Chapter 24: Evolutionary Genetics & Human Origins

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Today we're tackling some pretty big questions, the kind the artist Paul Gauguin put on a painting back in 1897.

Where do we come from?

Where are we going?

Big stuff.

Huge questions.

But we're approaching them from maybe a different angle today, the scientific one,

specifically evolutionary genetics.

Right.

So this is the geneticists take on those existential questions.

Exactly.

We're doing a deep dive into the core ideas, the mechanics, you know, the discoveries that show how life evolves at the genetic level.

We're basically tracing the whole idea of evolution, starting with Darwin, hitting that key moment with Mendel and bringing it right to modern molecular biology.

Sounds like a plan.

Let's start at the beginning then.

1859,

Darwin, the origin of species.

Which just completely shook things up, the idea that species weren't fixed, that they changed over time.

Yeah, he saw this constant struggle, competition for resources, and argued that species adapt.

They change slowly, driven by this relentless competition.

And his big mechanism was natural selection.

Basically, if you've got traits that help you survive and reproduce better in your environment, your fitness is higher.

Yeah.

Then those traits tend to get passed on, become more common.

That's how species change.

Simple, yet revolutionary.

It was incredibly insightful.

But there is a major hole in the theory.

Okay.

Darwin couldn't explain where the variations came from in the first place.

Or, crucially,

how they were inherited.

He knew they were passed down somehow, but the how was a complete black box.

Right, he didn't have the inheritance part.

That was the big stumbling block.

Absolutely.

For decades, people had ideas like blending inheritance traits, mixing like paint.

Which would dilute any good new trait pretty fast.

Exactly.

It wouldn't work with selection.

So the theory, while powerful, lacked a solid foundation until, yeah, until Mendel.

Around 1900, when Gregor Mendel's work on pea plants was rediscovered, that was the key.

That was the absolute key.

Suddenly, it clicked.

Traits are passed on by discrete things.

Genes carried in eggs and sperm.

They don't blend away.

Ah, so that provided the mechanism Darwin was missing.

Precisely.

And merging Mendel's rules with Darwin's selection principle.

That was the birth of evolutionary genetics.

Later, folks like Wright, Fisher,

Haldane really formalized it mathematically.

Okay, so foundation laid.

Genetics and evolution combined.

Where did the researchers go next?

Logically, they went back to Darwin's starting point.

Variation.

Darwin himself stressed how important variation was without it.

Natural selection has nothing to choose from.

Right.

So the focus shifted to actually measuring and understanding this variation.

People started documenting it at sort of three main levels.

What's the first level?

The most obvious one.

Phenotypic variation.

The physical differences you can see.

We call it polymorphism when there are multiple distinct forms in a population.

Like different colored snails or coat colors and squirrels.

Perfect examples.

Or think about human blood types.

A, B, A, B, O.

That's polymorphism too.

Just visible or easily testable differences within a species.

Got it.

Observable stuff.

What's level two?

Moving deeper.

Chromosomal variation.

This was pioneered by Theodosius Dobzhansky working with fruit flies, Drosophila.

Fruit flies again.

Why them?

They have these amazing giant chromosomes in their salivary glands.

Polythene chromosomes where you can actually see banding patterns that correspond to genes.

So you could literally see the genetic structure.

You could.

And Dobzhansky found different populations had different chromosome structures like segments flipped around in versions.

He even named them like Arrowhead and Pike's Peak.

And these different structures mattered.

Oh yeah.

Their frequencies changed with the seasons or over decades.

It was direct evidence that the genetic makeup of a population was shifting, evolving in response to the deepest level.

Molecular variation.

This really took off in the mid sixties.

Lewontin, Hubby, Harris.

They used protein gel electrophoresis.

Right.

Separating proteins based on charge and size.

What did that tell them?

It let them see subtle differences in the amino acid sequences.

A small change can make a protein move faster or slower through the gel.

So they could detect hidden variation.

Variation that didn't necessarily show up in the phenotype.

Exactly.

It was the genome wide.

