lot bigger.

We've got domestic house cats, lynxes, cheetahs, cougars.

They all live on Felidae Street.

But the street is in a city.

The city is the order.

Carnivora.

Now it's getting really crowded.

We have dogs, bears, weasels, seals, raccoons.

They are all meat eaters, mostly living in the same vast city.

The city is in a state.

The state is the class.

Mammalia.

Now we've got anything with fur, that produces milk.

Whales, bats, humans, mice.

The state is in a country.

The country is the phylum.

Chordata.

Basically, animals with a backbone or a similar structure.

Now we're including fish, birds, reptiles, amphibians.

And the country is on a continent.

The continent is the kingdom.

Animalia.

All animals.

Sponges, insects, jellyfish, worms.

And finally, the planet itself.

The domain.

Eukarya.

Any organism with complex cells.

So that's plants, fungi, animals, and protists.

So it goes from...

Most specific to most broad.

Species, genus, family, order, class, phylum, kingdom, domain.

It's a perfectly nested system.

Exactly.

Each box fits neatly inside a bigger box.

But, and here is the massive...

The text highlights Linnaeus was doing all of this in the 1700s.

Right.

Charles Darwin had not written on the origin of species yet.

Linnaeus wasn't thinking about evolution at all.

He was thinking about divine order.

He was just trying...

He was just trying to tidy up God's creation.

That is a critical historical point.

Linnaeus grouped things together based on how they looked.

Pure physical resemblance.

But as we just learned with the glass lizard and the snake, looking alike doesn't always mean you are related.

So sometimes the Linnaean filing cabinet is just flat out wrong.

Sometimes, yes.

For example, Linnaeus might have put two very similar looking worms in the same genus, but evolutionarily speaking, they might have separated hundreds of millions of years ago.

That's why modern systematists, the scientists who do this, today, are proposing a major shift.

They want to move away from these rigid ranks like class and order.

They bring up the phylo code in the chapter?

Right.

The phylo code.

The idea there is to stop worrying about whether something is officially a family or an order.

Just name the group's based entirely on their common evolutionary ancestor.

If they share a history, they are a group.

Period.

Ranks don't matter, history does.

Which brings us to the visual representation of that history.

We aren't just filing things in static drawers.

anymore.

We are drawing a tree.

The phylogenetic tree.

Okay, let's visualize this together.

The text walks through figure 26 .5.

It describes a tree.

But this isn't an oak tree growing up from the ground in your backyard.

It's usually drawn horizontally on its side.

Think of it more like a river system flowing backward, maybe.

Or a branching roadmap.

Let's go to the roadmap.

You start at the far left of the page.

There's one single unified road.

That is the root.

The root represents the ancestral lineage.

The great -great -great -grandparent of every single organism on that specific chart.

The most recent common ancestor of all taxa in the tree.

As you travel to the right, the road splits.

That split is a branch point, or what biologists call a node.

That node is a historical event.

It's a speciation event.

It is the exact moment where one lineage diverged and became two.

It represents the common ancestor of the two lines that diverge from it.

And the lines keep going.

Going to the right, maybe splitting again and again, until you get to the far right edge of the paper.

The ends of the lines.

Those are the taxa.

The species living today, or the groups we are currently studying.

Exactly.

Now, there is some very specific vocabulary we need to nail down here to talk about these relationships accurately.

The first concept is sister taxa.

Sister taxa?

Yeah.

It sounds like a sorority.

It means two groups of organisms that share an immediate common ancestor.

They are each other's absolute closest relatives.

Yeah.

In the diagram in the book, chimpanzees and humans are listed as sister taxa.

Because if you trace their two individual lines backward to the left, they meet at a node where nobody else joins in.

They share a private intersection.

Correct.

Contrast that with a basal taxa.

Basal usually means bottom or the base of something.

In this context, it refers to a lineage that diverges extremely early in the history of the group you're looking at.

So in the vertebrate tree shown in figure 26 .5, the fishes branch off first, way back at the beginning on the left.

They are the basal taxon to the rest of the terrestrial vertebrates.

They set the baseline.

Okay.

Now, reading these trees seems pretty straightforward once you know the terms.

But the text has a huge warning label here.

It says students mess this up constantly.

There are visual traps that our brains just naturally fall into.

The do not list.

These are crucial because our human brains desperately want to read the tree like a ladder of progress.

