Chapter 5: Laws of Variation

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Welcome back to the Deep Dive.

Today, we are cracking open what you could call the engine room of evolution.

We really are.

We're stepping right into the middle of Darwin's monumental work on the origin of species.

And we are tackling the absolute foundational element that makes the whole theory of natural selection possible.

This is chapter five, which he called laws of variation.

You know, this chapter, maybe more than any other, is where Darwin moves beyond just observing change and really tries to understand its rules.

It's a critical bridge in his whole argument.

He's already shown that variability exists.

I have been shaping breeds for centuries, but natural selection, this whole mechanism he's proposing, it just can't operate.

It can't build up advantageous traits unless there's a constant steady renewable supply of this raw material.

Exactly.

The variations themselves.

If every single organism were a perfect fixed copy of its parents, evolution would just stop dead in its tracks.

So our mission today is to move past that, that kind of dismissive phrase you hear all the time.

That variation is just due to chance.

Because that's scientifically lazy, isn't it?

And Darwin acknowledges our deep ignorance about the ultimate sort of primary cause of any single variation.

Yeah.

He's not pretending to know why one flower suddenly appears blue instead of red or why a lamb is born with slightly shorter legs.

He's not claiming to know the chemical mechanism behind it.

But, and this is the key, he argues that while we might not know the ultimate cause, we can analyze the fixed laws that govern how, when, and where that variation shows up.

So it's the difference between asking why did this one change happen, which he admits he doesn't know, and asking under what conditions will differences be produced and what patterns will they follow?

And that second question, the systemic one, that's what chapter five is all about.

And Darwin immediately kicks it off by defining the two primary elements that govern variability, the things that kind of stir up the changes.

Right.

First, you've got the external environment, the nature of the external conditions.

Things like climate, food, soil, whether you're near the sea.

But then, and this is so crucial, he says the far more important factor is the nature of the organism itself.

So think of it like this.

The organism has this innate inherited tendency, a kind of constitution that dictates how it's going to respond to those external pressures.

That's a great way to put it.

If you take a thousand different plants and you expose them all to slightly saltier soil,

their responses aren't going to be random.

One might get thicker leaves, another might change when it flowers, and a third might just die.

The response is governed by the inherited structure of that species, its built -in plasticity.

So this deep dive, we're going to follow Darwin's own systematic approach and explain the patterns these variations follow, how they get excited, how they're inherited, and this is fascinating, how they can even revert to ancient long lost forms.

This whole process is basically what keeps the conveyor belt of evolution constantly stocked with new options for natural selection to pit from.

Okay, let's unpack this first idea then.

Changed external conditions.

Darwin says that conditions, a new climate, a shift in diet, moving to a new place, they act in two main ways.

They either act directly on the organism's whole body, or on certain parts, causing some kind of immediate change.

Or they act indirectly through the reproductive system, which then stirs up variability in the next generation, in the offspring.

And if we focus on that direct action first, that itself can have two very different outcomes, what he calls definite or indefinite results.

And this distinction seems vital for figuring out when the environment is actually shaping the organism versus when it's just sort of opening the door for selection to do its work.

Exactly.

So if the result is definite, it means the organism is basically structured to yield in a really predictable way to that specific condition.

So all or almost all of the individuals change in the same way, a uniform shared modification.

Right.

Darwin brings up this example from Malkin Tandon, who noted that plants grown near the sea often develop slightly fleshy leaves, even if they're not fleshy at all when they grow inland.

That's a predictable,

direct and definite environmental effect on the whole population.

But the indefinite result is, for evolution, much more interesting.

This is when a change in conditions makes the organism what Darwin calls plastic.

So instead of a uniform change, you just get this general fluctuating variability.

The conditions basically upset the system, and that leads to a much wider range of differences across the population,

which, and this is critical,

gives natural selection a much bigger pool of unique traits to choose from and then accumulate.