And it gave us standard measures like the proportion of genes that are polymorphic or how heterozygous individuals are on average,

quantifying variation.

From visible traits to chromosomes to proteins.

But the ultimate resolution has to be the DNA itself, right?

Absolutely.

DNA sequencing changed everything.

Martin Kreitman's work on the add gene alcohol dehydrogenase in fruit flies was a landmark.

What did sequencing that gene reveal?

It revealed every single nucleotide difference between different copies of the gene.

And interestingly, most differences were in the non -coding bits, the introns and flanking regions.

Okay.

But what about the coding parts, the exons?

There you see two main types of changes.

First, synonymous substitutions.

These nucleotide changes may be in the third position of a codon that don't change the amino acid sequence because the genetic code is redundant.

The wobble position.

Okay.

And the other type.

Non -synonymous substitutions.

These do change the amino acid.

And the balance between these two tells us something important.

It tells us a huge amount.

Synonymous changes are usually neutral.

They don't affect the protein.

So they accumulate pretty freely by random drift.

Okay.

But non -synonymous changes alter the protein.

If that change is bad for function,

natural selection weeds it out really fast.

So you see far fewer non -synonymous changes accumulating in most genes.

So the ratio of non -synonymous to synonymous changes is like a footprint of natural selection acting at the DNA level.

Precisely.

It's selection made visible.

When a gene is under strong functional constraint,

non -synonymous changes are rare.

It's an incredibly powerful way to see evolution happening at its most fundamental level.

That's amazing.

It really connects the molecular details back to Darwin's big idea.

So this ability to read DNA sequences leads into molecular evolution using DNA as a historical record.

Exactly.

DNA molecules are like documents of evolutionary history.

They accumulate changes, mutations, rearrangements over time.

We can read that history.

How do you read it?

How do you reconstruct the history?

We build phylogenetic trees.

These are diagrams showing evolutionary relationships.

It usually involves four main steps.

First, you align the DNA or protein sequences from different species.

Then you calculate how similar or different they are.

Third, you group the most similar sequences together.

And finally, you arrange these groups onto a tree diagram that represents their evolutionary descent.

Can you give an example?

Sure.

Take the hominode primates,

humans, chimps, gorillas, orangutans, gibbons.

If you sequence their mitochondrial DNA,

you can build a tree.

And how do you decide which tree is the right one?

There could be many possibilities.

We often use the principle of parsimony.

It basically means we prefer the simplest explanation,

the tree that requires the fewest evolutionary changes, mutations, to explain the differences we see in the sequences today.

Occam's razor for evolution.

Kind of, yeah.

And when you apply parsimony to the primate mitochondrial DNA,

the result is consistently clear.

Humans and chimpanzees are each other's closest relatives.

Okay.

And this idea of change is accumulating over time.

That links to the molecular clock, right?

Yes.

The idea here is that for a given gene or protein,

mutations might accumulate at a relatively constant rate over long evolutionary time scales, like a clock ticking.

So if you know when two species split based on fossils.

Like humans and mice, maybe 80 million years ago?

You can count the differences in a specific protein, say alpha globin, figure out the rate of change, and then use that rate to estimate divergence times for other species where the record is patchy.

That's the core idea.

It was revolutionary for dating evolutionary events.

But I remember reading that this clock isn't perfect.

Different molecules evolve at wildly different speeds.

That's a crucial point.

It's not one universal clock.

Some proteins, like fibropeptides involved in blood clotting, evolve super fast.

Others, like histones that package DNA,

evolve incredibly slowly.

Why the huge difference?

It comes down to functional constraint.

How much can that molecule change without messing up its vital job?

Histones are critical for DNA structure.

Almost any change is harmful and gets eliminated by selection.

They're highly constrained.

So they evolve slowly.

Very slowly.

On the other hand, pseudogenes broken,

non -functional copies of genes have basically zero constraint.

They accumulate mutations rapidly because changes don't matter.

So the speed limit on evolution depends on how important the part is.

Exactly.

Functional importance dictates the evolutionary rate.

