Lecture 6 v2

What does “Evo-Devo” stand for?

Evolutionary Developmental Biology — the study of how developmental processes evolve and contribute to evolutionary innovation.

What is development in biological terms?

The process by which a single cell (zygote) becomes a multicellular organism with specialized structures and functions.

What is the main focus of evolutionary developmental biology?

How changes in development create evolutionary diversity and novel traits.

What is heterochrony?

Evolutionary change in the timing or rate of developmental events.

Give an example of heterochrony.

Axolotls retain juvenile traits (gills) into adulthood — a case of neoteny.

What is modularity in development?

Organisms are built from semi-independent units (modules) that can evolve separately.

Give an example of a developmental module.

Vertebrate limb — forelimb and hindlimb modules can evolve different shapes and sizes.

What is continuing variance?

Ongoing quantitative variation in an existing trait (e.g., beak depth in finches).

What is an evolutionary innovation?

The origin of a qualitatively new structure or function not present in ancestors.

Example of continuing variance?

Variation in limb length among mammals.

Example of evolutionary innovation?

The evolution of feathers from ancestral reptilian integuments.

How does continuing variance differ from innovation?

Variance modifies existing structures; innovation produces entirely new ones.

Why are innovations important evolutionarily?

They open new ecological opportunities and body plans.

Why are feathers considered an innovation rather than a modification of scales?

Feathers are tubular structures with complex branching development, not flattened scales.

What type of keratin are feathers made of?

Beta-keratin (β-keratin).

What type of keratin are mammalian hairs made of?

v

How do feathers develop embryologically?

From epidermal placodes that grow into tubular outgrowths.

What are the developmental stages of feather evolution according to Prum (1999)?

Hollow cylinder → tuft → rachis and barbs → planar vane → closed pennaceous feather.

What is modular about feather evolution?

Separate control of shaft, barbs, and pigmentation allowed diversification.

What original function might primitive feathers have served?

Insulation or display, not flight.

Why does feather evolution exemplify Evo-Devo principles?

It shows how developmental innovation leads to new evolutionary possibilities.

What is genetic sex determination (GSD)?

Sex determined by specific genes or chromosomes at fertilization.

What is environmental sex determination (ESD)?

Sex determined by external cues such as temperature or population density.

What are the two main types of chromosomal sex determination systems?

XX/XY and ZZ/ZW.

In mammals, which sex is heterogametic?

Males (XY).

In birds, which sex is heterogametic?

Females (ZW).

Give an example of an organism with environmental sex determination.

Many turtles and alligators — temperature-dependent sex determination.

What determines sex in crocodiles?

Incubation temperature of the eggs.

What determines sex in some fish species?

Social environment or dominance hierarchy.

What determines sex in geckos?

It varies — some use XX/XY, others ZZ/ZW, and some temperature-based systems.

What does SRY stand for?

Sex-determining Region on the Y chromosome.

What is the function of SRY?

It triggers the development of testes from the embryonic gonad.

Which gene does SRY activate?

SOX9, which promotes testis differentiation.

What happens if SRY is absent?

The gonad develops into an ovary.

Where is SRY located?

On the short arm of the Y chromosome.

Is the female pathway simply the “default”?

No—female development also requires active signaling (e.g., WNT4, RSPO1).

What pathway drives ovary development?

WNT4/RSPO1 → β-catenin signaling.

What pathway drives testis development?

SRY → SOX9 → AMH/testosterone production.

What is AMH and its function?

Anti-Müllerian Hormone; causes regression of female ducts in male embryos.

What hormone converts testosterone to DHT?

5α-reductase.

What is DHT’s function?

Promotes development of external male genitalia (penis, scrotum).

What happens in individuals lacking 5α-reductase?

Incomplete masculinization — “guevedoce” phenomenon in humans.

How fragile is the SRY system evolutionarily?

Highly fragile — prone to mutation, translocation, and gene loss.

What is the “Red Queen” concept as applied to SRY?

Continuous compensatory evolution to maintain function amid Y-chromosome degradation.

What does “homologous” mean in anatomy?

Structures derived from the same embryonic tissue but differing in adult form.

The clitoris and penis are homologous structures — from what embryonic tissue?

The genital tubercle.

The labia majora and scrotum are homologous — from what?

Genital swellings.

The labia minora and penile shaft are homologous — from what?

Urogenital folds.

What determines which homologous structure forms?

