Fossils & Evolution - Ch. 7 1
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 fromWright (1932)
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Example: Coiling in cephalopods
adjacent whorlsnot in contact
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Frequency distribution of coiling types (405 genera of ammonoids)
90% of measured taxa fallwithin outer contour
“Adaptive peak”— optimalcoiling geometry
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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. Cross-sectional shape of the cone2. Rate of expansion of the cone3. Tightness of the coil4. Whorl translation
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Coiling attributes
1. Shape of cone (circular)3. Tightness of coil
2. Rate of expansion (R2 = 2 × R1)
r1
r2
4. Translation
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Translation of the whorls
low translation high translation
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Computer-simulatedgastropod 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 non-
overlapping 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
perfect lens (all rays focusedon a single point)
imperfect lens
negative s.a.
positive s.a.
zero s.a.
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actual trilobite lenses
optimum aplanatic lens
Functional morphology of trilobite lenses
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Functional morphology of trilobite lenses
Estimation of visual field allows interpretations of life orientationand 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 leg2: shape if wing connected to knee3: shape if wing connected to ankle
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New reconstruction& new interpretationof flight
Small pterosaurs (if wingnot connected to leg)
Small pterosaurs (if wingconnected to ankle)
wingspan2
wing area
(narrow wings)
(broad wings)
weightwing area
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Functional analysis in saber-toothed 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 saber-
toothed 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