Parts under strong negative selection change slowly.

Parts under weak or no selection change faster.

Okay, that makes sense.

So we have variation, selection acting on it, DNA changing over time.

But how do these molecular changes actually create new features, new adaptations we see in organisms?

Right.

Connecting the molecular to the phenotypic.

The chapter highlights three key molecular mechanisms.

First, gene duplication.

Making an extra copy of a gene.

Yep.

The classic example is the globin gene family.

Started with one ancestral oxygen carrying gene maybe 800 million years ago, a duplication event happened.

And the copies went their separate ways.

Exactly.

One copy could maintain the original function, while the other was free to mutate and evolve a new related function.

That's how we got myoglobin for muscle oxygen and hemoglobin for blood oxygen transport from one common ancestor via duplication and divergence.

Creates novelty by freeing up a copy to experiment.

Okay, what's mechanism two?

Exon shuffling.

This is more a eukaryotic thing because our genes are split into exons, coding bits, and introns, non -coding bits.

Right.

Evolution can shuffle these exons around, combining exons from different genes to create entirely new proteins with novel combinations of domains.

Like building with molecular Lego bricks.

That's a great analogy.

The TPA gene tissue plasminogen activator involved in dissolving blood clots looks like it's built from exons borrowed from at least four other distinct genes.

A mosaic protein created by shuffling.

Clever.

And the third mechanism.

Changes in gene regulation.

This might be the most powerful for big evolutionary leaps.

It's not necessarily changing the protein the gene makes, but changing when and where that gene is turned on or off.

Controlling the timing and location of gene expression.

Precisely.

Small changes in the regulation of important developmental genes, like homey box genes that control body plan, can have massive effects on morphology, changing limb structures, body segments, things like that.

So subtle tweaks in regulation can lead to major physical changes.

Definitely.

Okay, so these molecular changes drive adaptation.

Now, how do we get entirely new species?

Let's talk speciation.

Right.

How does evolutionary genetics define a species?

Fundamentally, it's about gene flow.

A species is a group of organisms that can interbreed and exchange genes.

They share a common gene pool, but they are reproductively isolated from other such groups.

Reproductive isolation, that's the barrier.

That's the key.

And we talk about two main types of barriers or isolating mechanisms.

Which are?

Presigotic mechanisms.

These act before fertilization happens.

Things that prevent mating or prevent a hybrid zygote from forming.

Living in different habitats, mating at different times of the year, having different courtship rituals.

Exactly.

Or even physical incompatibility of reproductive organs.

Okay.

All prevent the sperm and egg from meeting.

Okay.

And the other type?

Poseigotic mechanisms.

Yeah.

These act after fertilization if a hybrid zygote is formed.

The hybrid might not survive or might be sterile, like a mule.

So the hybrid is a dead end, genetically speaking.

Right.

It breaks the chain of gene flow.

When these barriers arise between populations, speciation can occur.

And how do these barriers arise?

What are the main modes of speciation?

The most widely accepted and probably most common mode is allopatric speciation.

Allo meaning different.

Patric meaning homeland.

Geographic separation.

Yes.

A physical barrier, a mountain range, a river, an ocean splits a population.

The two isolated groups then evolve independently.

They accumulate different mutations, adapt to slightly different conditions.

Until eventually, even if they came back together, they couldn't interbreed anymore.

They've become separate species.

That's the idea.

Makes intuitive sense.

Is there another way?

Speciation without being geographically separated.

Yes.

That's sympatric speciation, sun meaning same homeland.

This is more controversial and harder to demonstrate, but the idea is that reproductive isolation evolves within a single coexisting population.

How could that happen?

Mechanisms aren't always clear, but may be strong selection for different resource use or changes in mating preferences.

The best examples often cited are some types of insects that become specialized on different host plants or certain fish.

Fish?

Yeah, the situlid fish in some small African crater lakes.

They show incredible diversity, dozens or hundreds of species in one small lake, which is hard to explain by purely geographic isolation.

It seems reproductive isolation evolved rapidly right there.

Fascinating.

