Chapter 12: Systematics: The Science of Biological Diversity

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Welcome back, Deep Divers.

Today we're diving headfirst into, well, a really fundamental question in biology.

How do we even begin to make sense of the millions and millions of species sharing our planet?

It's all about life's incredible diversity, really, and how science tries to untangle it all.

Our deep dive today is focused on systematics.

That's the science, the whole field dedicated to identifying, naming, classifying, and maybe most importantly, understanding the evolutionary relationships between all these living things.

Think of it like trying to map out the ultimate family tree for everything alive.

And that family tree is, well, it's absolutely monumental.

I mean, the grand ambition here for systematists is mapping the entire phylogenetic tree of life, tracing every single organism back to one common ancestor.

We're talking potentially 10 million eukaryotic species, give or take, plus just countless prokaryotes.

So yeah, it's a huge ongoing task connecting all those dots.

Absolutely.

And you, our listener, you're about to get a great shortcut to being really well informed about this family tree of life.

We're going to cover everything from the very first attempts people made to organize life all the way up to these cutting edge molecular techniques that are frankly completely reshaping how we see things.

So let's start at the beginning, shall we?

Okay.

First up is taxonomy.

This is sort of the practical side of systematics, actually giving species a name and a place so we can identify them and talk about them coherently.

Right.

And the modern system we use really kicked off with Carl Linnaeus, an 18th century Swedish naturalist.

Incredible ambition, this guy.

He wanted to name every known plant, animal, and before him, species had these incredibly unwieldy Latin descriptive names called polynomial, sometimes a dozen words long.

Just imagine trying to remember those for everything.

Wow.

So Linnaeus' big breakthrough wasn't just naming things.

It was how he named them.

He took those super cumbersome descriptions and boiled them down to a simple, really elegant two word system,

binomial system.

It seems simple now, but it basically changed everything, didn't it?

How did that small shift make such a huge impact?

Oh, it was revolutionary.

Purely because of convenience, those long polynomials, they pretty much vanished overnight.

The binomial system is just so elegant.

The first part is the genus, that's the generic name, and the second part is the specific epithet.

Put them together and you get the unique species name.

For example, you might have ornitharabienus, the evening permrose.

Ornitharathera, the genus, can stand alone to refer to the whole group, but biennus.

By itself, it's meaningless.

And there are rules, of course.

The names are always italicized or underlined, and crucially, the earliest valid binomial name published has priority.

There are international codes that govern all this.

That brings so much clarity.

Okay, and to make sure that clarity lasts, scientists use something called a type specimen, right?

Like a reference point.

Precisely.

Yeah, a type specimen is usually a physical preserved organism, often a dried plant pressed in a museum or hobarium collection.

It acts as the definitive benchmark for that species name.

It's the standard everyone refers back to.

And sometimes, you know, even within one species, we find more distinctions.

We call these subspecies or varieties.

They share specific features that other groups within the same species don't have.

A classic example is the peach, prunus persicavar, persica, and the nectarine, prunus persicavar, nectarine.

Same species, prunus persica, but distinct varieties.

Right, okay.

So from those specific level species, varieties Linnaeus and the scientists who followed him built up these much broader hierarchical categories, like those nested Russian dolls, right?

Moving from specific to very general.

They start with species, then group them into genus, then family, order, class.

Phylum used to be division for plants, but mostly phylum now, then kingdom.

And the very broadest level, a domain.

Exactly.

And any group at any of these levels, like the genus, prunus, is called a taxon.

The level itself, like genus or family, that's the category.

This hierarchy packs in a lot of information.

For instance, plant family names usually end in aishi, like febiche for the pea family, and orders often end in ales.

If you look at, say, the classification for maize, zea maize, or an edible mushroom, like Agaricus misperus, you see just how much evolutionary context is embedded in that series of names, from species all the way up to domain.

Now these early systems, even Linnaeus, is based on flower parts.

They were helpful for identification, but they weren't necessarily reflecting actual evolutionary relationships, were they?

They were more artificial.

That's right.

Early systems were often artificial systems.

Theophrastus grouped plants as trees, shrubs, herbs.