And here is where the complexity really starts, the difficulty of attribution.

Yes.

There's this fundamental challenge in telling the difference between the definite action of conditions where the environment directly causes a change.

And the accumulative action of natural selection where the environment is just favoring a change that was already there.

Darwin gives a few examples that suggest these slight definite effects are real, but they're just hard to pin down.

He mentions observations from Forbes that shells living at their southern limit or in shallow water are often more brightly colored.

It looks like the direct action of a warmer, sunnier environment.

And Gould saw something similar with birds, that they tend to be brighter under a clear atmosphere.

But then you get this great example, the furrier's test.

It's common knowledge that animals of the same species have thicker, better fur the further north you find them.

And the furrier, just relying on simple observation, would say, well, obviously it's the severe cold climate acting directly on the animal causing it to grow thicker fur.

But Darwin immediately steps in with the selective He asks how much of that thicker fur is really due to the cold's direct action on that one animal and how much is due to selection constantly favoring and preserving the warmest clad individuals generation after generation.

So in that scenario, the climate isn't the cause of the thicker fur variation.

It's just the rigorous selector that determines which existing variation survives to reproduce.

And this whole line of thinking leads him to lay, as he puts it, less weight on the direct action of conditions.

Why?

Because the evidence suggests the direct action is usually pretty slight, and it often just creates that indefinite variability.

Because you see similar varieties pop up under totally opposite conditions.

And on the flip side, you see many species that stay perfectly constant.

They don't vary at all, even when they're living under the most opposite climates.

If the external conditions were the main driver, the results should be way more predictable.

So this is his big intellectual pivot.

It is.

Since the conditions don't consistently produce uniform results, the key factor must be that innate tendency to vary that he talks about, whose ultimate cause is a mystery.

The conditions might just be the trigger that excites the variability that makes the organism plastic, but they don't write the blueprint for the change that results.

So just to clarify this for everyone listening,

conditions can cause variability, which is the raw material, but the conditions also include the engine of natural selection.

Because the conditions determine which variety gets to survive.

When you look at nature, it's selection, the survival of the fittest, that's the agent accumulating variations in a specific advantageous direction, even if the initial spark of change was stimulated by some external factor.

And that brings us perfectly to the internal actions, the effects of use and disuse.

This is a huge one for Darwin.

He observed in domestic animals that use strengthens and enlarges parts, while disuse diminishes them.

And crucially, these modifications are often inherited.

This concept, even though genetics would later refine it, was foundational for him as a source of gradual directional change.

It flows right from the idea that the organism's own nature is the dominant factor.

Exactly.

We can't really observe parent forms in nature easily, but we can look at biological structures that strongly suggest this has been happening for a long, long time.

And the classic example that always comes to mind is the flightless bird.

It seems like such a paradox.

A bird that can't do the one thing we associate with birds.

Right, look at the South American logger -headed duck.

The adults, they lose the ability to fly completely, but the young ones can still fly.

That loss in the adult suggests disuse over its lifetime, which then, through inheritance, gets passed down as a diminished structure in the next generation.

And you see this especially clearly in birds on oceanic islands, like the Galapagos, where there are no ground predators.

Why would you maintain the incredibly complex and metabolically expensive machinery of flight?

You wouldn't.

The nearly wingless state of so many of these island species is almost certainly caused by long -continued disuse.

Natural selection isn't actively killing off the fliers, but it's certainly not rewarding the resources they're spending on strong wings.

Then there's the ostrich, which is a continental example.

It can't fly, but it thrives even with predators around.

Its survival mechanism isn't flight.

It's running and kicking for defense.

So here, the reduction of the wing is likely a combination of two things.

Disuse, it rarely flies, and positive selection favoring the traits that make it a better runner.

Like large, strong legs.

Exactly.

So you can kind of deduce this evolutionary history.

The ancestor of the ostrich was smaller, it could probably fly, but as the bird's size and weight increased over generations, its legs were used more for running and defense, and its wings were used less.

