Download Studying Adaptation

Zoology 445
Fall 2006
Reading Sections 9.1 to 9.4 and 9.6
Studying Adaptation: Handout
I. Introduction
Why is the giraffe’s neck so long?
II. Studying adaptations
A. Observation
B. Hypothesis
C. Analysis
 Observational studies (Text section 9.3)
 Comparative Method (Text section 9.4)
 Experimental manipulations (Text section 9.2)
Best work includes more than one of method of analysis when making a case for a trait being
adaptive.
AM Jarosz
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Example: Snail shell morphology along the coast of Maine.
Species Littorina obtusata an intertidal marine snail found along the east coast of US.
Observation: Temporal change of many populations in shell morphology along coast of Maine.
Shell Morphology.
When scaled for overall size, found that spire decreased in height and shell thickness
increased through time.
Fig 2A Seeley (1986) PNAS 83:6897-6901. Paper is postec on the course web page as an
OPTIONAL reading.
Temporal pattern:
Spatial pattern:
B. Hypothesis: Decrease in spire height and increase in shell thickness adaptive because they
reduce predation by crabs.
C. Analysis:
1. Comparative method: Shell height also found to vary spatially among areas where crab
densities differed.
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2. Experimental manipulation: Placed 18 pairs of tethered snails in water at three locations that
differed in crab density.
Location
Crab density
Gleason Point
High
Sipp Bay
Low
Timber Cove
Absent (or very rare)
Fig 3.
 = snails with high spire
 = snails with low spires
III. Trade-offs and Constraints on adaptation (Section 9.6)
Example of an evolutionary trade-off: Female flower size in the plant Begonia involucrate.
(Pages 351 to 353)
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We have already talked a bit about evolutionary or historical constraints. At one level we are
constrained by the number of limbs we have.
Book examines two other constraints on pages 354 to 358.
Take Home message:
1. Must be cautious when stating that a trait is adaptive.
2. Trade-offs and constraints may limit a traits ability to approach optimal adaptive state.
\Evolution Zol 445\2006\Studying adaptation handout.doc
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