Tuesday, February 21

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Tuesday, February 21 7pm Center for Creative
Photography Free to the Public Biological
Evolution What It Is and What It Isn't Joanna
Masel Assistant Professor Ecology and
Evolutionary Biology
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Animal Form
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The Cambrian Explosion 540-490 mya Rapid
diversification of multicellular life forms,
including animals All major animal body plans
appear
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Trilobites

• appear in early Cambrian (540 mya)
• extinct 300 million years later
• contains 5000 genera very diverse!

Millimeters to .7 meters long
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Trilobites were exclusively marine
Locomotion -- walking -- swimming
Enrollment -- for protection
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Life in The Burgess Shale 505 mya
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The Burgess Shale British Columbia, Canada
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Wiwaxia
Marella
Sanctacaris
Ottoia
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Nectocaris
Marella
Odontogriphus
Hallucigenia
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Hallucigenia
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Hallucigenia
Oops this side up!
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Hallucigenia
Oops this side up!
Now considered to be in Phylum Arthropoda
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No Obvious Link to Surviving Taxa
Opabinia
Amiskwia
Wiwaxia
Anomalocaris
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Traditional view A cone of increasing diversity
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Alternative View Diversification, decimation,
more diversification
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Domain Eukarya, Kingdom Animalia An animal is a
multicellular heterotroph Multicellular
consisting of assemblages of cells Heterotroph
organism that obtains energy from pre-formed
organic molecules (usually by ingesting other
organisms)
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Multicellularity
Evolved numerous times, including in animals
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Origin of life (bacteria)
Origin of Earth
4000
5000
Millions of Years Ago (mya)
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3000
4000
Millions of Years Ago (mya)
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2000
3000
Millions of Years Ago (mya)
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1st Multicellular Eukaryotes (red algae)
1st Eukaryotic Cells
1000
2000
Millions of Years Ago (mya)
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1st Animals
Cambrian Explosion
600
0
1000
Millions of Years Ago (mya)
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Why Be Multicellular?
Organisms can BE LARGE by adding cells. Being
LARGE means -- able to consume small things --
able to avoid large predators -- able to move
long distances
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Why not be a LARGE one-celled organism?
Answer Transport Constraints
Cells transport nutrient and O2 molecules in,
and waste and CO2 molecules out. There are two
ways to transport molecules across cell membranes.
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• Passive transport ( diffusion)
• Molecules diffuse down concentration or
  electrical gradient.
• No energy expended.

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2. Active transport Molecules pumped against
or electrical gradient. Pump requires energy
to run.
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Required rate of transport proportional to
Volume, V Feasible rate of transport
proportional to Surface Area, S
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S/V declines with L, meaning that V increases
faster than S.
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As cell gets larger, V ? faster than S ?
Hence, as cells get larger, desired rate
increases faster than feasible rate.
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Strategy change shape of cell cylinder has
more surface area per unit volume than sphere.
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Data from unicellular plant species
(algae) Cells more cylindrical as they get
larger, keeping S/V constant.
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Multicellularity introduces a new problem in
transport!
In a large cluster of cells, many cells would
have no surface exposed to the environment at all.
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In multicellular forms, each cell has less S
exposed to outside environment.
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Solution 1 Arrange cells in hollow sphere or
cylinder.
Phylum Porifera Sponge
Phylum Cnidaria Hydra
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Cylinder shapes are everywhere
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Solution 2 cell specialization and division of
labor cell specialization elaboration of one
function and reduction of others division of
labor different cell types specialize on
different tasks
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Why cells specialize "A jack of all trades is
master of none"
Doing it all means not doing any one thing really
well.
Example of tradeoff Fully-featured Swiss Army
knife does many jobs, but each tool can be
awkward to use.
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Cell Specialization in Sponges

• belong to Phylum Porifera
• similar to ancestor common to animal kingdom

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Cell Types in a Sponge
Outer Epithelial Cells
Porocytes allow water into sponge
Choanocytes (collar cells) circulate water,
filter food particles
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Collar consists of microvilli. Food trapped
and ingested.
flagellum
Each is microvillus.
Cylindrical shape of microvillus increases S/V.
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More recently evolved animals (such as arthropods
vertebrates) have HUGE numbers of cell types.
Mammal Cell Types skin cells, T-cells, muscle
cells, neurons, red blood cells, B-cells,
sensory receptors, sperm, egg, etc.
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Q Worlds Largest Cell?
A Ovum, or Egg Cell, of Ostrich ( yolk of
unfertilized egg)
Ovums size is specialization for storage of
nutrients for embryo.
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versus
Multicellular organisms do NOT have larger cells
on average than unicellular organisms. They have
more cells. And highly specialized cells.
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Multicellularity Cooperation
In multicellular organisms, groups of cell types
cooperate. But there can be conflict. Consider
cells which serve no useful function, but exploit
cooperators and replicate wildly. We call
these.
cancer cells.
Michod lab, EEB conflict mediator genes
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• Themes in Animal Form
• Cell Specialization and Division of Labor

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• Themes in Animal Form
• Cell Specialization and Division of Labor
• Hierarchical Organization of Cells

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Cell basic unit of organismal structure Tissue col
lection of cells of same type Organ collection of
tissues that serves a specific function Organ
System collection of organs that serves a
specific function
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Examples
Cell neuron Tissue nerve Organ brain or spinal
cord Organ System nervous system
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Stomach as an Organ Within Digestive System
Figure 41.2
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• Themes in Animal Form
• Cell Specialization and Division of Labor
• Hierarchical Organization of Cells
• Control of Organ Systems
• Homeostasis
• Negative feedback

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Homeostasis Maintenance of constant conditions in
internal environment
Example Control of Room Temperature
Example involves negative feedback.
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Thermoregulation as Homeostasis
capillary dilation ? sweating
skin
? blood T
Thermostat in brain involves negative feedback.
brain
? blood T
capillary constriction ? sweating
skin
T Temperature