And it is absolutely not a ladder.

Trap number one, rotation.

This one blows people's minds a bit.

Imagine that branch point the node is a pivot point, like a mobile hanging over a baby's crib.

You can freely spin the branches around that node.

So if I have a tree with humans at the top branch and chimps on the branch below them, and I flip it so chimps are on the top.

It is the exact same tree.

The relationship has not changed one bit.

The vertical order of the names listed on the far right side doesn't matter at all.

What matters is the branching pattern.

Who connects to whom and where.

I feel excited.

I think that's hard to accept emotionally.

We want top to mean best or most evolved.

And that is exactly the psychological trap.

We think evolution is a progression aimed directly at making us.

But in a phylogenetic tree, the order from top to bottom is completely arbitrary.

You could list the species alphabetically or by weight.

And as long as the horizontal lines connect to the correct historical nodes, the tree is 100 % valid.

Trap number two, evolutionary sequence.

I hear this one at dinner parties all the time.

If humans evolved from chimps, why are there still chimps?

And that question reveals a fundamental misunderstanding of what the tree is showing.

The tree does not show that humans evolved from chimps.

It shows their cousins.

Exactly.

If you look at the node where the human line and the chimp line split, that node represents an ancestor that was neither a chimp nor a human.

It was a completely different primate that lived maybe six or seven million years ago.

That population split.

One group went down an evolutionary path to become...

modern chimps.

The other group went down a different path to become modern humans.

So we share a grandparent.

We didn't descend from our cousin.

Precisely.

And that shared grandparent is long extinct.

Trap number three, phenotypic similarity.

Just because two things look completely different doesn't mean they aren't close relatives.

The crocodiles and birds example is the absolute best one here.

It seems insane on its face.

A crocodile is a lizard -y, scale -y, crawling swamp monster.

A bird is a fluffy, singing, flying, warm -blooded creature.

In my head, crocs go in the box with lizards and snakes.

But the phylogenetic tree says no.

Crocodiles are actually more closely related to birds than they are to lizards.

How is that even possible?

Because the lineage that eventually led to birds underwent a massive, rapid explosion of physical change.

They developed feathers.

They developed powered flight.

They evolved a high metabolism.

They changed their look, their phenotype, incredibly dramatically.

Meanwhile, the crocodile lineage stayed relatively...

conservative in its basic body plan.

But if you trace the genetic history back, the crocodile and the bird share a more recent common ancestor than either one does with the lizard.

That's a really good reminder that evolution isn't moving at a steady, uniform speed for everyone.

Some lines change incredibly fast, others just chill out for millions of years looking basically the same.

Exactly.

You can't judge age or relationship by appearance alone.

So reading these trees allows us to see the actual historical truth behind the physical appearance.

And the text brings up a real -world application of this that I think is super cool.

It's like CSI, marine biology, the whale meat mystery.

This is a classic textbook case of using phylogeny for real -world forensics.

So set the scene for the listener.

Fine.

We're in Japan.

Whale meat is sold in local markets.

Now, hunting certain whales, like mink whales, is legal in small numbers for scientific purposes, and that meat ends up in the market.

But hunting fin whales or humpback whales is strictly illegal.

Internationally, they are protected species.

But here's the major problem for law enforcement.

Once you chop a massive whale into little steaks and freeze it, it all just looks like dark red meat.

You cannot tell if that steak came from a legal mink or a highly endangered humpback just by looking at it in a display case.

So researchers went undercover.

They actually bought samples of this whale meat from various markets across Japan.

And they took those samples back to the lab.

They sequenced the mitochondrial DNA, the mtDNA, from the meat.

Why use mtDNA specifically?

It's very abundant in cells.

It mutates relatively quickly, so you can see recent differences.

And we already have a massive library of reference sequences for it from known animals.

So they took this unknown market meat DNA and compared it to known DNA sequences of living, identified whale species.

They built a phylogenetic tree, figure 26 .6.

Yes, they constructed a gene tree.

They placed the unknown market meat samples as new branches on the tree based on their genetic similarity.

And what did they find?

Well, a lot of the meat clustered exactly where it should with the legal mink whales.

But a shocking number of the market samples fell squarely onto the branches belonging to fin whales, humpback whales, and even deeply protected sperm whales.

So they were catching poachers red -handed using evolutionary history.

They proved definitively that an illegal trade was happening using a phylogenetic tree as hard scientific evidence.