Presence or absence of androgens (testosterone/DHT).

Are chromosomes alone sufficient to determine sexual anatomy?

No—hormones and local cell-cell signaling are also required.

What is a DSD (Difference in Sexual Development)?

Natural variation in chromosomal, gonadal, or anatomical sex.

Are DSDs considered “errors” in development?

No—natural variations within a continuous developmental spectrum

Give examples of DSDs.

Androgen Insensitivity Syndrome, 5α-reductase deficiency, Turner (XO), Klinefelter (XXY).

What is Complete Androgen Insensitivity Syndrome (CAIS)?

XY individuals with nonfunctional androgen receptors who develop as phenotypic females.

What is Turner Syndrome (XO)?

Only one X chromosome; typically short stature, ovarian insufficiency.

What is Klinefelter Syndrome (XXY)?

Male with extra X; reduced fertility and androgen levels.

How do DSDs illustrate developmental biology’s principles?

They show that sex is not binary but a developmental process influenced by multiple pathways.

What is the main lesson about “female as default”?

Misleading—female development is actively regulated, not passive.

What does Evo-Devo reveal about sexual differentiation?

Sex arises from complex, coevolved developmental networks.

Why is sex determination said to evolve rapidly?

Because it depends on small, mutable regulatory switches rather than new structures.

What does the evolution of multiple sex-determining systems across species show?

Convergent evolution — many ways to produce two sexes.

What is an example of sex-determining system turnover?

Geckos — evolved genetic, environmental, and mixed systems repeatedly.

What is the biological significance of developmental flexibility in sex?

It allows adaptation to environmental or social pressures.

Why are developmental systems considered “anti-essentialist”?

They demonstrate that biological categories (like “male/female”) are processes, not fixed essences.

What role do hormones play in sexual development?

Coordinate differentiation of reproductive organs and secondary sexual traits.

What is a paracrine signal?

A signal acting locally between nearby cells.

What is an endocrine signal?

A hormone traveling through the bloodstream to distant targets.

What is the interaction between genes and hormones called?

Gene–hormone interplay — both genetic and biochemical regulation shape development.

What is an example of evolutionary innovation produced by developmental novelty?

Feathers, eyes, or mammalian placenta.

How do modularity and gene networks facilitate evolution?

Allow parts of the body to change without disrupting others.

What does “deep homology” mean?

Different structures share the same ancestral genetic toolkit (e.g., limbs, wings, fins).

What genes underlie deep homology in animal body plans?

Hox genes.

What is the evolutionary importance of Hox genes?

They determine body segment identity and are conserved across animals.

How does Evo-Devo explain innovation without “new genes”?

By repurposing and recombining existing genetic pathways.

What is an example of gene repurposing in evolution?

Crystallins in the vertebrate eye lens evolved from stress proteins.

How do developmental systems evolve?

Through gradual changes in timing, regulation, and interaction of developmental genes.

What is the relationship between evolution and development?

Evolution modifies development; development constrains and channels evolution.

Why are “constraints” in development important for understanding evolution?

They limit possible forms but also guide evolutionary innovation.

How does Evo-Devo reinterpret “variation”?

As changes in developmental pathways, not just in genes.

What is the ultimate message of Lecture 6?

Evolutionary novelty arises from changes in how organisms develop, not just from selection on existing variation.

What do SRY, SOX9, and WNT4 pathways collectively illustrate?

Sex determination as a dynamic developmental network.

How does the Red Queen hypothesis apply to developmental systems?

They must continually evolve to maintain function amid change.

What do homologous structures between sexes demonstrate?

Shared developmental origins despite divergent outcomes.

What is the evolutionary consequence of modularity?

Increased evolvability — the capacity to evolve new traits.

What does the evolution of feathers and genitalia have in common?

Both exemplify developmental remodeling and evolutionary innovation.

What is the link between sex and innovation in Evo-Devo?

Both depend on flexibility and coevolution of developmental processes.

What is meant by “sex as a developmental process”?

It’s a sequence of regulatory decisions, not a binary genetic switch.

What do Differences in Sexual Development (DSDs) reveal about evolution?

They show that variation is natural and that “norms” are statistical, not absolute.

How does Evo-Devo challenge essentialism in biology?

By showing that organisms are dynamic products of processes, not static types.

How do development and evolution together explain diversity of life?

Development provides the raw pathways; evolution modifies them, creating the variety of forms we see.