A double action.

Disuse shrinking the wings, selection enlarging the legs, and eventually no more flight.

Now for a smaller and frankly weirder case, you have to look at beetles.

Specifically, dung -feeding beetles.

Okay.

The naturalist Kirby noticed that the anterior tarsiso, the front feet of many male dung -feeding beetles, are often just broken off.

They just break off.

That sounds awful.

It sounds painful.

But it happens habitually, which means they must be losing them early in life.

And in some species, like the Egyptian sacred beetle Atuchus, these front feet are actually just rudimentary or totally missing from birth.

So what's Darwin's take on this?

His analysis is key.

The absence of these feet isn't because of inherited mutilations, the injury itself isn't passed down.

It's due to the effects of long continued disuse.

If the feet aren't that important and they're not used much while they're burrowing, then selection has no reason to maintain them.

And the tendency to shrink caused by disuse just carries on through the generations.

Now we get to a really critical point where we see natural selection actually overmastering disuse.

This is what proves selection is the dominant force.

This is the Madeira beetle paradox discovered by Wollaston.

It's amazing.

Out of 550 species of beetles living on the island of Madeira,

a shocking 200 of them are so deficient in wings that they cannot fly.

200 species.

200.

And of the genera that you need to the island, 23 out of 29 have all their species in this flightless condition.

That is a massive targeted evolutionary change.

So what's the logic here?

It can't just be disuse.

It isn't.

Darwin's cause and effect logic here is just so powerful.

Beetles in many parts of the world fly around a lot, but in a windy exposed place like Madeira, the ones that fly are frequently blown out to sea and they die.

Oh, I see.

So the wind is the selecting agent, not a predator.

Precisely.

The risk of being destroyed at sea is higher than the risk of being attacked on land.

So individuals that flew the least, either because their wings were naturally a bit weaker, which is variation, or because they just had a more ground hooking indolent habit, which is disuse.

They had the best chance of surviving because they weren't getting blown into the ocean.

Right.

The ones that readily took to the air were the ones most often killed.

So selection actively preserved the non -flyers and their wingless descendants.

And Darwin has a brilliant counterpoint to back this up.

He does.

He points out that flower feeding insects, like certain moths on the island, have to use their wings to live.

They have to fly even in the wind to find food.

And for those species, unlike their ground feeding beetle neighbors, their wings are often normal or even enlarged.

Because for them, flying is a condition of survival,

and selection acts accordingly.

It's the ultimate lesson in what fitness means.

If you're a ground feeder on a madera, wings are a dangerous liability.

If you're a flower feeder, they're essential.

It's like his analogy about the mariners.

It's better for good swimmers to swim farther, but for bad swimmers, it's better to just stick to the wreck.

Exactly.

The selective pressure dictates the right strategy.

This combined action of disuse and selection also explains what happens to the eyes of animals that live in total darkness.

Burrowing animals and cave animals.

Take the tucotucco, a burrowing rodent in South America.

Its eyes are rudimentary.

They're often covered by skin and fur.

Since it lives underground all the time, its eyes are useless.

And Darwin was told by a local that these tucotuccos were often found blind because of severe eye inflammation.

So now you have a situation where the eyes are not only useless, they're actively injurious.

Frequent inflammation is a problem, so natural selection would actually aid the disuse.

Reducing the eye size, having the eyelids stick together, fur growing over them, all of that is an advantage because it saves resources and reduces the risk of injury.

But then you have the pure cave animals, the blind crabs and fish in the caves of Carniola and Kentucky.

Here the eyes are just useless in the total darkness.

It's hard to argue they're actively injurious or that they waste a significant amount of energy.

So their loss is attributed mainly to just long continued disuse over countless generations.

And the existence of these blind animals lets Darwin attack the creationist view with a really important comparative argument.

Because if these animals were all separately created for dark conditions, you'd expect them to be very similar all around the world, right?