Okay, so that brings us back to Gauguin's questions, particularly, what are we?

Let's talk human evolution.

The human story is really interesting from a genetic perspective.

There's this striking paradox.

Which is?

Morphologically, anatomically, we're quite different from our closest relatives, chimpanzees.

Bipedalism, big brains, language, tool use.

Major differences.

But genetically,

our genomes are incredibly similar, over 99 % identical at the sequence level.

Wow, only 1 % difference accounts for all that.

Well, it's likely that a lot of the key differences lie in that small fraction, particularly in gene regulation, like we talked about earlier.

Small changes in when and where genes are expressed during development can have big effects.

So that 1 % packs a big punch.

How does the genetic story align with the fossil record?

Pretty well.

Fossils trace our lineage, the hominins, back in Africa around four to five million years ago, maybe earlier, with things like Ardipithecus, then Australocodificus like Lucy, then the Homo lineage like Homo habilis and Homo erectus.

And genetics add more detail now, like relationships with Neanderthals.

Oh, absolutely.

Sequencing ancient DNA from Neanderthals and another archaic group, the Denisovans, has shown clearly that our ancestors interbred with them after leaving Africa.

Many modern humans, especially outside Africa, carry small amounts of Neanderthal or Denisovan DNA.

So our family tree is a bit more tangled than we thought.

A little bit, yeah.

But another striking genetic finding about modern humans is how little overall genetic variation we have compared to many other species, even other apes.

We're genetically quite uniform, why is that?

It strongly suggests our ancestors went through a population bottleneck relatively recently,

a period where the effective population size was quite small.

This reduces overall diversity.

Okay, and how do we trace our deeper origins using genetics?

We use something called the coalescent principle.

Imagine tracing gene lineages backward in time.

If you take a specific piece of DNA, say mitochondrial DNA, which is only passed down maternally.

Right, from mother to child.

And you look at all the different versions present in people today, you can trace them back until they all merge or coalesce into a single common ancestral sequence in a single woman.

The mitochondrial Eve.

Exactly, and you can do the same thing with the Y chromosome, which is passed only from father to son, tracing back to a Y chromosomal atom.

And what do these analyses tell us about where and when?

They consistently point to these common ancestors living in Africa, somewhere between 100 ,000 and 200 ,000 years ago.

This provides really strong support for the idea that all modern humans originated relatively recently in Africa.

An African origin model.

Yes.

It doesn't mean there were only two people alive then, just that all current mitochondrial DNA traces back to one woman, and all current Y chromosomes trace back to one man from that African population.

That's quite a journey we've covered, from Darwin wrestling with inheritance, to seeing selection in DNA code, to the mechanisms of speciation, and finally, tracing our own species' genetic roots back to Africa.

It really ties together so many threads of biology.

Understanding the genetics is fundamental to understanding the entire evolutionary story.

So, to wrap up, key takeaways seem to be, Darwin needed Mendel, variation is the fuel, DNA is the historical record, evolution has speed limits set by function, and speciation creates the branches on the tree of life, including our own relatively recent branch.

That sums it up pretty well.

And maybe a final thought to chew on related to those evolutionary rates.

We mentioned living fossils like crocodiles or coelacanths,

organisms that look like they haven't changed much in millions of years.

But interestingly,

studies show their DNA sequences are often evolving at rates similar to rapidly changing groups like mammals.

So their molecules are changing, but their body aren't.

Pretty much.

Which raises the question, if the molecular clock is ticking away in these lineages just like in others, what's the real bottleneck?

What's preventing that molecular change from translating into dramatic phenotypic change in these seemingly static lineages?

What's the key difference compared to groups that diversify rapidly?

Hmm.

That is a puzzle.

The link between molecular change and observable change isn't always straightforward.

Something to think about.

Definitely.

Well, this has been a fantastic deep dive.

Thanks for walking us through the core of evolutionary genetics.

My pleasure.

It's a fascinating feel.

And thank you all for joining us.

We hope this gives you a solid handle on the genetic underpinnings of life's incredible history.