Lec 11 Teaching Prep: Part 3 — Cases 5-8, Continuum, Closing
2026-05-05 06:39:35 • 31:37
Lecture 11 Teaching Prep Part 3.
The first TPS cases 5 through 8, the continuum, and the closing.
Welcome back, Michael.
Part 3 covers the first think-pair share, the answer cases that follow it,
the second TPS, the continuum slide, the why-mixing dominates synthesis,
the project workflow, and the exit ticket.
About 20 minutes of audio for around 20 minutes of class time.
This is the payoff.
Everything in parts 1 and 2 has been infrastructure for the moves you make in the last 20 minutes.
The continuum slide is the single most important slide in the lecture.
If you have to choose what to cut, do not cut it.
Cut bonus material on cephalopods or birds of paradise, not the continuum.
Start with the first TPS.
Predict the mechanism mix.
This is 4 minutes total, 3 minutes of discussion plus 1 minute of cold calls.
The setup for animals.
Tree frog.
Mandrel face.
Red and blue.
Neon damselfish flink.
Hummingbird gorge it.
Magenta head on.
Black at angle.
For each, predict the pigment contribution, the structural contribution,
and what diagnostic experiment you would run.
The pedagogical purpose of this TPS is twofold.
First, it is a comprehension check on the framework.
Can students apply grinding and bleaching predictions to novel cases?
Second, the hummingbird is a trick question.
Hummingbird gorge it is not mixed.
It is pure structural.
Hollow flattened melanosomes per Nordenette all 2021.
The point of including it is to make sure students are willing to say this one
is not mixed rather than reflexively assigning a mixed mechanism to every animal you list.
The hints on screen.
Three layers of skin and tree frogs.
Soft tissue mechanism for mandrel red.
I rid of forzaanthophore stacking and damselfish.
Angle dependence in hummingbird.
These are designed to be enough to orient a thoughtful student without giving the answer.
When you launch the discussion, do it crisply.
Three minutes.
Talk to your neighbor.
Go.
Then watch the room.
If it is quiet at minute one and a half, walk in among them.
Don't sit at the lectern.
Light interventions.
Ask one pair what would you predict for the tree frog?
That moves the room.
When you cold call, take four pairs, one organism each.
Don't worry about correctness.
The answers come from the next four slides.
The TPS purpose is to get them committed before the answer reveals so the reveal lands.
The trick question debrief is short.
The hummingbird is the one that is not mixed.
Pure structural.
At normal incidence, the multi-layer peak sits in the rat, so the gorge it looks magenta.
Hold the head, the peak shifts blue out of the visible, and the gorge it looks black.
The point of including it was to make sure you noticed that not every striking color is
a mixed mechanism.
Some are pure pigment.
Some are pure structural.
The continuum will tell us which.
That is your transition into the answer cases.
Before the case slides, there is one quick anatomy primer slide.
The Feather Barb.
Hooper at all 2021.
Scientific reports.
About 90 seconds.
Pre-Zones and a typical color-bearing Barb.
The cortex, where pigment lives.
The Medulla, the air voids that scatter.
The core, the melanin that absorbs.
Most Feather color mechanisms live in the Barb or Barbule, not the racus.
Mixed mechanism means different zones of the same 50 micron column of biology working
as one optical system.
Don't dwell.
The slide exists to give students a vocabulary, so the case studies that follow make sense
at the right level of detail.
Then advance.
Now cases 5 through 8.
These are the project relevant ones.
Paste on a little.
Students have just done the TPS.
Attention is sharp, and you can deliver content fast.
Case 5.
Tree Frogs.
Bagnara 1968.
Foundational paper on dermal chromata-4 architecture.
The three layers.
Santa for is on top with yellow taran or keratinoid pigment.
Iridophores in the middle with guanine platelet stacks producing structural blue.
Melonophores at the bottom with melanin to absorb transmitted light.
The optics.
White light passes through the yellow filter, losing short wavelengths.
Hits the Iridophore stack, reflects blue back up.
Reflected blue passes back through the yellow filter.
Net result, green.
Same blue plus yellow recipe as parrots, but executed in skin tissue with cells instead
of feathers with subcellular nanostructure.
The melanophore at the bottom catches transmitted light, preventing wash out.