That is incredibly cool.

It's not just abstract lines on a textbook page.

It's actively catching criminals.

It really shows the practical power of the tool.

But this begs a pretty big procedural question.

How do they actually build that tree?

How do we know which DNA sequence goes on which branch?

This moves us into segment three, building the tree.

And this brings us right back to our glass lizard versus snake problem.

Right.

To build a valid, accurate tree, you need data.

But not all biological data is created equal.

You have to be able to distinguish between homology and analogy.

We've used these words in passing, but let's define them strictly right now, because this is where all the major mistakes in classification happen.

Homology is similarity due to shared ancestry.

That is the gold standard for tree building.

That's having the family nose or the family chin because you inherited it from your actual grandmother.

And analogy.

Analogy is the fool's gold of biology.

It is similarity due to convergent evolution.

And virgin evolution.

That's when two totally different, unrelated species end up looking at each other.

They end up being exactly the same because they have the exact same job in nature.

Because they faced very similar environmental pressures, yes.

Natural selection shaped them in the exact same way independently.

The text gives a fantastic visual example of this in figure 26 .7.

The tale of the two moles.

I love this one.

On one side of the world, in North America, you have the common mole.

It's a placental mammal, a eutherian.

Its babies develop fully inside the mother.

Evolutionarily, it's fairly closely related to us.

To dogs, to whales.

And on the exact opposite side of the world, in Australia, you have the marsupial mole.

Which is related to kangaroos and koalas.

Its young are born very prematurely and finish growing inside an external pouch.

These two entire lineages, placental mammals and marsupials, split roughly 140 million years ago.

They are not close relatives at all.

But if you look at the picture of them side by side in the book.

They look like identical twins.

They both have these long, cylindrical, torquedo -shaped bodies.

Incredibly soft, velvety fur.

Tiny little blind eyes.

And these massive, thick, shovel -like front paws.

Why?

If they aren't related, why are they identical?

Because dirt is dirt.

Whether you are living in the soil of Australia or the soil of North America.

If you want to survive by tunneling rapidly through dirt to catch worms, you need a shovel.

Natural selection solved the digging problem the exact same way.

Twice.

Completely independently.

So those giant digging paws are analogous structures.

Correct.

If you tried to build a phylogenetic tree based purely on the trait has shovel paws.

You would put the marsupial mole and the North American mole right next to each other.

And your tree would be completely wrong.

You have to look past the superficial analogy to find the true underlying homology.

How do systematists actually do that?

Is there a reliable trick to tell them apart?

The text suggests a very strong clue.

Look at complexity.

Explain that a bit more.

The more complex two structures are.

And the more individual elements they share in precise detail.

The more likely it is that they are homologous.

Think about the skull.

Okay, the skull.

Look at the human skull and the chimpanzee skull.

They aren't just solid round bowling balls.

They are incredibly complex structures made of many specific individual bones that are fused together in a very specific intricate pattern.

The sutures, the actual squiggly lines where the individual bones join together, match up almost perfectly between a chimp and a human.

It's like comparing a highly complex jigsaw puzzle.

Exactly.

The mathematical odds of two completely unrelated animals independently evolving that exact same complex 50 -piece jigsaw puzzle just to solve a generic environmental problem are basically zero.

High complexity strongly suggests shared evolutionary history.

And we do this exact same thing with DNA now too, right?

Molecular homologies.

This is the modern frontier of taxonomy.

We don't just look at bones.

We sequence the actual A, C, T, and Gs of the genetic code.

But even that is tricky.

Why?

DNA seems like it would be perfectly clear cut.

It's literally a code.

Because of mutations that accumulate over millions of years.

Specifically insertions and deletions.

Typos in the code.

Think of it like this.

Imagine two massive books that start off as identical copies of each other.

But over time, in one book, someone randomly tore out a page.

That's a deletion.

And in the other book, someone randomly pasted in a brand new paragraph.

That's an insertion.

If you try to read them side by side, line by line, the sentences won't match up anymore.

Even if 99 % of the words are the same, the spacing is ruined.

Exactly.

The alignment is broken.

So as shown in figure 26 .8, scientists use sophisticated computer algorithms to realign them.

The computer looks at the sequences and mathematically inserts gaps, basically blank placeholder spaces, to push the DNA sequences back into alignment so the matching parts line up with each other again.

