fossils & evolution - ch. 71 chapter 7—key concepts and terms: adaptive landscape convergence...

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