Linnaeus had his sexual system based on stamen number.

Useful for sorting, but not for understanding history.

But then Darwin published On the Origin of Species, and the goal shifted dramatically towards natural classifications.

The aim became creating systems that accurately reflect phylogeny, the true evolutionary history of organisms.

And we depict these hypothesized relationships using phylogenetic trees, those branching diagrams you often see.

Okay, but figuring out those true evolutionary connections, that family tree, must be kind of tricky.

What's the biggest trap scientists need to avoid?

I'm thinking about things that just look like versus things that are genuinely related.

That is absolutely the critical challenge.

You have to distinguish between homologous features and analogous features.

Homologous features are those that share a common evolutionary origin, even if they look different or do different jobs now.

Think about, say, regular foliage leaves, the first leaves of a seedling called cotyledons, bud scales protecting buds, even flower parts like petals and sepals fundamentally.

They're all modified leaves.

They trace back to a common ancestral leaf structure.

Homology is the key to building evolutionary trees.

Okay, so homology points to shared ancestry.

What about analogy?

Analogous features are the tricky ones.

They look similar or perform a similar function, but they evolved completely independently.

The classic example is bird wings versus insect wings.

Both are for flight, but their underlying structure and evolutionary origin are totally different.

This often happens through convergent evolution.

That's where unrelated species living in similar environments face similar pressures and end up evolving similar traits.

Think about desert environments.

You find plants in the spurge family, the cactus family, the milkweed family that look remarkably alike with succulent stems and spines, even though they're from completely different evolutionary lineages.

Looks can definitely be deceiving.

All right, so we've got the naming system down with taxonomy, and we know we need to focus on homologous traits, not just lookalikes.

The next step was developing a rigorous method to actually build that evolutionary tree, and that's where cladistics comes in, right?

Exactly.

Cladistics is the dominant method used today for figuring out phylogenetic relationships.

It works by identifying those true evolutionary groups, which we call clades or monophyletal groups, and the key is finding shared derived characters.

Technical term is synepomorphies.

These are basically new traits, new character states that popped up in a common ancestor and were then inherited by all of its descendants, like say the evolution of wood in plants.

To know if a character is derived or new, you need a point of comparison.

So you use an outgroup that's a taxon you know is closely related to your study group, but definitely outside of it.

The outgroup helps you figure out which character states are ancestral versus which ones are the new derived ones.

Okay, so you analyze these shared derived characters, and the result is a cladogram, which isn't quite a full phylogenetic tree, but more like a hypothesis of relationships.

Precisely.

A cladogram is a branching diagram showing how groups are related based on those

synepomorphies.

Really important point.

It doesn't mean one group on the diagram gave rise directly to the next one along the branch.

It just shows that groups branching off from the same point, called sister groups, share a more recent common ancestor with each other than they do with groups further down the tree.

And when you have different possible cladograms from your data, you generally apply the principle of parsimony.

Parsimony, meaning the simplest explanation.

Exactly.

The cladogram that requires the fewest evolutionary changes, the fewest appearances of new traits or reversals back to old ones, is generally preferred as the best hypothesis.

The one that assumes the most homology and the least analogy, basically.

Can you walk us through a quick example, like with those plants mentioned,

hornworts, ferns, pines, oaks?

Sure.

Let's say we use hornworts as our outgroup, because they lack a lot of features the others have.

Okay, so first we see that ferns, pines, and oaks all share vascular tissues, xylem, and phloem, which hornworts lack.

That unites them as a monophyletic group, a clade.

Then we look further.

Pines and oaks both have wood and seeds, which ferns lack.

So that tells us pines and oaks share a more recent common ancestor with each other than either does with ferns.

They're sister groups relative to ferns.

Got it.

And then if we add flowers, only oaks have those.

So that further resolves the relationships within that seed plant group.

Each shared derived character helps us build a more detailed picture step by step following that parsimony principle.

Okay, cladistics using physical traits was a huge step forward.

But the real game changer, especially recently, seems to be molecular systematics, right?

Looking directly at the genes.

Oh, absolutely.

It's been a revolution.

