LS2 lect3

LS2 lect3 - Life Sciences 2 September 8, 2010 Embryology...

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Life Sciences 2 September 8, 2010 Prof. Daniel E. Lieberman (Peabody 53)
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ANNOUNCEMENTS Labs : Start this week! Problems : Dr Carr (carr@fas.harvard.edu) My office hours: Tuesdays 1-2 & by appt (Peabody 53)
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embryos represent basic body plan that can be widely modified ...an old idea whose origins helped Darwin formulate his theory Phylotypic stage
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Last lecture: 2. What is the basic body plan? 1. How do we go from a single cell to a whole body? TODAY: How does the basic body plan evolve ? How did we go from being simple chordates to complex humans? How does the basic body plan work ? How does it all fit together and function?
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A reminder 3 Germ Layers: ectoderm mesoderm endoderm Basic body plan: neural tube central axis (notochord) segmented mesoderm (somites) central body cavity (coelom) gut tube
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Believable for “simple” organism? What about more complex organisms?
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This “problem” remains a major basis for critiques of evolution “Blind Watchmaker” Analogy - Evolution by Natural Selection is agentless (blind) - Organisms are complex (like a watch) - Having nothing “guide” evolutionary changes to development is like assembling a watch “blind”
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Two solutions to the “blind watchmaker” problem 1. Evolutionary changes that generate complexity occurred from many, many intermediate steps.
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Two solutions to the “blind watchmaker” problem promote evolutionary change and complexity Body has many parts (modules ) that develop via a hierarchical (branching) process with many interactions between modules Modules Genes Cells Germ layers Segments Etc… Interactions Growth Movement Induction Etc. . Interactions between modules cause integration: parts become combined into whole
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An example of integration Folding of body walls to create tube-shaped embryo coelom Gut tube Neural tube Paraxial mesoderm Splanchnic mesoderm Somatic mesoderm
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AN IMPORTANT POINT: Variation happens Variation in modules Variation in how modules interact e.g. # of somites e.g. interactions among somites and other tissues Genetic mutations
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An analogy Modules (different Lego™ blocks) Interactions (how you snap them together) Because Legos all share basic bits that snap together, they can be endlessly recombined in novel ways to create VARIATIONS that often fit together and “work” Selection can then act on this variation
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And, since development is hierarchical with many stages/units/interactions, there is great opportunity for change (descent with modification)
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-bones, muscles & nerves that interconnect -limbs -heads Let’s see how evolutionary changes to development created:
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This note was uploaded on 03/29/2012 for the course LS 2 taught by Professor Andrewa.biewener,petert.ellison,anddaniele.lieberman during the Fall '10 term at Harvard.

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LS2 lect3 - Life Sciences 2 September 8, 2010 Embryology...

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