Chapter 7—Key concepts and terms: Adaptive landscape Convergence / divergence Theoretical morphology Morphospace Functional morphologic analysis Fossils & Evolution - Ch. 7

Fossils & Evolution - Ch. 7 Outline Concept of adaptive landscape Theoretical morphology Functional morphologic analysis Fossils & Evolution - Ch. 7

“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 Fossils & Evolution - Ch. 7

Fossils & Evolution - Ch. 7 “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 Fossils & Evolution - Ch. 7

Fossils & Evolution - Ch. 7 “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 Fossils & Evolution - Ch. 7

Fossils & Evolution - Ch. 7 Adaptive landscape concept from Wright (1932) Fossils & Evolution - Ch. 7

Example: Coiling in cephalopods adjacent whorls not in contact Fossils & Evolution - Ch. 7

Frequency distribution of coiling types (405 genera of ammonoids) “Adaptive peak”— optimal coiling geometry 90% of measured taxa fall within outer contour Fossils & Evolution - Ch. 7

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.) Fossils & Evolution - Ch. 7

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) Fossils & Evolution - Ch. 7

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 Fossils & Evolution - Ch. 7

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 Cross-sectional shape of the cone Rate of expansion of the cone Tightness of the coil Whorl translation Fossils & Evolution - Ch. 7

Fossils & Evolution - Ch. 7 Coiling attributes 1. Shape of cone (circular) 3. Tightness of coil r1 2. Rate of expansion (R2 = 2 × R1) r2 4. Translation Fossils & Evolution - Ch. 7

Translation of the whorls low translation high translation Fossils & Evolution - Ch. 7

Computer-simulated gastropod shell Fossils & Evolution - Ch. 7

Fossils & Evolution - Ch. 7 Morphospace of coiled shells: A = gastropods; B = cephalopods; C = bivalves; D = brachiopods Fossils & Evolution - Ch. 7

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! Fossils & Evolution - Ch. 7

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 Fossils & Evolution - Ch. 7

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) Fossils & Evolution - Ch. 7

Fossils & Evolution - Ch. 7

Spherical aberration negative s.a. perfect lens (all rays focused on a single point) zero s.a. imperfect lens positive s.a. Fossils & Evolution - Ch. 7

Functional morphology of trilobite lenses actual trilobite lenses optimum aplanatic lens Fossils & Evolution - Ch. 7

Functional morphology of trilobite lenses Estimation of visual field allows interpretations of life orientation and other aspects of functional morphology in trilobites Fossils & Evolution - Ch. 7

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 Fossils & Evolution - Ch. 7

Pteranodon (old reconstruction) Fossils & Evolution - Ch. 7

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 Fossils & Evolution - Ch. 7

Fossils & Evolution - Ch. 7 Flying speed vs. sinking rate (estimates from wind tunnel experiments with old reconstruction) Fossils & Evolution - Ch. 7

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 Fossils & Evolution - Ch. 7

Fossils & Evolution - Ch. 7

Fossils & Evolution - Ch. 7

Fossils & Evolution - Ch. 7 1: shape if wing not connected to leg 2: shape if wing connected to knee 3: shape if wing connected to ankle Fossils & Evolution - Ch. 7

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 Fossils & Evolution - Ch. 7 (broad wings)

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)? Fossils & Evolution - Ch. 7

Fossils & Evolution - Ch. 7 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 Fossils & Evolution - Ch. 7

Fossils & Evolution - Ch. 7

Fossils & Evolution - Ch. 7 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. 7