Moving beyond morphology, beyond what things look like, down to the level of DNA and RNA sequences.

Molecular data has some massive advantages.

For one, it's much easier to quantify your comparing sequences of A's, T's, C's, and G's.

You also get vastly more characters to analyze, potentially thousands or millions of base pairs compared to maybe dozens of morphological traits.

And maybe the biggest advantage is that it lets you compare organisms that look incredibly different, organisms you'd never think of comparing based on looks alone.

Plus, we know that differences in the sequences of homologous genes, especially mutations that don't really affect function neutral mutations, tend to accumulate over time at a roughly predictable rate.

They act like molecular clocks, helping us estimate when different lineages diverge from each other.

And the power of combining this molecular data with the traditional morphological studies, it's incredible.

You mentioned the angiosperm phylogenic group, the APG.

Yes, the APG is a fantastic example.

It's this huge international collaboration of systematists.

They have synthesized vast amounts of molecular data, primarily DNA sequences, along with morphological data.

And the result is this consensus phylogeny of this family tree for flowering plants that has really well -supported positions for almost every single plant family.

It's fundamentally rewritten our understanding of flowering plant evolution.

And led to some real surprises, I bet.

Oh, definitely.

Some groupings are just mind -blowing based on what we thought before.

Get this, the giant parasitic flower rafflesia, you know, the huge stinky one.

Molecular data puts it in the same order, Malpurgialis, as the poinsettia, which has these tiny inconspicuous flowers.

Morphology alone would never, ever have suggested that.

Wow.

Another one, there are about 10 families of flowering plants that can fix nitrogen.

People used to think this ability evolved independently many times, but molecular data shows almost all of them actually belong to a single large clade.

So the predisposition likely evolved just once, much earlier than we thought.

That's fascinating.

And the water lotus example.

Right.

The water lotus nolumbo.

Looks so much like a water lily.

Everyone assumed they were close relatives.

Nope.

Molecular data clearly shows its closest relatives are actually trees like sycamores, placnus, and the protease family, which includes things like macadamia nuts.

Totally unexpected.

So if we're talking plants, where does most of this molecular data come from?

You mentioned chloroplast DNA.

Yes.

For plants, the chloroplast genome has been the workhorse.

It's a separate circle of DNA within the chloroplast, much smaller than the main nuclear genome, maybe 135 ,000 to 160 ,000 base pairs.

It's just more to sequence and analyze.

It has a distinct structure, often with two sections called invoided repeats separating large and small single copy regions.

And within that chloroplast DNA,

is there a specific gene that's been particularly useful?

There is.

The RBCL gene has been incredibly important, especially for looking at broad relationships among large groups of plants.

It codes for part of the rubisco enzyme, the key enzyme in photosynthesis.

So pretty much all photosynthetic eukaryotes and cyanobacteria have it.

It also evolves relatively slowly, which is good for comparing ancient groups.

It's usually present as a single copy, doesn't have introns which complicate things, and it's long enough about 1 ,428 base pairs to hold a lot of useful information.

Another useful chloroplast gene is at PB.

Mitochondrial genes in plants tend to evolve too slowly for resolving relationships between closely related species, though they're useful sometimes.

And nuclear genes are being used more and more now, too.

This explosion of genetic data also led to something called DNA barcoding, didn't it?

Like a genetic fingerprint for species.

Exactly.

It was inspired by the universal product code you see on products.

The idea, pioneered by Paul Hebert for animals, was to find a short, standardized gene region that could reliably distinguish most species.

For animals, a mitochondrial gene called COX1, or C -O -1, works really well for the most part.

But not for plants.

Right.

COX1 generally evolves too slowly in plants to tell closely related species apart.

So after a lot of research, the consensus for plants landed on using two regions from the chloroplast DNA, parts of the RBCL gene we just talked about, and another one called MAPK.

Using these two together, researchers found they could correctly identify about 72 % of species and, importantly, distinguish between 100 % in genera.

It's not perfect for every single species, but it's a very powerful tool.

And the big advantage is speed and convenience.

Absolutely.

You can potentially identify a plant from just a tiny fragment, a piece of leaf, root, even pollen without needing flowers or fruits, which might only be present for a short time.