Same architectural role as price walled men's hidden black under feathers in songbirds.
Different lineage, same architectural solution.
Convergence again.
Worth saying that out loud.
The students just heard the price walled men result 30 minutes ago.
Tying the mechanism back to a paper they have already heard about builds coherence.
Project Tion.
Refish skin uses this exact recipe.
Most green rasses, parrots fishes and surgeon fishes are three layer chromatofore systems.
Cyrillabrine, group four and aconthurti, group five, your color diversity is largely a story
of Iridophore plus pigment cell stacking.
Not pigment chemistry alone.
Say that out loud.
Your students may have assumed refish color comes from clever pigments.
It mostly comes from clever cell stacking.
The diagnostics are beautiful.
Bleach the xanthophore pigment, the animal turns blue.
Disrupt the Iridophore stack, the animal turns yellow.
Block the melanophore, the animal turns pale.
Three layers, three diagnostic tests, three mechanisms in collaboration.
The tree frog is the cleanest demonstration of the chromatofore stacking framework that
you will see in a vertebrae.
Case six mandrels.
Two minutes.
From and Torres 2004, Journal of Experimental Biology.
The year is 2004, not 2003.
The earlier deck had the wrong year.
Use 2004 if a student asks.
Mandrel blue facial ridges are produced by ordered collagen fiber arrays that scatter
short wavelengths coherently.
Same physics as a blue j-feather in the malean skin tissue.
The red is dense subdermal vasculature plus oxygenated hemoglobin.
Not a pigment cell color at all.
Just blood made visible by a thin transparent overlay of skin.
This is mixed in the patchwork sense, not the same tissue sense.
Blue ridges from collagen geometry.
Red nose from hemoglobin.
Yellow beard from filmelinin in hair.
Three different mechanisms, three adjacent patches, all coordinated for a single sexual
signal and all under hormonal control.
Mirror-ranked males have brighter blue ridges and redder noses simultaneously.
The mandrel face is a multidimensional, honest signaling display.
The teaching moment is also a correction to a generalization you have made earlier.
Mammals only have melanin mostly.
They do only have melanin in their hair.
But in skin, primates have rediscovered structural color by a collagen architecture.
A small primate-specific structural color come back after the nocturnal bottleneck.
Today after the nocturnal bottleneck out loud, that phrase is one of the recurring themes
of the course and your students will appreciate the callback.
Case 7.
Turricos.
Two minutes.
The pigment exception.
Most green birds use the paratric structural blue plus yellow pigment.
Turricos break the rule.
Their green is pure pigment.
Turricoverdin, a copper porphyrin, found in no other animal lineage on earth.
Their red is also pigment.
Turricin, also a copper porphyrin.
No structural component required.
The diagnostic is decisive.
Grind a turicofether, you get green powder.
Grind a parat-fether, yellow powder.
The grinding test discriminates turicos from every other green bird.
White turicos.
Two hypotheses.
One ancestral access to dietary copper supported copper porphyrin biosynthesis and the pigment
innovation preempted the need for structural color.
Two, porphyrin-based color is faster to deploy developmentally than nanostructure.
And turicos are highly social birds where rapid plumage display matters.
Don't pick between hypotheses.
Say we don't know which is right, possibly both, and move on.
The teaching point is that every rule has exceptions.
Next mechanisms are the most common solution, but pure pigment green and pure structural
green both exist in birds.
The grinding test diagnoses, which one you are looking at.
Project tie-in for group 7, holocorin, and group 4, Cyril Lebrini.
If your fish is green, predict whether it is mixed or pure pigment.
Then check the literature.
Most reef fish green will be mixed.
But you will get a much better grade if you actually verify rather than assume.
Case 8, Blue Tang.
Paracanthorus Hepatus.
Group 5's clade, acantherity.
The body is pure iridipore.
Guanine multilayer in the dermis.
The yellow tail is zanthophores with carotenoid plus tern pigments, no iridipore beneath.
The black mask is melanophores in dense aggregation.
Different cells, different densities, different body regions, all in the same fish.
The teaching beat.
Transition zones where blue meets yellow are not pigment mixing.
They are different cell types tiled in different densities at the boundary.
This is what Crototochwell and Malarino 2023, your reading later in the course, would
call developmental geography of the chromatophore stack.