You might.

But that's not what we see at all.

The entomologist Shiate pointed out that these subterranean faunas are really just small ramifications of the local faunas from the surrounding areas that have penetrated the earth.

So the American cave animals are still clearly related to other North American species, and the European ones are related to European species.

They didn't just appear out of nowhere, perfectly designed for darkness.

They migrated in, they adapted to the darkness, and disuse, probably helped by selection which might have favored longer antennae or palpi to find food,

gradually wiped out their vision.

Parallel but distinct evolutionary paths based on who their ancestors were.

Okay, next up is acclimatization.

How organisms adapt to different climates.

Darwin starts by noting that habit -like a plant's flowering period is definitely hereditary.

Yes, but then he argues that we often overrate how perfectly adapted species are to their native climate.

We just assume a species only lives where it's perfectly suited.

But he says we can tell this is an exaggeration because we're so often wrong when we try to predict if an imported plant will survive.

And on the other hand, many imported species are wildly successful, coming from totally different countries.

This is a major insight.

He stresses that in nature, a species range is often limited by competition with other beings just as much, if not more than, by adaptation to a specific climate.

It's the aggressive neighbors, the parasites, the lack of the right food that limits you, not just the temperature.

But constitutional adaptation is absolutely real.

Dr.

Hooker grew pines and rhododendrons in England from seeds he'd collected at different heights in the Himalayas.

And the plants showed different abilities to resist cold, proving that the constitutional resistance was inherited, not just a response to the English climate.

And domestic animals give us overwhelming evidence of flexibility.

They can withstand incredibly different climates and, crucially, stay fertile, which Darwin calls a far severe test of adaptation.

Because fertility is so sensitive to stress from the climate.

He concludes that this adaptation to a specific climate is probably just a quality that's easily grafted onto an innate, wide flexibility of constitution that's common to most animals.

Like the rat and the mouse, which have been transported everywhere and now thrive from the cold Falklands to northern Norway.

That fundamental flexibility has to be innate.

So the question for this section becomes a synthesis again.

How much of a climatization is just habit or custom?

And how much is the natural selection of varieties that have different innate constitutions?

The ancient agricultural writers, even from China, were very cautious about moving plants and animals, which suggests habit has a strong influence.

But then you look at modern agriculture, like the fruit trees in the United States.

Certain varieties are specifically recommended for the northern states and others for the south.

Since these varieties are so new, their constitutional differences can't be from long habit.

They must be from selection, human selection of innate variations.

And Darwin addresses the skeptics of his day, who pointed to the kidney bean as proof that acclimatization was impossible because it was still so tender to frost after centuries of cultivation.

And Darwin just dismisses this.

He says the experiment has been conducted unfairly.

For selection to work, you need a rigorous selective environment.

He says the experiment hasn't been properly tried until someone plants the beans so early that most of them are killed by frost and then diligently collect seeds only from the few survivors.

And you have to repeat that process for many generations and prevent any accidental crosses.

Right.

Constitutional differences do appear in seedlings and selection has to act on them consistently and aggressively for acclimatization to happen.

So the final conclusion here is that habit and use -disuse have played a part, but their effects are often combined with, and sometimes overmastered by, the natural selection of innate variations.

Variation and selection are the primary drivers.

Habit is the secondary inherited effect.

Alright, let's shift gears now to what Darwin calls correlated variation.

This feels like one of the most complex parts of the chapter.

It is, because it's about unintentional changes.

Correlation means the whole organism is so tied together during its growth that when selection builds up a variation in one part, other parts get modified unintentionally.

Often in ways that are useless, or maybe even harmful.

And this is so important because it explains modifications that arise without selection acting directly on them.

It's like an internal rule of growth that selection then has to either work with or weed out later.

So what are the predictable patterns of this real correlation?

Well, you see it strongly in homologous parts.

