Lecture 7: Developmental Bias I: Constraint to Macroevolution

Question 1

Von Baer’s Laws of Embryonic Development

Answer 1

These laws describe the pattern of embryonic development and how embryos of different species become distinct over time.

1. General Features Appear First: Traits common to large groups (e.g., vertebrates) appear earlier than species-specific traits.
2. Specialization Progression: Development goes from general to specific traits, adding unique features at later stages.
3. Species Divergence: Each embryo diverges from a general form without passing through other species’ adult stages.
4. Embryo Comparison: An embryo resembles the embryo, not the adult form, of another species.

Question 2

What is the contrast between Von Baer’s Laws and Haeckel’s Theory?

Answer 2

Haeckel suggested that development stages recapitulate evolutionary history. Von Baer disagreed, proposing that embryos diverge and don’t mimic the adult forms of ancestral species.

Question 3

What are examples of developmental constraints in Chordates?

Answer 3

Chordates, such as zebrafish, chicks, and mice, follow a conserved vertebrate body plan early in development.

• Chordate Body Plan: Amphioxus and fossils like Pikaia (505 million years old) exemplify early vertebrate structure.
• Vertebrate Phylotypic Stage: A conserved stage where core traits like the basic body structure of vertebrates emerge.
• Hox Genes in Body Plans: Hox genes create a common “body plan code” across species, guiding the structure’s formation at this phylotypic stage.

Question 4

Hourglass Model

Answer 4

This model suggests that the middle stages (phylotypic stage) are most conserved, while early and late stages show more variability, supporting that constraints at the middle stage limit evolution.

Question 5

Developmental Constraints in Plants

Answer 5

Flower development in plants also follows constraints, with specific floral structures (stamen, petal, carpel, sepal) appearing in diverse species.

1) Floral Homoeotic Mutations: Mutations in floral development genes (Class A, B, and C) can modify flower parts, but these changes occur within set constraints, preserving the basic floral structure.

2) Evolution Across Plant Types: Gymnosperms (e.g., pine trees) and angiosperms (flowering plants like water lilies and poppies) have developmental pathways limiting how flower structures evolve.

Question 6

Pleiotropy and Developmental Bias in Evolution

Answer 6

1) Pleiotropy: A single gene influencing multiple traits, often resulting in correlated trait evolution.

2) Role in Development: Pleiotropic genes in core gene regulatory networks (GRNs) can restrict evolutionary changes, as they impact fundamental structures (e.g., body plans or floral organs).

3) Macroevolution Impact: Developmental biases create boundaries, causing evolution to follow certain paths and limiting diversity within major clades.

Question 7

What are the key takeaways from this lecture/main concepts?

Answer 7

• Developmental Constraints and Biases: These restrict evolutionary possibilities, making certain forms and body plans more likely than others.
• Diagnosis of Large Clades: Developmental constraints help distinguish broad groups of organisms (e.g., vertebrates) due to consistent body plans.
• Evolutionary Limitations: Developmental biases, reinforced by pleiotropy, limit how much evolution can vary from established patterns