For group 5 specifically, every acantheris, naso, and zebra-soma color pattern can be decomposed
into this framework.
Which cells are present in what layer at what density?
Color pattern is not pigment chemistry.
Color pattern is the developmental geography of where chromatophores end up.
This is going to be a major theme for the second half of the course.
Your diversity at macroevaluationary scales is often diversity in chromatophore deployment,
not diversity in chromatophore biochemistry.
The molecules are old and shared.
The patterns are new and lineage specific.
Pause after delivering that sentence.
It is a load-bearing claim and you want it to land.
Two bonus slides come next.
They are both cuttable if you are running short.
Slides 28 and 29.
Slide 28 cephalopods.
About 60 to 90 seconds if you have it.
Pigment sacs that expand and contract in under a second.
Reflectance, structural proteins that retune Bragg stacks facing via phosphorylation.
Look a force that scatter broad band white.
Same skin patch goes yellow, green, blue, red, in seconds.
Bird feathers are static mixed mechanisms.
Optical state fix to growth.
Fish chromatophores are semi-dynamic.
Both pigment and structural state change in real time under neural control.
Most sophisticated optical machinery in the animal kingdom.
Today is the framework.
Demo 5 on Wednesday is the live observation.
Slide 29 superblocks.
McCoy at all 2018.
Nature communications.
About 60 seconds.
Bird of paradise superblocks.
Reflectance under 0.05%.
Vantiblock equivalent.
Biological.
Modified barbules form a forest of vertical micro spikes.
Light entering bounces between cavities.
Absorbed across many passes.
Mixed mechanism engineers absence.
As well as presence.
Superblock flanking patches around bird of paradise.
Display amplify the iridescent display by improving visual contrast.
Selection produced an entire optical category just to make adjacent display colors look better.
If you are running short on time, skip these two slides entirely.
The continuum is non-negotiable.
The bonus slides are cutable.
Now the second TPS.
Design a mixed mechanism color.
Three minutes total.
Two minutes of discussion plus one minute of cold calls.
The setup is generative rather than diagnostic.
You are an evolutionary engineer.
Your imaginary animal needs to look iridescent purple.
The constraints are explicit on this slide.
Purple shifts with viewing angle, so it must have a structural component.
It is saturated and bright, so it likely needs pigment amplification.
It is visible in low light conditions, so it can't be too directional.
The student's job is to design a recipe.
Which pigment, which structural element, which arrangement, which hidden underlayer.
The pigment palette is melanin, carotenoid, tarin, porphorin.
There are no purple pigments.
They have to make purple from a mix or from physics.
Structural mechanisms.
Then fill multi-layer.
Photonic crystal.
Coherent scatter.
Cell types, melanophore.
Zanthophore, iridophore.
Look-a-for.
This TPS does two things pedagogically.
First, it forces students to apply the framework in reverse
from desired phenotype back to mechanism.
That is a deeper test of understanding than identifying mechanisms in known species.
Second, the constraint that there are no purple pigments forces students to confront the conjugation trap.
Pigments cannot easily make purple.
The chemistry doesn't allow it.
If you want purple, you almost have to use structure.
Two minutes.
Launch it.
Watch the room.
Walk around.
Light interventions if a pair is stuck.
Ask them what color the structural component might produce.
Then what pigment they could pair with it.
When you cold-call, take two or three pairs.
The good answers will look like.
Structural blue from a multi-layer plus a red pigment as a filter.
Or violet structural color from a tilted multi-layer plus a melanin absorber underneath.
Or fluorescent dye plus photonic crystal cavity for directional emission.
There are at least four legitimate solutions.
Don't grade for correctness.
Grade for whether the student articulated a mechanism for each contribution.
The debrief is short and biological.
Nature has solved this problem multiple times.
Purple-throated sunbirds.
Violetier hummingbirds.
Various lampornous species.
Almost always with a structural solution because conjugation link constraints make purple pigments very hard to evolve.
Pigment alone almost never delivers a saturated angular violet.
It requires a structural component.
Say that explicitly.
The TPS converges on a real biological generalization.
And the convergence is the lesson.
Slide 31 is fluorescence amplifies the mix.
About 90 seconds.
All feather under visible and UV.
Same as the vucu-cic papillomechanism we covered earlier.
