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


Outline

• Concept of adaptive landscape

• Theoretical morphology

• Functional morphologic analysis


“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


“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


“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


Adaptive landscape

concept fromWright (1932)


Example: Coiling in cephalopods

adjacent whorlsnot in contact


Frequency distribution of coiling types (405 genera of ammonoids)

90% of measured taxa fallwithin outer contour

“Adaptive peak”— optimalcoiling geometry


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.)


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)


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


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


Coiling attributes

1. Shape of cone (circular)3. Tightness of coil

2. Rate of expansion (R2 = 2 × R1)

r1

r2

4. Translation


Translation of the whorls

low translation high translation


Computer-simulatedgastropod shell


Morphospace of coiled shells: A = gastropods;B = cephalopods; C = bivalves; D = brachiopods


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!


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


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)


Spherical aberration

perfect lens (all rays focusedon a single point)

imperfect lens

negative s.a.

positive s.a.

zero s.a.


actual trilobite lenses

optimum aplanatic lens

Functional morphology of trilobite lenses


Functional morphology of trilobite lenses

Estimation of visual field allows interpretations of life orientationand other aspects of functional morphology in trilobites


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


Pteranodon (old reconstruction)


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


Flying speed vs. sinking rate (estimates from wind tunnel experiments with old reconstruction)


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


1: shape if wing not connected to leg2: shape if wing connected to knee3: shape if wing connected to ankle


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


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)?


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


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

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

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