Think about identifying seedlings or timber or powdered herbs.

It's incredibly useful.

And connecting this back briefly, tools like DNA barcoding combined with things like high -resolution satellite imagery, like Google Earth, are really changing biodiversity discovery and conservation.

People spot potential new habitats from space, like the Mount Mabu rainforest in Mozambique discovered via satellite.

Then scientists go in, collect samples, use DNA barcoding.

It's integrating technology in amazing ways.

Hashtag, tag, tag, tag, four, the major groups of organisms and the origin of eukaryotes.

Okay.

Let's pull back from the specific methods and look at the really big picture again.

How has our understanding of the major divisions of life itself changed over time?

I mean, for a long time after Linnaeus, the dominant view was basically two kingdoms of life, plants and animals.

If it wasn't obviously an animal, it got lumped in with the plants, including fungi, algae, bacteria, everything else.

Kind of a messy arrangement.

Very messy.

But then in the 20th century, advances like electron microscopy and better biochemical techniques made the huge difference between prokaryotic cells like bacteria lacking a nucleus and eukaryotic cells like plants and animals with the nucleus really clear.

That led to recognizing a separate kingdom, Monera, for the prokaryotes.

But the really big shakeup came later, right?

With Karl Woese.

Exactly.

In the 1970s, Karl Woese and his colleagues started comparing the sequences of ribosomal RNA, specifically the small subunit RNA, across all sorts of organisms.

This molecule is fundamental to life and changes very slowly, making it great for looking at deep evolutionary history.

Their findings completely rewrote the bacteria, archaea and eukarya.

Okay, so bacteria and archaea are both prokaryotes, no nucleus, and eukarya are the eukaryotes.

Right.

But here's the kicker.

Well, Woese's work showed that archaea, despite being prokaryotes, are actually more closely related evolutionarily to eukarya than they are to bacteria.

That was a huge surprise.

We share a more recent common ancestor with archaea than archaea do with bacteria.

That's fundamental.

So within the eukarya domain, we used to have those four familiar kingdoms, protista, fungi,

anemolia, and plantae.

How has that held up?

Well, fungi, anemolia, and plantae are still considered valid kingdoms, largely in monophyletic groups.

But protista, not so much.

Molecular systematics has shown very clearly that the organisms traditionally lumped together as protists, mostly single -celled eukaryotes, are not a monophyletic group.

They're scattered all across the eukaryotic tree.

So now, the thinking is that the domain eukaryote is composed of maybe seven major supergroups.

Think of it as a level between domain and kingdom.

Most of these supergroups consist entirely of organisms we used to call protists.

Fungi and anemolia are actually found together within one supergroup, the orpistoconta, and the kingdom plantae, the land plants, along with their closest algal relatives, form the core of another supergroup.

Wow, so the whole kingdom -protista concept is basically obsolete.

Okay, let's dive into that transition then, the origin of the eukaryotic cell itself, from simpler prokaryotic ancestors.

How did that incredible transformation happen?

The leading hypothesis for the origin of two key eukaryotic organelles, mitochondria and chloroplasts, is the serial endosymbiotic theory.

The core idea is that these organelles were originally free -living bacteria that were engulfed, taken up by an ancient host cell, probably an early archaeon or something related.

But instead of being digested, they established a permanent residence inside the host.

They became endosymbionts, organisms living inside another.

And serial implies it happened in steps.

Yes, definitely.

The evidence strongly suggests that mitochondria came first, before chloroplasts, so step one was acquiring mitochondria.

Step two, in the lineage leading to plants and algae, was acquiring chloroplasts.

But even before that, the host cell itself likely underwent some major changes.

It probably lost its rigid prokaryotic cell wall, allowing it to become larger and change shape more easily.

It likely developed the ability to engulf particles from the outside endocytosis, maybe initially for feeding.

This required a more flexible plasma membrane, probably incorporating sterols, and an internal scaffolding, a cytoskeleton, to support the larger size and movement, and the nucleus.

That might have formed from infoldings of the plasma membrane that surrounded the DNA.