Pigment fluorescence amplified by photonic structure.
Examples.
Budgerigar yellow head with fluorescence and spongy keratin.
Several reef fish with GFP-like proteins layered with iridophores.
Mantis shrimp-telson.
Scorpion cuticle.
Honest signal angle.
Fluorescent plus structural mix signals are particularly hard to fake.
You need both pigment biochemistry and structural integrity.
60 to 90 seconds and move on.
Slide 32 is why mixing wins three ways.
About 60 seconds.
Three properties.
Saturation extremes.
Super black bot plus ultra white syphocolous.
Heutinability.
Peacock four colors from one molecule.
Multi-axis honest signal.
Mandrel face.
Common threat.
Mixing expands the optical and informational space accessible to evolution.
This is a transition slide.
Don't linger.
Now the continuum slide.
This is the slide of the lecture.
Everything before the slide has been infrastructure.
Everything after the slide is application.
About three minutes.
Open with the verbal frame.
Throw out the binary table.
Instead, think of pigment and structure as a continuum with six positions.
Say throw out the binary table out loud.
Students need to feel the framework shift in real time.
The slide has six photographs across the top.
Turico, Western Tanager, Papilio, Amazon Parrot, Peacock I spot Morpho.
Then a numbered table underneath with the same six labels and exemplars.
Walk down the six positions deliberately.
Slow your pace by 20% compared to the case studies.
Each position deserves its own breath.
Position one.
Pure pigment.
Turico.
Grind.
The color survives entirely.
The mechanism is selective absorption.
Full stop.
Diagnostic.
Grinding leaves the color intact.
Bleaching destroys it.
Position two.
Pigment plus reflective backing.
Western Tanager.
Karatanoid layer with white spongy keratin underneath.
The white layer amplifies the pigment by sending transmitted light back through it.
Grind.
Duller color.
But present.
The structure amplifies.
Doesn't produce.
Position three.
Pigment plus structural amplifier.
Papilio swallowtail.
Papilio chrome plus cighten multi-layer.
Multi-layer cavity changes how the pigment radiates light.
Grind.
Pigment survives.
Brightness drops.
The structural element changes how the pigment radiates light.
Position four.
Pigment plus structural producer.
Green parrot.
Cidico-fulvin plus structural blue from spongy keratin.
Grind yellow only.
Bleach blue only.
Each component independently removable.
Each contributes its own color and the two combines subtractively.
Position five.
Structural plus pigment building block.
Peacock.
Dual beetle.
The pigment I.S. the structure.
Grind.
Vanishes.
Bleach also vanishes.
Both diagnostics destroy the color because pigment and structure are not separable.
Position six.
Pure structure.
Morpho butterfly.
Hummingbird gorge it.
Grind.
Vanishes.
Bleach.
No effect.
Geometry alone produces the color.
No pigment is involved.
Then deliver the synthesis sentence at full energy.
The grinding and bleaching test pair locates an animal on this continuum.
Two diagnostics.
Six positions.
That is your framework for analyzing any animal color from now until the end of the course.
Pause after that sentence for at least three full seconds.
Don't rush.
The students are looking at the slide and processing the framework.
Let them have it.
If you have time and only if you have time, do a brief review by pointing to two cases and asking the room where on the continuum.
The peacock should land at five.
The tree frog should land at three or four depending on whether they emphasize the synergistic interaction.
There is no single correct answer.
The continuum is meant to support a productive discussion, not deliver a single fact.
If you do this review, keep it to 90 seconds.
Don't let it expand.
Next slide.
Why mixed mechanisms dominate?
For reasons.
This slide deserves about three to four minutes.
It is the evolutionary logic synthesis.
Reason one.
Each mechanism alone has limits.
Pigments can't easily make blue.
That is the conjugation trap.
Students saw it in LEC7.
Pure structures can fade or wash out without absorber backing.
Combining them lifts both ceilings.
Reason two.
Combinations expand the achievable color space.
Two mechanisms with different physical bases multiplicatively combine.
Yellow filter times blue reflector equals green that no pigment alone can deliver and no structure alone can deliver.
The math is multiplication, not addition.
That is why mixed mechanism colors can be brighter and more saturated than the sum of their parts.
Reason three.
Each component is independently tunable.