Parts that were identical in the early embryo, like the right and left sides of the body or the front and hind legs, they tend to vary in a similar way.

So a mutation in a front leg might show up in the back leg too.

There's a tendency for that, yes.

You also see hard parts affecting soft parts.

Darwin notes some people believe the shape of a bird's pelvis influences the shape of its kidneys.

Or in humans, the shape of the mother's pelvis might influence the shape of the baby's skull.

The soft growings, but the really fascinating ones are the obscure links, where the connection is a total mystery to us, but it's statistically real.

Yes.

What on earth links complete whiteness and blue eyes to deafness in cats?

You can select for white fur and blue eyes, and you get deafness along for the ride.

Or the fact that the tortoise shell color in cats is almost exclusively found in females.

Or his favorite example, pigeons.

He notes this correlation between having feathered feet and having skin between the outer toes.

That's so specific.

It suggests some shared developmental pathway is controlling both feathers and membrane growth, even though one trait is visible and the other is hidden.

Another case is in mammals.

This correlation between abnormal skin, like the lack of fur in whales or the armor of armadillos, and having abnormal teeth.

The parts are physically distant, yet their development seems tied together.

So this brings us to a really crucial point for his theory correlation that's independent of any use or utility.

Right.

And for this, he uses two families of plants.

The compositus plants, like daisies, and the umbiliferous plants, like carrots.

In these plants, the outer and inner flowers can differ dramatically, and those differences are correlated with parts that seem totally unimportant.

He notes that the seeds often differ in shape between the outer ray flores and the central florets, and this is often independent of any visible difference in the

Specifically, in the umbiliferae, the seeds might be what's called orthospermus in the outer flowers, meaning straight seeds, and colospermus in the central flowers, meaning hollow seeds.

Okay, so why does a tiny difference between a straight seed and a hollow seed matter so much?

Because the elder de Candel, who was one of the top botanists of the day, based his main divisions of that whole plant order on these seed characters.

He saw them as fundamental, highly important traits.

He assumed they must be essential to the plant's design.

But Darwin argues it seems impossible that these tiny differences, straight versus hollow or slightly different sculpting on the seed, could offer the slightest service to the plant.

Therefore,

modifications that are considered of high value for classification can be wholly due to these internal laws of variation and correlation,

arising without any influence from natural selection.

That is a total mic drop moment for the creationist view.

It really is.

Darwin is saying that a major structural feature, the very thing systematists use to define large groups, can just be an accidental, internal byproduct of the organism's own developmental rules, not an active choice by a designer.

It just completely undermines the idea of perfect design, where every single feature has to have a perfect purpose.

But selection is still the ultimate editor.

Darwin finishes the section by explaining a rule.

Winged seeds are never found in fruits that don't open.

This isn't a biological correlation like a genetic link.

It's a phytonal limit imposed by selection.

Exactly.

Winged seeds can't provide any advantage unless the capsule opens to let them disperse.

Selection simply can't favor a variation that provides zero benefit to fitness.

Okay, so following that idea of linked parts, Darwin moves to the conservation of resources within an organism, starting with compensation.

He begins with Gerst's law of compensation, or balance of growth.

In order to spend on one side, nature is forced to economize on the other side.

It's this old idea that energy spent in one place has to be pulled from somewhere else.

You see it in domestic animals all the time, right?

Clearly.

A cow gives less milk when you're fattening it for meat.

If a fruit gets much bigger and tastier, the seeds inside tend to shrink.

In poultry, if you breed for a big crest of feathers on the head, the comb usually gets smaller.

Nature reallocates.

Darwin suspects this idea of compensation is actually part of a much broader principle.

He does.

He thinks it merges into the principle that natural selection is always trying to economize resources in every part of the organization, whether or not another part is getting bigger at the same time.

The economy principle is wider than just compensation.

And what's the logic behind that?

It's pure survival of the fittest.

If a structure becomes less useful because of a change in habits or conditions, its reduction will be favored.

