10-Evolution and Development (1)

Evolutionary Developmental Biology (Evo-Devo)

• Concept: Elevates the importance of developmental biology in evolutionary theory.

• Key Ideas:

  • Mutations are not the only source of new traits; activating different genomic parts can create novel changes.

  • Natural selection acts on phenotypes regardless of their origin.

Homeotic Transformations

• Definition: Developmental changes that result in different phenotypes.

• Significance: Can be a source of evolutionary change, e.g., extra vertebrae.

Historical Perspective

• William Bateson: Discovered abnormalities in fruit flies and vertebrates; recognized transformations linked to genetics but did not understand the mechanisms (1894).

Ontogeny

• Definition: Refers to the developmental stages of an organism.

• Baer’s Law: General traits develop before specialized traits.

• Haeckel’s Recapitulation Theory: Stated that ontogeny recapitulates phylogeny (development stages reflect evolutionary history).

Developmental Stages

• Stages: Evidence shows that amphibian embryos are most similar across species, while later stages (Stage 4) exhibit more divergence (reptiles and mammals).

Timing of Development

• Heterochrony: Variation in the timing of developmental events.

  • Recapitulation: Earlier appearance of ancestral traits in descendants.

  • Paedomorphosis: Ancestral traits retained but appear later in descendant stages.

Types of Heterochrony

• Four Types:

  • Accelerated: Somatic traits appear earlier.

  • Unchanged: Traits remain consistent across generations.

  • Retarded: Developmental timing delayed.

• Illustrative comparison using Eurycea (salamanders).

Introduction to Evo-Devo

• Hox Genes: Homeotic selector genes that dictate body structure patterns.

• Mutations in Hox genes can lead to significant changes in morphology.

Gene Regulation in Development

• Gene Control Regions: Sections that regulate gene expression; includes up-regulatory and down-regulatory elements.

• Key Proteins:

  • Repressor: Inhibits gene expression.

  • Transcription Factor: Binds to DNA to modulate gene activity.

Hox Genes in Drosophila

• Functionality: Segment identity is determined based on the expression of Hox genes within specific developmental regions of the organism.

Conservation of Developmental Pathways

• Common Framework: Similar regulatory networks underpin diverse anatomical structures across species (e.g., limbs of tetrapods).

Evolutionary Reversal and Development

• Example: Loss of molar teeth in certain Felidae species.

Eye Development & Evolution

• Evolutionary Complexity: Eyes appear less complex when scrutinized through an evolutionary perspective.

  • Initial opsin gene duplications led to various light-sensitive proteins.

Evolution of Structures

• Opsins and Crystallins: Key components in the evolution of the eye—opsins are light-sensing proteins, while crystallins developed into lens structure.

Developmental Processes in Eye Evolution

• Photoreceptors: Cells that process light signals, shared across species, showcasing a conserved evolutionary pathway.

• Ectopic Eye Formation: Abnormal eye development in different regions.

Co-option of Genes in Evolution

• Regulatory Modularity: Changes in gene enhancer regions can shift expression patterns, leading to novel structures.

Constraints on Development

• Physical, Selective, Genetic: Barriers limit evolutionary pathways; e.g., vertebra count in neck extension demonstrates antagonistic pleiotropy.

Conclusion on Gene Duplication

• Outcomes: Duplication can lead to either loss, pseudogenes, or functional divergence in gene roles, supporting evolutionary innovation.