Evolution can adjust the pigment without changing the structure and vice versa.
Small change in carotenoid concentration shifts the hue without re-engineering the nanostructure.
Small change in lattice spacing shifts the structural peak without changing the pigment.
This is the modularity argument.
Modular systems evolve faster than monolithic ones because mutations can target one module without breaking the hole.
Reason four.
Honest signaling becomes multi-dimensional.
Pigment quality reports on diet and parasite load.
Structural quality reports on developmental stress at molt.
Next colors carry richer information about the animals overall condition.
This is the costly signaling argument applied to color.
And remember, this morning's deep dive on price-walled men gave you a specific empirical instance of this argument.
Sexual dichromatism in tanager carotenoid patches is reporting on hidden layer architecture, not pigment chemistry.
The acromatic layer is a developmental signal.
The carotenoid is a dietary signal.
Mixed colors can carry both at once.
The take home.
Mixed mechanism color is easier to evolve, easier to tune, and richer in information than either pigment or structure alone.
Selection has discovered this thousands of times across animal lineages.
Selection doesn't care about the categories we draw.
It cares about what works.
And what works in lineage after lineage is to combine pigment and structure rather than pick one.
Pause after that paragraph.
It is the conclusion of the analytical content of the lecture.
After the pause, advance to the project workflow slide.
Project workflow, three steps.
About two minutes.
This is the project tailored slide.
Step one, classify each color patch on the continuum using spectrum shape, the angle test, and the IR signature.
Step two, predict mechanism from continuum position.
Positions one and two are pigment dominated.
Three and four are mixed.
Five and six are structural dominated.
Step three, test phylogenetically.
Do sisters species share the mechanism or is their turnover?
Common pitfalls.
Green is not always mixed.
Turricostyle pure pigment exists.
Blue is not always structural.
Some pipefish and lizards use billiverdon.
And always tech UV.
Many reef fish have invisible to you patches that are functionally important for conspicfic recognition.
Bonus slide on bird of paradise.
Slide 36.
Cuttable if you are short on time.
About 90 seconds if you have it.
Eridescent throat patches from hollow flattened melanosomes, convergent with hummingbirds, Nordenet all 2021.
Update from the prior deck, that citation was fixed from a misidentified Eliasin 2020 hummingbird paper.
Yellow ornamental feathers from dietary carotenoids.
Super black surrounding patches from light trap nanostructure, McCoy 2018.
Eridescent breast shields from 2D photonic crystal melanosomes.
Pure white display feathers from spongy keratin without pigment.
One bird, five mechanisms.
Sexual selection at maximum intensity.
Mandrels and birds of paradise, old world primates and Australasian songbirds,
independently evolved multi-mechanism display systems for the same functional reason.
Sexually selected mate signals.
Convergence again.
Now block five.
Looking ahead.
Tell students about Thursday.
LEC 12.
Color patterns eye.
A field guide to color in space.
Note the new tagline.
The earlier tagline was genetics of stripes and spots.
The new tagline is a field guide to color in space.
We zoom out from how is one patch of color made to how is a pattern of color made.
The genetics of melanin pattern.
Zebra stripes.
Lepard spots.
Butterfly eye spots.
Turing models of color pattern formation.
And the chromata fortiling logic from today.
Applied at body scale.
The framework you built today is what makes LEC 12 possible.
Demo 5 on Wednesday dynamic color.
Live cephalopod chromata 4 activity.
Hopefully some live fish color change from the campus Aquaria.
Today's chromata 4 stacking framework is the substrate for everything demo 5 will show.
Tell students explicitly.
What you saw today as a static optical sandwich, you will see Wednesday in motion.
The cells you read about today are the cells that flicker on the squid skin you will see in section.
Project Tion.
Tell students to start thinking about their fish group through the continuum framework.
Which color patches are pigment?
Which are structural?
Which are mixed?
They will need this vocabulary for their spectral analyses in demo 6 through 8.
Especially group 4, Cyril Lebrini and group 5, Akinthority and group 7, Halakorn.
These groups have the most chromata fort diversity and the richest opportunities to position their species on the continuum.
This week's reading is Kara O'And Conor 2021.
Protective coloration.
The function side of the question.
Tell students to start at this week.
Thursday's LEC 12 begins the function block.
Now the exit ticket.
