Chapter 6
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Comparative anatomy
Correspondence between parts and comparison of forelimbs among four vertebrates.
Although the forelimb in all is used differently—human (grasping), bird (flight), lion
(running), seal (swimming)—all have the same basic underlying structure of bones and soft
tissues.
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Homology and analogy
(a) Homology. As vertebrate forearms, the bat wing, mouse forearm, and human arm are
homologous structures as all are composed of similar bones inherited from a recent
common ancestor. (b) Analogy. The wings of bats, butterflies, and birds evolved
independently, not from a recent common ancestor. But they have a similar function, flight,
and so are analogous.
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By human design
Functions vary and designs vary. Unlike biological organisms, these humanly designed and
built machines show no correspondence of parts from planes (flight), to mining machines
(burrowing), or to boats (swimming).
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Evolution of horses
Evolution of horses
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Morphological series—evolution of jaws
Vertebrate jaws evolved from the front set of gill arches of jawless ancestors.
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Morphological series—evolution of limbs from fins
Note that the bones of the fish hip and shoulder correspond to bones in this early tetrapod
(amphibian). Here, the morphological series carries from one type of vertebrate (fish) to
another (amphibian), and from one environment (water) to another (land).
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Diversity of type, unity of pattern
Although the vertebrate species differ, the underlying pattern of the forelimb is
fundamentally the same.
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Vestigial features I
a) Whales evolved from terrestrial ancestors with four legs. But in whales, the hips and
hindlimbs are reduced to small bones with no function. b) Snakes evolved from lizards with
four legs. But in primitive snakes, the remnants of hindlimbs persist (forelimbs are absent).
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Vestigial features II
The human appendix is a vestigial structure, reduced from the cecum of primate ancestors,
where it was involved in digestion of significant amounts of plant material. Shown here is
the human stomach leading into parts of the small intestine (duodenum, jejunum, ileum),
which empties into the large intestine (ascending colon). Note the appendix at this
juncture.
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Continental drift
Changing continental positions through most of the Phanerozoic era. Time, in millions of
years, is approximate.
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Dinosaur distribution
During the middle of the Mesozoic era, the dinosaur Allosaurus occupied the large, single
continent of Pangaea (see also “Continental drift”). Subsequently, as this continent broke
apart, populations of Allosaurus became isolated from each other and speciated into other
derivative carnivorous species (Gigantotosaurus, Carcharodontosaurus, Acrocanthosaurus).
The forming continents drifted into their present positions today. The location of these
fossil remains, now carried into distant locations, are indicated by dots.
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FIGURE 6.1 Paleontology
Appearance of animals and plants, early (bottom) to recent (top). Random (left)
appearance of organisms-humans, some plants, more mammals, fishes, microorganisms,
birds. Ordered (right) appearance from singlecelled, to multicelled, to fishes, amphibians,
reptiles, birds, and mammals, including humans.
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FIGURE 6.3 Different Functions but Similar Underlying Forelimb Anatomy
Forelimbs of bat, mole, and dugong. Each limb performs a different function-flight, digging,
and swimming, respectively-and all are superficially different, but all three share a
common, underlying anatomical plan.
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FIGURE 6.5 Morphological Series
From four-toed to single-toed modern horses, this morphological series illustrates the
correspondence between parts (feet, teeth, skull) and their modifications. Here, the
stratigraphic position of these species is added.
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FIGURE 6.7 Diversity of Type, Unity of Pattern
(a) Similarities. Parts may be similar in ancestry, function, and/or appearance.
Respectively, these are defined as homology, analogy, or homoplasy. (b) Although the
vertebrate species differ, the underlying pattern of the forelimb is fundamentally the same.
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FIGURE 6.8 Convergence of Design
Groups of animals often adapt to habitats that differ from those of most other members of their group. Most birds fly, but
some, such as ostriches, cannot, and live exclusively on land; others, such as penguins, live much of their lives in water. Most
mammals are terrestrial, but some fly (bats) and others live exclusively in water (whales, dolphins). “Flying” fishes take to the
air. As species from different groups enter similar habitats, they experience similar biological demands. Convergence to similar
habitats, in part, accounts for the sleek bodies and fins or flippers of tuna and dolphins, because similar functions (analogy) are
served by similar parts under similar conditions. Yet tuna and dolphins come from different ancestries and are still fishes and
mammals, respectively. Common function alone is insufficient to explain all aspects of design. Despite current similar habitats,
each design carries evolutionary features of the past into the present.
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FIGURE 6.9 Comparative Embryology
Embryonic retention of ancestral characteristics. Note the persistence of gill slits in early
stages of embryonic development in the mammals (right) that of course, do not possess
functional gills in the adult. Also note that a tail is present early in humans, but lost before
birth.
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FIGURE 6.10 Vestigial Features
(a) Whales evolved from terrestrial ancestors with four legs. But in whales, the hips and
hindlimbs are reduced to small bones with no function. (b) Snakes evolved from lizards
with four legs. But in primitive snakes, the remnants of hindlimbs persist (forelimbs are
absent). (c) The human appendix is a vestigial structure, reduced from the cecum of
primate ancestors, which was involved in digestion of significant plant material.
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FIGURE 6.11 Atavistic Features
Extra toes in modern horses. (a) Modern horses have only one enlarged digit on each foot, a single toe. The one toe
evolved from ancestors with three or four toes. During the course of their evolution, the peripheral toes—IV, II, I—
were lost and the central toe—III—emphasized. (b,c) On rare occasions, however, these “lost” toes or their
remnants reappear, testifying to the lingering presence of the underlying ancestral developmental pattern. (d) On
rare occasion, a modern horse, such as the one illustrated, is born with additional toes. Such toe remnants in
modern horses apparently represent the partial reemergence of an ancient ancestral pattern. For more information
on extra toes in modern horses, see S. J. Gould, Hen’s Teeth and Horse’s Toes: Further Reflections in Natural
History (New York: W. W. Norton, 1983).
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FIGURE 6.12 Biogeographic Realms
Continental land areas support characteristic assortments of plants and animals, which in
turn define six biogeographic realms. The number of mammalian families are indicated,
along with the number of endemic families.
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FIGURE 6.13a Continental Drift
(a) Plate tectonics. Shown are the suture zones of abutting major current crustal plates
and their respective movement directions.
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FIGURE 6.14 Bridges
Connections between geographic areas allow for or restrict dispersal of plants and animals. (a) Corridors
and filter bridges. Corridors allow for the relatively uninterrupted spread of organisms. The doubleheaded
arrows indicate such open expanses across Eurasia and North America. Filter bridges permit selective
transit of organisms that pass, either because of inhospitable climate or ecological obstruction. One major
selective filter has been across the Bering Strait; another is in the Middle East, restricting reptile species.
(b) Filter bridges between North and South America occur in the narrow land connection between these
continents. Some species have crossed this filter, but others have not.
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Table 6.1 Center of Origin
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FIGURE 6.15 The Panda’s Thumb
The panda has five digits like most mammals; however, opposing these is another digit, a
“thumb, “ which is actually not a thumb at all but an enlarged wrist bone. The furred and
padded panda right paw is shown; next to it, the underlying skeleton of hand and wrist
bones are sketched.