That makes sense.

But is there a trap here too, just like with the moles?

Always.

It's called a molecular homoplasy.

Or simply, a coincidental match.

Because if I have an alphabet with only four letters A, C, T, G, and I, write out a long enough string of completely random letters.

Eventually you will get a match just by pure statistical chance.

If you compare the DNA of a human and a totally unrelated mushroom, you might find a short sequence of DNA that looks identical.

But it's not because we share a recent ancestor who had that DNA.

It's purely a statistical accident.

So the computer software has to be smart enough to look at the alignment and say, is this a real historical relationship or just a random coincidence?

Right.

It uses complex statistical models to rule out pure luck.

Okay.

So let's say we've gathered all our solid data, we've thrown out the analogies, the fake clues like the moles' paws, and we've kept the true homologies, the real clues like the complex skull bones.

Now we actually play the game.

And the game is called cladistics.

Cladistics.

This is the primary standard method that systematists use today to reconstruct phylogeny.

The text breaks this down into some very strict logical rules.

The ultimate goal is to create valid clades.

A clade is a group of organisms that includes an ancestral species and all of its descendants.

All of them.

No exceptions allowed.

This leads to some necessary geometry in the book.

Figure 26 .Noe shows three different circles drawn around branches on a tree.

And only one of them is considered a good, valid clade.

The good one is called a monophyletic group.

Mono meaning single, phyletic meaning tribe.

Single tribe.

It is the common ancestor plus every single descendant that came from it.

That is a valid clade.

Then we have the bad groups, the taxonomic mistakes that we try to fix.

The first mistake is a paraphyletic group.

Para meaning beside the tribe.

This is when you draw a circle that includes the common ancestor and some of its descendants, but you deliberately leave one or more descendant lineages out of the circle.

The text gives a classic, real -world example of this mistake.

The artiodactyls.

Right, the even -toed ungulates.

This historically included hippos, cows, deer, giraffes, pigs.

We traditionally grouped them all together as one big family of hoofed land animals.

But genetic evidence revealed something incredible.

It turns out whales actually evolved from right within that exact group.

Whales are basically highly modified aquatic hippos.

So if you use the word artiodactyls and you only mean the hoofed animals walking around on land and you exclude the whales.

You have accidentally created a paraphyletic group.

You cut out a direct descendant, the weird cousin, just because he moved to the ocean and lost his legs.

Cladistics says that is an invalid grouping.

You cannot exclude descendants just because their phenotype changed radically.

And the third type of grouping is polyphyletic.

Poly meaning many tribes.

This is when you group things together that do not share a recent common ancestor.

Like if you grouped dolphins and sharks together into a marine predator's clade just because they both have dorsal fins and swim in the ocean.

You're grabbing the very tips of completely different branches and completely ignoring the deep evolutionary history that separates them.

Okay, so finding true monophyletic clades is the ultimate goal.

To find these clades, the text says we have to look for shared derived characters.

This is a vital distinction to grasp.

Shared ancestral characters versus shared derived characters.

It sounds incredibly technical, but it's really all about context.

Let's use the backbone example from the book to clear this up.

Okay, perfect.

Let's say we are trying to define the clade of mammals.

Does the trait having a backbone help us define what makes a mammal special?

Well, mammals definitely have backbones.

Yes, but so do reptiles and birds and amphibians and fish.

The backbone evolved in an ancestor that lived way, way before the first mammal ever existed.

For mammals, the backbone is the same.

It has a shared ancestral character.

It is inherited from the deep past, so it doesn't help us distinguish mammals from reptiles.

It's old news.

It doesn't tell us where the mammal branch specifically starts.

Right.

But what about hair?

Only mammals have hair.

Exactly.

Hair is a shared derived character.

It is an evolutionary novelty that appeared specifically in the immediate ancestor of all mammals.

And it is entirely unique to that specific clade.

So, to build a precise tree, we kind of ignore the broad ancestral stuff because everyone in the room has it.

And we intensely track the derived stuff, the brand new inventions.

Yes.

We track when the new features appear.

And systematists use something called the out -group method to do this systematically.

This part of the text, figure 26 point tool, felt like solving a logic puzzle.

It literally is a biological logic puzzle.

Let's solve it right now.

You start with your in -group, the specific animals you actually want to classify.

Let's use the animals from the book's data matrix.