Other internal membranes probably formed the endomembrane system, like the ER and GULTI.

Okay, so you have this evolving host cell becoming more complex.

Then comes the really interesting part, adopting these bacteria.

Right.

Instead of just eating them, it formed a stable partnership.

For mitochondria, the evidence is very strong that they originated from an alpha proteobacterium that was engulfed by an ancestor of essentially all eukaryotes living today.

This bacterium was probably aerobic, good at using oxygen to generate energy, which would have been a huge advantage for the host.

And chloroplasts came later, in some lineages.

Exactly.

Chloroplasts are descendants of cyanobacteria, photosynthetic bacteria, that were engulfed.

And this seems to have happened multiple times in different ways.

We talk about primary endosymbiosis.

That's when a eukaryotic cell engulfs a cyanobacterium directly.

The resulting chloroplast, or plastid, typically has two membranes, its own original two.

This happened in the ancestor of red algae, green algae, and a small group called glocofites.

This lineage includes the ancestor of all land plants.

Okay, primary is one eukaryote eating a cyanobacterium.

Then you have secondary endosymbiosis.

This is wild.

It's when a different eukaryotic cell engulfs another eukaryotic cell that already has a primary plastid.

The engulfed cell becomes the new plastid, often retaining three or even four membranes around it, remnants of the multiple engulfing events.

This happened independently in several lineages, like euglinids, dinoflagellates, and the straminopiles, which include brown algae and diatoms.

Whoa.

And tertiary - eukaryote that has a secondary plastid.

This is known in some dinoflagellates.

Plastids in these cases can have more than two membranes too.

A key point in all these symbiosis is that over evolutionary time, most of the genes from the original endosymbiont bacterium got transferred to the host cell's nucleus.

So modern mitochondria and chloroplasts can replicate themselves, but they've lost most of their own genes and can no longer live independently outside the host cell.

You can actually see modern parallels, like the ciliate bordicella, which sometimes hosts the green alga chlorella inside it, a living example of endosymbiosis happening today.

Hashtag hashtag five.

The protists and eukaryotic kingdoms, revisited for characteristics.

Okay, so that explains the origin of key eukaryotic features.

Let's quickly recap the defining traits of those three big multicellular eukaryotic kingdoms we still recognize.

Fungi, anemolia, and plantae, starting with kingdom fungi.

Right.

Fungi are typically non -modal filamentous eukaryotes.

They absorb their nutrients from the environment.

They don't ingest food like animals or photosynthesize like plants.

Their cell walls contain chitin, which is different from plants.

They often have quite complex reproductive cycles.

And as we mentioned, molecular data shows they're actually more closely related to animals than to plants, which still surprises some people.

Kingdom animalia.

Animals are multicellular, and they crucially lack cell walls, plastids, and photosynthetic pigments.

Their nutrition is primarily ingestive.

They eat things.

They generally show a high degree of tissue specialization and differentiation.

They're typically modal at some stage of life, and reproduction is predominantly sexual.

And finally, kingdom plantae.

Kingdom plantae includes the land plants, that's the three phyla of bryophytes like mosses, and the seven phyla of vascular plants like ferns, conifers, and flowering plants.

They are multicellular photosynthesizers adapted for life on land.

Their cell walls are primarily made of cellulose.

They show significant structural differentiation into organs like roots, stems, and leaves with specialized tissues.

But the single unifying feature that defines plantae, the land plants, is the presence of a multicellular embryo that develops within the tissues of the Okay, so fungi, enamelia, plantae are relatively well -defined kingdoms, which leaves the protists.

Right.

As we said, protists is no longer a formal kingdom because it's not a monophyletic group.

It's basically a catch -all term for all the eukaryotes that aren't fungi, animals, or plants.

It's an incredibly diverse, paraphyletic grouping, meaning it includes a common ancestor, but not all of its descendants.

It includes unicellular organisms, colonial forms, simple multicellular ones.

It lumps together heterotrophic organisms, often called protozoa, and various groups of autotrophic algae, like the green algae, which we now know are the direct ancestors of land plants.

So yeah, a very mixed bag, hashtag, tag, tag six, life cycles, and deploy.

