Transcript Chapter 1 Preservation and the fossil record

Chapter 7—Key concepts and terms:
• Adaptive landscape
• Convergence / divergence
• Theoretical morphology
– Morphospace
• Functional morphologic analysis
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Outline
• Concept of adaptive landscape
• Theoretical morphology
• Functional morphologic analysis
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“Adaptationist” view of
functional morphology
• Assumption: morphology is adaptive: i.e.,
morphologic features are present in an
organism because they are useful to the
organism
– Functionally neutral features may exist, but
they are probably rare
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“Adaptive Landscape”
• For any array of morphologic characters,
certain character-states or combinations of
character-states are more adaptive
(advantageous to the organism) than others
• Adaptive landscape (for two characters)
– Peaks = character combinations that are highly
advantageous (optimal morphology)
• In reality, adaptive landscape is
multidimensional
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“Adaptive Landscape”
• On an “adaptive landscape” map, a single individual plots
as a point and a population plots as an area
• Within any population, some individuals will possess
character combinations that are higher up the adaptive
peak than others
• Over time, because of natural selection, the population will
climb the adaptive peak
• Different adaptive routes lead to convergence and
divergence
• There can be no route from peak to peak involving a path
through an adaptive valley
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Adaptive landscape
concept from
Wright (1932)
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Example: Coiling in cephalopods
adjacent whorls
not in contact
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Frequency distribution of coiling types
(405 genera of ammonoids)
“Adaptive peak”— optimal
coiling geometry
90% of measured taxa fall
within outer contour
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Adaptive landscape (cont.)
• Question: Does evolution cease when a
population reaches an adaptive peak?
• Answer: No!
– Adaptive landscape is constantly changing!!!
(environmental change, introduction of new
predators/prey, competitors, disease, etc.)
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Theoretical morphology
• Loosely defined as the study of
morphospace and the preferential
occupancy of certain regions
– Example: shell geometry in coiled
invertebrates (gastropods, cephalopods,
bivalves, brachiopods)
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Theoretical morphology
• Morphospace = the total spectrum of all
morphologies that could possibly exist
• Most morphospace is unoccupied and has
never been occupied
– Only a relatively few basic morphologies have
actually evolved, and these “designs” have been
used by large numbers of taxa
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Shell geometry in coiled
invertebrates
•
•
Coiled shells can be thought of as a
tapered cone that is coiled about an axis
Geometry of the cone can be described by
four attributes
1.
2.
3.
4.
Cross-sectional shape of the cone
Rate of expansion of the cone
Tightness of the coil
Whorl translation
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Coiling attributes
1. Shape of cone (circular)
3. Tightness of coil
r1
2. Rate of expansion
(R2 = 2 × R1)
r2
4. Translation
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Translation of the whorls
low translation
high translation
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Computer-simulated
gastropod shell
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Morphospace of coiled shells: A = gastropods;
B = cephalopods; C = bivalves; D = brachiopods
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Coiled shell morphospace
• Note that:
– Most morphospace is vacant
– Four evolutionary groups occupy mostly nonoverlapping regions of the block
– Four evolutionary groups have different
functional and environmental requirements,
therefore four different adaptive peaks!
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Functional morphologic analysis
• Structures in fossils are most commonly
interpreted by comparison with similar structures
in living animals
• Homologous structures have a common
evolutionary origin (but not necessarily the same
function)
– e.g., fore-limbs in tetrapods
• Analogous structures have the same function
(but not the same evolutionary origin)
– e.g., wings in birds and flies
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Functional morphologic analysis
• Example: Vision in trilobites
• Through natural selection, trilobite eye
lenses became optimized to eliminate
spherical aberration (“aplanatic” lens)
• Moreover, calcite in each lens is oriented
with optical axis perpendicular to visual
surface (to eliminate birefringence)
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Spherical aberration
negative s.a.
perfect lens (all rays focused
on a single point)
zero s.a.
imperfect lens
positive s.a.
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Functional morphology of
trilobite lenses
actual trilobite lenses
optimum aplanatic lens
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Functional morphology of
trilobite lenses
Estimation of visual field allows interpretations of life orientation
and other aspects of functional morphology in trilobites
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Functional morphologic analysis:
Example: Flight in pterosaurs
• Pterosaurs had wingspans of 7 meters up to 15
meters (larger than any bird)
• A bird with a 7-meter wingspan would weigh 100
kg, but Pteranodon weighed only 15 kg
– Therefore, Pteranodon was thought to have lacked the
musculature necessary for powered flight
– It was interpreted as a glider
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Pteranodon (old reconstruction)
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Functional analysis in
Pteranodon
• Wind tunnel experiments suggested that
Pteranodon had a lower optimal flying
speed than extant large birds or man-made
gliders
– Less energy required for take-off
– Easy to glide and soar
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Flying speed vs. sinking rate (estimates from wind
tunnel experiments with old reconstruction)
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New reconstruction and new
interpretation of flight
• Pterosaurs fit all criteria of fliers and none of
gliders!
– Down-and-forward flight stroke (as in birds and bats)
• Inferred from structural features of sternum and shoulder girdle
– Recovery stroke similar to that in birds
– Wing membrane supported and controlled by a system
of stiff fibers oriented like the main structural elements
in birds and bats
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1: shape if wing not connected to leg
2: shape if wing connected to knee
3: shape if wing connected to ankle
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New reconstruction
& new interpretation
of flight
wingspan2
wing area
(narrow wings)
Small pterosaurs (if wing
not connected to leg)
Small pterosaurs (if wing
connected to ankle)
weight
wing area
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(broad wings)
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Functional analysis in sabertoothed cats
• Saber-toothed carnivores have evolved
independently at least four times
– What is function of large canine teeth?
– No living animal occupies ecologic niche of sabertoothed cats
• How did saber-toothed cats kill prey?
– Attack to the back (like lions)?
– Throat slashing?
– Ambush, then attack to abdomen (like monitor lizard)?
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Saber-toothed cats
• Smilodon (extinct 10,000
ybp) was about 1 foot
shorter than a modern
lion, but twice as heavy
• Smilodon had a bobtail,
not a long balancing tail
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Saber-toothed cat
•Gape as much as 95°
•Bite force not as great as
in modern big cats
•Canines relatively dull
•Upper and lower canines
designed to shear against
one another
•Probably killed by a
slashing bite to abdomen
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