A leopard, a turtle, a frog, a base, and a lamprey.

Okay.

And to solve the puzzle, you need a control subject, the out -group.

A species that is related to the in -group but is known to have diverged before any of them.

The text uses the lancelet.

It's this tiny, primitive, filter -feeding marine chordate.

It has no backbone, no jaws, no limbs, no hair.

It represents the original primitive condition.

So, we basically line everyone up and compare them to the lancelet.

Step one, who has a backbone?

Well, the lancelet doesn't.

But the lamprey, base, frog, turtle, and leopard all do.

So, backbone is our very first branch point.

The lancelet stays on the left side of a node.

Everyone else moves to the right.

Step two, who has hinged jaws?

The lamprey is a jawless fish, so it doesn't.

But the base, frog, turtle, and leopard do.

So, hinged jaws is the next derived character, the next branch point.

The lamprey drops off the main line.

Step three, four limbs.

The base is a fish.

It just has fins, no legs.

The frog, turtle, and leopard have four limbs.

The base drops off.

Step four, the amnion, which is the watertight membrane, inside an egg that lets animals lay eggs on dry land.

Frags are amphibians.

They have to lay their jelly -like eggs in the water.

They don't have an amnion.

Turtles and leopards do.

So, the frog drops off.

Final step, hair.

The turtle has scales.

No hair.

The leopard has hair.

And there you have it.

You have built a perfect staircase, a phylogenetic tree entirely defined by the chronological appearance of new derived evolutionary traits.

It's incredibly satisfying when you see the logic actually work out like that.

It's so much cleaner and more objective than the old Linnaean looks -like method.

It is much more rigorous.

But we have to be careful because nature isn't always that clean.

Sometimes we have huge data sets with thousands of DNA -based pairs and the data contradicts itself.

Maybe one gene suggests tree A, but another gene suggests tree B.

So how do computers choose the best tree when the data is messy?

Scientists supply two main principles.

The first is maximum parsimony.

Which is basically Occam's razor, right?

Exactly.

Keep it simple.

Nature is usually, though not always, efficient.

Parsimony assumes that the evolutionary explanation that requires the absolute fewest evolutionary events, the fewest structural changes, or the fewest DNA mutations is the most likely to be the correct one.

The text uses a great beetle example for this.

Right.

If you're looking at two possible phylogenetic trees for a group of beetles, if tree A requires you to assume that wings evolved, were completely lost, and then re -evolved a second time, which is three major evolutionary events, and tree B only requires you to assume that wings evolved exactly once one event, you place your bet on tree B.

Because it's statistically much more probable.

Unless you find overwhelming fossil evidence to the contrary, yes, you assume the simplest path.

The second method is called maximum likelihood.

This is heavily computational.

It uses complex probability rules about how DNA changes over time.

For example, knowing that certain DNA -based changes are more likely to happen than others.

The computer runs millions of times a day, and it's very difficult to find the tree that is statistically most likely to have produced the exact DNA sequences we see today.

Before we move on from reading trees, I really want to touch on phylogenetic bracketing.

Because to me, this feels like the closest thing biology has to an actual time machine.

It is exactly how paleontologists predict the behavior of extinct animals that we will never see alive.

Dinosaurs.

Yes, dinosaurs.

We obviously can't sit in a blind and watch a Tyrannosaurus rex raise its kids, but we can look at its exact position on the phylogenetic tree, and look at its closest living relatives.

On one side of the dinosaur branch you have modern birds, which technically are highly derived living dinosaurs, and on the other side, branching off just before them, you have the crocodiles.

So we look at the traits that birds and crocodiles both share today.

Exactly.

Both birds and crocodiles build physical nests.

Both of them brood their eggs, meaning they sit on them or actively guard them to keep them warm and safe.

Both of them vocalize birds' sing, crocodiles bellow to communicate with their offspring, and biologically both have four chambered hearts.

So using phylogenetic bracketing, we sandwich the extinct dinosaurs between them and predict.

We predict that their shared common ancestor had all those traits, and therefore all the extinct dinosaurs sandwiched in the middle of that bracket likely had those traits too.

And the text points out that we actually found physical proof of this prediction.

We did.

Paleontologists discovered spectacular fossils of a dinosaur called Oviraptor, preserved exactly in a brooding position, sitting directly on top of a clutch of its eggs, just like a modern bird.