Why?

Because the individual avoids wasting nutriment on a useless part.

And even the smallest saving of resources can help in the fierce struggle for life.

Darwin gives this incredible example from his own work on barnacles or syrupedes.

When a barnacle is a parasite living inside another animal, it's protected.

And so it loses, more or less completely, its own complex shell, its carapace.

He talks about the extreme case of a parasite called proteolepis.

In all other barnacles, the carapace is formed by these enormously developed front segments of the head.

It's a highly complex structure.

But in the protected parasitic proteolepis, this entire complex front part of the head is reduced to a tiny, mere rudiment.

So the saving of that large complex structure, which is now totally superfluous because of its parasitic life, is a decided advantage.

Yes.

Selection promotes it simply because it saves energy.

The law of economy is broader than

because selection will reduce a part not to build up another, but simply because getting rid of waste is a good thing in itself.

The resources are just saved, which helps overall fitness.

Now we get into the internal laws of variation, looking at which parts of an organism tend to vary the most.

Darwin sees a couple of key patterns here, starting with multiple and rudimentary parts.

Right.

There seems to be a general rule.

When an organ is repeated many times in the same animal, like the segments of a centipede, the vertebrae in a snake, or the stamens in some flowers,

the number and structure of those parts are highly variable.

But if a part occurs in smaller numbers, it tends to be more constant.

And this variability in multiplied parts is linked to what he calls low organization, or less specialization.

A structure that has to do very, very general work is less rigidly controlled by selection than one that's specialized for a single specific job.

It's like the difference between a general purpose screwdriver and a specialized tool for one specific screw.

That's a great analogy.

The general tool can vary a bit in shape and still work.

Natural selection is just less strict on those non -specialized repetitive parts, so deviations can persist.

And similarly, rudimentary or useless organs are known to be highly variable.

We mentioned the useless eyes in cave fish.

And the reason for that is just so simple and elegant.

If an organ is useless,

natural selection has no power to check any deviations in its structure.

It doesn't matter if the tiny, useless pelvis bone of a whale is a bit bigger or smaller.

It gives no advantage or disadvantage, so the part is free to just fluctuate wildly.

And then Section 7 introduces this paradox that connects right back to his main theory.

The most important, or extraordinarily developed, parts are also highly variable.

That seems backwards.

It does.

The rule is that a part developed in an extraordinary way, compared to the same part in its close relatives, tends to be highly variable.

So we have to be careful with the context.

It's not about a structure that's abnormal for the whole class, like a bat's wing is for mammals.

That's constant across all bats.

Right.

It only applies if one species has a structure that's remarkably developed compared to others in the same genus.

And he illustrates this with the opercular valves of the rock barnacle pergoma.

These valves are really important structures, and they usually differ very little, even between distinct genera.

They're structurally vital.

However, in the various species within the genus pergoma, these valves show a marvelous amount of diversification.

The degree of variation within a single species of pergoma is so great that its varieties differ more from each other than do the species of other distinct genera.

Okay, so on the view of independent creation, that makes no sense.

Why would the most important and most modified part be the most unstable?

You'd think the most specialized future would be the most fixed.

But on the view of descent with modification, it makes perfect sense.

An extraordinarily modified part implies a huge amount of changes happen since that species branched off from its common ancestor.

And that change is relatively recent, geologically speaking.

Exactly.

And since the change is still ongoing, the generative variability, as he calls it, is still high.

Natural selection hasn't had enough time yet to completely fix the organ and stamp out the innate tendency to revert or vary again.

So there's a constant struggle between selection trying to keep the breed true and the tendency for variation to keep popping up.

What is newest is what is least stable.

That logic leads perfectly into the difference between specific and generic characters.

It does.

So generic characters are what all species in a genus share,

the defining features of the group.

Specific characters are the subtle differences between

the unique color or size that sets one apart from another.