This is the last two minutes of the lecture and it serves three pedagogical purposes.
First, it is a comprehension check on the continuum and the diagnostics.
Second, it forces every student to commit to an answer.
Students who get away with passive listening cannot get away with the exit ticket.
Third, it gives you a quick read on which concepts didn't land so you can adjust LEC 12 if needed.
Three questions.
Students answer one of three.
The first question is the parrot test.
A green parrot feather is treated with a solvent that dissolves only its pigment.
Predict the color and explain using two mechanisms.
The expected answer is structural blue because the pigment that produces yellow has been removed and the structural blue alone remains visible.
The second question is continuum placement.
Place flamingo pink, peacock blue eye spot and green tree frog skin on the continuum.
Justify with diagnostics.
The expected answers.
Flamingo pink at position one.
Pure pigment.
Carotenoid in keratin.
Grinding leaves it pink.
Peacock blue eye spot at position five.
Pigment as structure.
Melonin rods and photonic crystal.
Grinding eliminates color.
Bleaching also eliminates.
Green tree frog skin at position three or four.
Pigment plus structural producer.
Blue eye rid of four plus yellow's anthophore.
Grinding partially eliminates color depending on which layer you grind.
The third question is the mandrel puzzle.
Identify the two mechanisms making the blue ridges and red nose.
Propose one reason selection might favor combining different optical strategies in adjacent patches.
The expected answer.
Collagen scattering for blue.
Hemoglobin vasculature for red.
The reason selection favors combination is multi-dimensional honest signaling.
Different mechanisms report on different aspects of male condition.
And that lets a receiver integrate over more dimensions of information.
Ty this back to the Price-Waldman dichromatism finding from block one if a student is articulate enough.
Tell students to flip their no card and answer one question.
Hand in on the way out.
Don't grade for correctness.
Grade for engagement.
Anyone who writes a coherent paragraph gets full credit.
Anyone who writes I don't know gets partial credit if they explain why they don't know.
The point is to make every student commit not to enforce mastery.
Then close the lecture.
Wednesday is demo five.
Dynamic color.
Thursday is LEC 12.
See you then.
A few last coaching notes for the whole hour.
Watch for the moment when students lock in.
There is usually one slide where the room shift scares.
Body's lean forward.
Fones go down.
Eyes go up.
For LEC 11, there are two candidate moments.
One is the green parrot reveal.
No green pigment.
Zero.
The other is the Price-Waldman dichromatism finding.
The male female difference is in the acromatic layer not the pigment.
If neither moment lands, your delivery on those lines was too soft.
Try them again next time you teach this lecture.
If you find yourself running short on time, the cuttable pieces in order of cutability
are the bird of paradise erudessence aside on slide 36.
The cephalopod bonus slide.
The super black slide.
The second cold call on the second TPS.
And case seven turicos.
Do not cut case one parrots.
Do not cut the continuum.
Do not cut the exit ticket.
If you find yourself running long, you can compress by skipping the angle shift detail in case three butterflies.
Skipping the convergence aside in case four jewel beetles.
And condensing the four reasons in the Y-mix dominates slide to two reasons.
The first two reasons, limits and combination space are the load bearing ones.
Reasons three and four are amplification.
A note specific to the Price-Waldman mini-deck.
If you are running long going into the mini-deck, the four methods slides eight through eleven can be compressed to a single sentence each.
Multispectral, micro spectrophotometry, hyperspectral, optical model, same pattern for scales.
Move on.
The dichromatism slide and the eight family phylogenies slide are the load bearing ones.
Don't cut those.
I contact discipline.
The lecture has a lot of citations.
Z2003.
Vucusic 2001.
Bagnara 1968.
Prum and Torres 2004.
Shaki and Hill 2005.
Hooper 2021.
Kratachwil and Malarino 2023.
Price-Waldman 2025.
Makoi 2018.
Norton 2021.
When you say a citation, look at the room, not the slide.
The slide has the citation.
Your job is to make eye contact with a student in the back row while you say the year.
That is how you signal that primary literature is the basis of the field, not the slide.
You have this.
The framework is clean.
The case is land in sequence.
The synthesis pays off and the price-waldman deep dive gives the recap real intellectual heft.
Walk in confident.
Trust the architecture you built.
See you on the other side.