The phylogenetic prediction was absolutely correct.

Dinosaurs weren't all just cold, solitary killers.

They were likely attentive parents who communicated with their young.

That is just such a radically different image than the mindless swamp -dwelling monsters we saw in old movies.

Science updates the picture using the tree.

Speaking of updating the picture, let's zoom all the way down to the molecular level.

Segment five, the genome as a history book.

Sections 26 .4 and 26 .5.

This is where things get really modern.

DNA doesn't just hold the blueprints for making eyes and hair.

It is a literal, readable archive of deep history.

And one of the biggest drivers of this genetic history, according to the text, is gene duplication.

This is a massively important mechanism for evolutionary change.

Sometimes, during the complex process of DNA replication, a molecular mistake happens.

An entire gene, or even a whole suite of genes, accidentally gets copied twice.

So now the organism has a redundant backup copy.

Exactly.

And that backup copy is incredibly powerful, evolutionarily speaking.

The original first copy of the gene can just keep doing its essential job, keeping the organism alive.

But the second copy, it's completely free.

It can accumulate mutations without killing the animal.

It can experiment.

The text distinguishes between two types of these homologous genes.

Orthologous and pyrologous.

Right.

Orthologous genes are the exact same vital gene found in different species because they inherited it from a shared ancestor.

Like the gene that codes for human hemoglobin and dog hemoglobin, they just slowly drift apart after the species separate.

Paralytous genes, on the other hand, are multiple copies of a gene existing within the exact same species, created by that duplication error.

Like our sense of smell.

That is the perfect example from figure 26 .18.

Humans have hundreds of different olfactory receptor genes that let us distinguish different smells.

We didn't start out with hundreds.

They all arose from repeated duplications of just a few ancestral genes.

Because of those redundant copies mutating over time, we can now smell coffee and rain and smoke.

It's like evolution's blank scratch pad.

That is a brilliant way to put it.

Now, if we can reliably track these specific DNA mutations, we can do something really wild.

We can tell time.

Molecular clocks.

The concept is wonderfully simple.

Scientists observed that certain specific genes seem to accumulate random mutations at an incredibly steady, constant rate.

Tick, tick, tick.

Like a genetic metronome.

Exactly.

If we can calculate the exact rate of that tick, say, we figure out that this specific gene averages exactly one mutation every one million years, and then we count the total number of genetic differences between two living species.

We can do the math and calculate exactly how long ago they split apart.

We calibrate this clock by checking it against known dates from the fossil record to make sure the time is accurate.

And the text gives a really high -stakes, real -world example of this in action, tracking the origin of HIV.

This part absolutely blew my mind.

We used evolutionary trees to date the AIDS epidemic.

We obviously don't have ancient fossilized remains of viruses and rock, but medical researchers had preserved blood samples containing HIV from patients in the 1980s and 1990s and one incredibly rare preserved sample from all the way back in 1959.

So they looked at how much the virus's RNA had changed, mutated,

between those exact dates.

Right.

They charted the differences and found the exact tick rate of the virus.

And it evolves incredibly fast.

Then they took that line on the graph and just projected it backward in time.

They basically asked the math.

When was the HIV -1M strain completely identical to its primate ancestor virus?

And the date the clock gave them?

Around 1930.

So decades before the global health crisis was even recognized by doctors in the 1980s, the virus had already jumped into the human population.

And was quietly spreading and mutating.

We only know that because of the molecular clock.

It's exactly like carbon dating, but using the mutation rate of genetic code.

That is both terrifying and amazing.

It is.

But we do have to be cautious.

The text warns that the clock is not perfectly smooth.

Natural selection can suddenly speed up mutations or slow them down depending on environmental stress.

But it gives us a vital chronological timeline where the fossil record totally fails us.

Which brings us to the final and perhaps the biggest revelation.

The revelation of the entire chapter.

Segment 6.

Shaking up the tree of life.

This is the scientific bombshell of the late 20th century.

When I was in middle school, I was explicitly taught the five kingdoms of life.

It was drilled into me.

Monera, which was the bacteria.

Then protists, plants, fungi, and animals.

It felt very tidy and permanent.

Monera was essentially a giant catch -all bucket for anything single -celled and prokaryotic.

If it didn't have a nucleus, it got thrown in the Monera bin.

But then scientists started actually saying, sequencing rRNA, ribosomal RNA.

And they found something genuinely shocking.

