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Evolutionary and Developmental Biology

Learning Objectives

• Understand how developmental patterns impact evolutionary history:

  • Analyze embryonic similarities

  • Identify atavisms and vestigial structures

  • Discuss examples of bad design

• Recognize how spatial and temporal patterns of gene expression contribute to body form variation.

• Explain the concept of deep homology in tool kit genes.

• Understand Hox gene structure and their influence on development, including leg development in amphipods.

Historical Context

• Charles Darwin: Quote from "The Origin of Species (1859)": "Thus, community in embryonic structure reveals community of descent."

Ontogeny and Phylogeny

• Debate on whether ontogeny (development) recapitulates phylogeny (evolution):

  • Ernst Haeckel's View: Developmental stages mimic adult evolutionary stages, exemplified in fish, frogs, and birds.

  • Von Baer's Counterargument: No recapitulation occurs; embryos become increasingly diverse.

Observational Examples

• Dolphins: Rare occurrences of dolphins being born with hindlimbs illustrate evolutionary vestiges.

• Embryonic Similarities: Related embryos show notable similarities (e.g., chickens, possums, cats, bats, humans).

Embryonic Clues

• Sea Urchin: Displays radial symmetry in adults and bilateral symmetry in larvae.

• Sand Dollar: Shares similarities with sea urchins in both larvae and adult forms.

• Human Embryo: Exhibits pharyngeal arches, a tail, and initially appears to have gills.

• Whale Development: Fetal whales show hindlimb buds consistent with evolutionary ancestry.

Atavisms and Vestigial Structures

• Dolphins: Instances of hindlimb flippers indicate atavistic traits.

• Chickens: Mutations leading to alligator-like teeth demonstrate the presence of vestigial structures.

• Whales: They retain pelvic bones, which have adapted for copulation.

• Kakapo Bird: Exhibits bizarre behavior when frightened despite being flightless.

Bad Design Examples

• Koala Pouch: Faces downward, inefficient for an arboreal lifestyle.

• Human Laryngeal Nerve: Takes an evolutionary 'roundabout' route to the larynx, affecting various species differently, such as giraffes.

Development and Evolution

• Role of Development: Genetics, inheritance, and natural selection intertwine to draw conclusions about evolutionary processes.

• Heterochrony: Variations in the timing of developmental events lead to phenotypic discrepancies.

Hox Genes

Structure and Function

• First identified in Drosophila, Hox genes establish body layout.

• Example: Mutations like biothorax in Drosophila illustrate drastic deviations in morphology due to Hox mutations.

Hox Gene Expression Patterns

• Different Hox genes dictate specific structures:

  • Patterns within Drosophila: Identified genes (e.g., Ant-C, BX-C, Ubx, Abd-A) determine segment fate.

• Influence on Crustaceans: Hox genes play crucial roles in leg development across species.

• Knockout Experiments showcase the redundancy and interactions between Hox genes, which can lead to morphological changes in crustacean species (e.g., Parhyale).

Hox Gene Interaction Insights

• abd-A Knockout: Results in protruding limbs in segments.

• Abd-B Knockout: Shows Hox gene interactions, impacting overall limb configurations.

• Triple Knockout of Hox Genes: Demonstrates evolutionary consequences when posterior genes are disabled, allowing anterior genes to dominate in previously inhibited regions.

Genetic Homology in Evolution

• Studies showed gene parallels across species: eyeless in flies, small eye in mice, and aniridia in humans.

• Promiscuous Pax6: Commonality of genetic sequences across diverse species shows evolutionary connections.

• Hox genes present in both invertebrates and vertebrates reinforce this homology, showing a shared genetic toolkit across phyla.

Conclusion

• Evolution’s Architecture: Evolution functions akin to an architect adapting prior designs to innovate new structures. This principle is encapsulated in the work of scientists such as Jerry Coyne.