All right, one last major concept to unpack.

The different types of sexual life cycles we see across these diverse eukaryotes, it seems there are three main patterns.

That's right.

There are three principal types of life cycles involving sexual reproduction, and the key difference between them is when meiosis, the cell division that halves the chromosome number, occurs relative to fertilization.

Let's start with what might be the simplest zygotic meiosis.

Here, the dominant phase of the life cycle is haploid having one set of chromosomes.

Two haploid cells fuse to form a diploid zygote, two sets of chromosomes, but that zygote is the only diploid cell.

It immediately undergoes meiosis to produce new haploid cells or individuals.

You see this commonly in fungi and some algae like chlamydomonas.

Okay, so zygote is the only diploid stage.

What's next?

Next is gametic meiosis.

This is the pattern familiar from most animals, including us.

Here, the dominant individual is diploid.

This diploid organism undergoes meiosis to produce haploid gametes, sperm, and eggs.

These gametes are the only haploid cells in the cycle.

They fuse pretty much immediately during fertilization to form a diploid zygote, which then grows into a new diploid individual through mitosis.

Diploid organism makes haploid gametes.

The third type is sporic meiosis, which is also known as alternation of generations.

This is the characteristic life cycle of all plants and also many algae.

In this cycle, there are two distinct multicellular generations.

There's a diploid generation called the scorophyte, which produces haploid spores through meiosis.

These spores don't fuse.

Instead, they divide by mitosis and grow into a multicellular haploid generation called the gamophyte.

The gametophyte then produces haploid gametes, sperm, and eggs through mitosis.

These gametes fuse during fertilization to form a diploid zygote, which then grows into a new diploid sporophyte, completing the cycle.

Exactly.

Sometimes these two generations look identical.

We call that isomorphic alternation of generations.

But often, especially in land plants, they look very different.

That's heteromorphic alternation of generations.

There's a clear evolutionary trend in plants.

In the bryophytes, like mosses, the gamophyte generation is the dominant photosynthetic phase.

But in vascular plants, like ferns and especially seed plants, the sporophyte generation became much larger, more complex, and nutritionally independent, while the gametophyte became highly reduced.

Think of the tiny pollen green and the embryo sac in flowering plants.

Looking at these life cycles, especially that trend towards a dominant diploid sporophyte in plants, what's the bigger evolutionary takeaway?

Why diploidy?

Well, the evolutionary advantage of being deployed for a larger portion of the life cycle is thought to be significant.

Having two sets of chromosomes allows for storing more genetic information.

It provides genetic redundancy if one copy of a gene is damaged, the other can still function.

It also allows for more complex patterns of gene expression, like dominance and recessiveness, potentially masking harmful mutations, or allowing for novel reactions between alleles.

This greater genetic complexity and stability might explain why the diploid sporophyte generation became the larger, more complex, and ultimately dominant phase in the most successful groups of terrestrial plants.

It seems to have conferred a real advantage for adapting to life on land.

Hashtag, tag, tag, outro, Westgale.

And just like that, wow, we've covered a huge amount of ground journeying through the fascinating science of how we understand biological diversity.

We've seen the progression from, you know, Linnaeus's early attempts at naming and classifying based on visible features, right up to the incredible power of sequencing DNA to redraw the entire tree of life.

It gives us such a profound and constantly deepening view of life's vast interconnectedness.

It really does.

And it highlights how dynamic Systematics is.

It's absolutely not a finished science.

It's an ongoing discovery process.

Every new fossil find, every new genome sequenced, helps refine our understanding of life's evolutionary journey and how all living things are related.

The tree keeps getting clearer.

Absolutely.

And that leaves us with maybe a final thought for you, our listener, to ponder.

Thinking about how rapidly technology is advancing molecular techniques, computing power, AI.

What currently unimaginable methods might future Systematists develop?

What new tools might they use to uncover even more hidden branches on the planet?

How might those future discoveries reshape, perhaps even fundamentally alter, our deepest understanding of biodiversity and our place within it?

Something to think about.

From all of us here at the Deep Dive, thank you so much for joining us on this exploration.