And the rule is that specific characters are way more variable than generic ones.

Which, from the view of independent creation, has no rational explanation.

Why should the very points of difference between separately created species also be the points most likely to vary now?

Why aren't the generic characters just as variable?

But the evolutionary explanation gives us a clear timeline.

Unary characters were inherited from a very ancient ancestor, common to the whole group.

They've been constant for a long, long time, so they're stable.

Specific characters, on the other hand, are the parts that have varied since the species branched off.

They're the features that are still actively being modified by selection.

And since they've varied recently, they're likely still varying.

This means a species is really just a strongly marked and fixed variety, and it still has the variability that helped define it in the first place.

The points of difference are the points of change.

Now we can apply a similar idea to secondary sexual characters.

Traits like elaborate plumage, antlers, things attached to one sex but not directly for reproduction.

Naturals all agree these are highly variable, even within one species.

And they differ hugely between allied species.

So why?

Why so variable?

Because they're accumulated by sexual selection, which Darwin says is less rigid in its action than ordinary selection.

Right.

Ordinary selection is life or death.

Sexual selection just means the less favored males have fewer offspring.

It doesn't necessarily mean they die.

So because the criteria for mating success, which can be subjective like fashion or just based on fighting, are less absolute than the criteria for survival, these characters stay less constant and are more prone to dramatic variation.

But here's the crucial length that ties it all back to the ancestor.

The parts that show these secondary sexual differences within a species are generally the very same parts that differentiate the species within the genus.

That is a powerful correlation.

It's huge.

For instance, in some beetle groups, the number of joints in their feet is normally constant across large groups.

But in this one group, it varies greatly and it's different in the two sexes of the same species.

And let me guess, it's also the specific point used to tell the different species in that genus apart.

You got it.

And this relationship has a clear meaning from Darwin's perspective.

Both the species and the sexes descended from a common ancestor.

Whatever part of that ancestor's body first became highly variable,

the hotspot variability was then seized upon by both natural selection to adapt the species to different niches and sexual selection to help with mating and competition.

The historical tendency to vary dictates the future path of evolution.

So we end with two really powerful related concepts that Darwin uses to just cement this theory of common descent.

Analogous variation and the evidence from reversion.

Analogous variation is when distinct species or races that come from a common parent tend to vary in similar ways.

The innate, inherited constitution guides the possible range of variations.

Pigeons,

again.

Widely separating breeds, tumblers, fantails, powders.

They all still sometimes produce sub -varieties with traits the original rock pigeon never had, like reversed head feathers or feathered feet.

This parallel variation happens because they all inherited the same deep -seated constitution, the same tendency to vary, which gets triggered by some unknown influence.

And you see it in plants, too.

The Swedish turnip and the rutabaga, they're closely related, and under cultivation they both develop these enlarged roots and stems.

So if you assume they were created separately, you have to attribute this similarity not to the veracaza, the real cause of common descent, but to three separate yet closely related acts of creation.

The evolutionary explanation is just vastly simpler.

And then there is reversion, the reappearance of long -lost ancestral traits.

This is maybe the most compelling evidence that species share a deep, hidden history.

The classic example is the pigeon, where all the different breeds will sometimes revert to the slaty blue color of the ancestral rock pigeon, complete with the two black bars on the wings, the white loins, and the bar on the tail.

And this tendency to revert is especially strong when you cross two distinct, differently colored breeds.

And the hypothesis here is the principle of latency.

The tendency to produce that lost character has been lying dormant or latent in each generation, for maybe hundreds or thousands of generations, and it occasionally prevails under certain conditions or when it's disturbed by being crossed with another breed.

Darwin argues that the idea of transmitting a tendency for a useless, long -lost color pattern is no more improbable than transmitting a useless rudimentary organ like a whale's pelvic bone.

It's just part of the inherited blueprint.

And this brings us to what is probably Darwin's greatest proof of dissent in this entire chapter, his ultimate mic drop moment.