The microscopic organisms inside that Monera bucket were not all the same thing.

In fact, genetically speaking, they were as completely different from each other as a human being is from a blade of grass.

The bucket was actually two entirely different biological worlds masquerading as one.

So biology had to completely scrap the five kingdoms model.

We moved up a level to the three domains.

Bacteria, archaea, and eukarya.

And here's the real kicker.

Look closely at figure 26 .21 in the book.

It is not a perfectly symmetrical three -pronged tripod.

No, it's not.

Archaea and eukarya branch off together on the right side of the tree.

Wait.

So archaea, those incredibly weird extremophile micros that live in boiling acid pools and deep -sea vents, they are more closely related to us, the eukarya, than they are to standard bacteria.

Yes.

We are the sister taxon to the archaea.

The everyday bacteria are the distant outcasts.

We are the central group to both of us.

That completely rewrites the mental map of life.

We used to think, you know, bacteria and archaea are just primitive germs and we are this special complex branch.

When in reality, we are basically just a highly specialized overgrown twig branching off of the archaean lineage.

But the chapter says it gets even messier than that.

It introduces this concept of horizontal gene transfer or HGT.

This is the ultimate curveball for the tree of life metaphor.

We are so conditioned to think of genes moving strictly vertically.

Parent passes DNA to child, child passes it to grandchild.

Down the branches of the tree.

Vertical inheritance.

But in the microbial world, genes constantly move sideways, horizontally.

A bacterium can literally shoot a plasmid, a little floating ring of DNA directly into a completely unrelated bacterium.

A virus can accidentally pick up a gene from one host and permanently inject it into the genome of a totally different species.

So it's like if I walked past you on the street, gave you a high five, and suddenly you had my exact eye color.

Basically, yes.

And the scale of this horizontal swapping is staggering.

The textbook mentions one major study that found that 80 % of the genes in 181 different prokaryotic genomes had moved horizontally between different species at some point in their history.

80%.

That's insane.

So the tree isn't really a tree with solid separate branches.

It's a tangled net.

The text literally calls it a tangled web.

Figure 26 .23 visualizes it.

It even suggests that we, the eukaryotes, might have originally arisen from an ancient fusion, an endosymbiosis where an archaean cell and a bacterial cell literally merge together.

We are essentially genetic chimeras, a biological mashup of the other two domains.

The roots of the tree are completely tangled together.

The text gives a really crazy specific example of this.

The red alga, called Galderia sulfuraria.

Figure 26 .22.

This particular alga lives in completely inhospitable volcanoes.

It has volcanic sulfur hot springs.

Normally, eukaryotic cells like an alga would die almost instantly in that extreme heat and acid.

So how does it survive?

It didn't slowly evolve the ability to survive there over millions of years.

It effectively stole it.

It horizontally acquired survival genes directly from the extremophile archaea and bacteria that were already living in the hot spring.

It basically downloaded the Genetic Survival Kit app directly from the locals.

That is horizontal gene transfer in real -time action.

It forces us to realize that the tree of life, especially at the very base of the trunk, might be better described as a ring of life.

A massive primordial pool of constantly swapping genes.

It really highlights that science isn't a static textbook of absolute facts.

We didn't just find all the answers in the 1700s and stop.

Not at all.

As the chapter concludes, we are constantly finding entirely new branches, like the lucky archaeota.

The newly discovered group of archaea found near those deep -sea hydrothermal vents.

Right.

Genomic sequencing suggests they might actually be our closest living microbial relatives.

The missing evolutionary bridge between simple prokaryotic cells and the complex eukaryotic cells that make up you and me.

Every single time we sequence a new genome, the focus of the tree gets a little bit sharper.

It's a working hypothesis that literally gets better with every single data point we collect.

Exactly.

Well, I don't know about you, but I am definitely going to look at the little lizards in my garden with a lot more suspicion.

Now, are they lizards?

Are they snakes?

Are they just tiny derived dinosaurs practicing their brooding behavior?

Phylogenetically speaking, they are fascinating living archives of history.

You just have to know how to read them.

And I think that is the ultimate takeaway here for you listening.

Nothing in biology just simply is.

Everything became.

Very well put.

Thank you for joining us on this exploration of Chapter 26.

Go out there and try building some phylogenetic trees of your own.

This has been a production of the Last Minute Lecture Team.

See you next time.