The complex case of the equine genus horses, asses, zebras.

He uses this evidence to connect the laws of variation directly to the fact of common ancestry.

And it starts simply.

Stripes, like a zebra's, can appear just through simple variation in a pure horse, a pure ass, or a pure hemeonus, especially if they have a dun colored coat.

In horses, the stripes are often clearest when they're a foal, and then they disappear.

He even bred a foal from two bay race horses that, when it was a week old, was covered in narrow, zebra -like bars that soon faded completely.

And in India, the Catiwar breed of horses is so often striped, with a stripe down the spine, barred legs, sometimes even triple shoulder stripes, that an unstriped horse is considered impure.

And these are not hybrids.

This is that pure latent tendency expressing itself.

But the decisive proof, Darwin says, is in hybridization.

Yes.

The common mule, a hybrid of a horse and an ass, is particularly likely to have bars on its legs.

Sometimes nine out of 10 mules in some regions have them.

Crossing two different species, which rarely show stripes themselves,

reawakens the latent trait in their hybrid offspring.

And then there's the really famous case of Lord Morton's mare.

An incredible story.

This mare, a chestnut, was first bred to a male quagga, a striped relative of the zebra, and she produced a hybrid that was strongly barred.

But the amazing part is what happened next.

The pure offspring that she subsequently had with a pure Arabian sire, an unstriped horse, were also faintly barred on their legs and shoulders.

The influence of the first sire, the quagga, had somehow awakened that latent striped pattern in the mare's own reproductive system.

So her subsequent pure offspring showed the ancestral trait.

That is impossible to dismiss.

It shows the tendency was dormant, the cross was the trigger, and the pattern proves that the Arabian horse, the ass, and the quagga all share a history.

And the final case involves a hybrid of an ass and a hemeonus.

Neither parent is usually striped.

But the hybrid had all four legs barred, three shoulder stripes, and even zebra -like stripes on its face.

This cumulative pattern stripes appearing in different pure species, appearing most strongly in hybrids, and being clearest in the young, it all points to one thing.

It points to a common progenitor, striped like a zebra, that existed thousands of generations ago.

The strikes aren't a new creation.

They are a deep ancestral memory written into the very constitution of the entire genus.

Darwin's final statement here is just devastating to the creationist view.

It forces a choice.

You either accept that these laws of variation and reversion prove descent from a common striped ancestor, or you have to attribute this pattern to separate creation, where each species was created with a tendency to mimic the others only when it varies or is crossed.

And Darwin says that second choice is to reject a real for an unreal cause.

He concludes that such a view makes the works of God a mere mockery and deception.

The law of reversion driven by these latent tendencies is proof of common ancestry.

So chapter five really serves as the complete scientific and philosophical defense of variability.

It does.

While we're still ignorant of the ultimate primary cause of any one variation, he says we can assign a reason in hardly one case out of a hundred, the overall process follows these fixed identifiable laws.

Laws conditioned by external factors like use and disuse, and internal constraints like correlation and economy of growth.

And all the laws we discussed, the high variability of recently changed parts, the stability of ancient generic parts, and this powerful mechanism of reversion, they all consistently point toward that single conclusion.

Species are descended from common ancestors.

This whole mechanism provides the constant steady stream of raw fluctuating material that natural selection needs to act upon, ensuring that life never stops diversifying.

Indeed.

And the existence of these latent characters, these ancestral memories, is maybe the most complex implication of all.

It is.

I mean, if the ancestral zebra -like pattern can lie dormant in the constitutional makeup of every horse, ass, and hymenus for thousands and thousands of generations, it raises this enormous question.

How complex must the mechanisms of inheritance be?

And how far back can these powerful specific tendencies persist, just waiting for the right unknown conditions, like a hybrid cross, to bring them forth?

A compelling thought to carry with you about the deep history written into every living thing.

Thank you for diving deep with us today.