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Principles of Evolution


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Title: Principles of Evolution

Principles of Evolution
  • Chapter 6 Evidence of Evolution
  • James F. Thompson, Ph.D., MT(ASCP)

Recall Darwins Evidence
  • The Origin of Species By Means of Natural
    Selection or the Preservation of Favoured Races
    in the Struggle for Life, documents that fact
    that evolution has occurred
  • gives examples of artificial and natural
    selection to explain the mechanism of evolution
  • includes considerable evidence from comparative
    anatomy and comparative embryology
  • relies very little on the fossil record, a record
    still quite meager in the 1850s

Darwin Artificial Selection
Darwin Artificial Selection
Darwin Artificial Selection
Darwin was a recognized expert on pigeon breeds.
Evidence of Organismal Change
Several Hundred Breeds Recognized
Darwins Pigeons
Darwins studies of pigeon breeds included a
tentative phylogeny
Darwins Pigeons
Darwins studies of pigeon breeds included
detailed comparative anatomy
However, Darwin did not mention the Galapagos
finches in The Origin of Species.
Comparative Anatomy
  • Edward Tyson (16501708) is regarded as the
    founder of comparative anatomy.
  • In 1680, he demonstrated that porpoises are
  • In 1698, he dissected a chimpanzee and the result
    was the book, Orang-Outang, sive Homo Sylvestris
    or, the Anatomy of a Pygmie Compared with that of
    a Monkey, an Ape, and a Man.
  • He concluded that the chimpanzee has more in
    common with man than with monkeys, particularly
    with respect to the brain.

Thomas Henry Huxley
  • Frontispiece to Huxley's Evidence as to Man's
    Place in Nature (1863), comparing the skeletons
    of various apes to that of man.

Macroevolution The Transition From Reptile to
Bird Microraptor gui
Microevolutionary Change in Soapberry Bugs
A shortened beak is an adaptation to an
introduced food source.
Micro-evolutionary Change in DarwinsFinches
Body size and beak characteristics vary in
response to El nino induced droughts.
The drought changes the plant community and the
birds respond by adapting to new proportions of
seeds of differing size and hardness (bigger
and harder during drought.)
Comparative Embryology
  • Aristotle Preformation, Epigenesis, Pangenesis
  • Renaissance (human/medical) anatomists continued
    to collect data based on the ancient concepts
  • Leonardo Da Vinci, William Harvey, Marcello
    Malpighi, many others, especially the Italians
  • Christian Pander, Heinrich Rathke, and Karl Ernst
    von Baer founded modern embryology in the 1820s
    in Germany
  • Advances in microsopes and stains and the
    establishment of the Cell Theory of Life
    contributed to the progress

Comparative Embryology
  • Christian Pander discovered the three primary
    germ layers ectoderm, endoderm, and mesoderm
  • Pander also discovered the reciprocal regulatory
    tissue interactions among embryonic cells we now
    call Induction.

Comparative Embryology
  • Heinrich Rathke described for the first time the
    vertebrate pharyngeal/branchial pouches/arches,
    which become the gill apparatus of fish but
    become the mammalian jaws and ears, and
    contribute to the formation of the vertebrate
    skull, and the origin of the vertebrate
    reproductive, excretory, and respiratory systems.

Comparative Embryology
  • Karl Ernst von Baer extended Pander's studies of
    the chick embryo.
  • von Baer discovered the notochord, the rod of
    dorsal most mesoderm that separates the embryo
    into right and left halves and which instructs
    the ectoderm above it to become the nervous
  • He also discovered the mammalian egg.

von Baer's Laws
  • The general features of a large group of animals
    appear earlier in development than do the
    specialized features of a smaller group.
  • All developing vertebrates appear very similar
    shortly after gastrulation.
  • It is only later in development that the special
    features of class, order, and finally species
  • All vertebrate embryos have gill arches,
    notochords, spinal cords, and primitive kidneys.
  • Less general characters are developed from the
    more general, until finally the most specialized
  • All vertebrates initially have the same type of
  • Only later does the skin develop fish scales,
    reptilian scales, bird feathers, or the hair,
    claws, and nails of mammals.
  • Similarly, the early development of the limb is
    essentially the same in all vertebrates.
  • Only later do the differences between legs,
    wings, and arms become apparent.

von Baer's Laws
  • The embryo of a given species, instead of passing
    through the adult stages of lower animals,
    departs more and more from them.
  • The visceral clefts of embryonic birds and
    mammals do not resemble the gill slits of adult
    fish in detail.
  • Rather, they resemble the visceral clefts of
    embryonic fish and other embryonic vertebrates.
  • Whereas fish preserve and elaborate these clefts
    into true gill slits, mammals convert them into
    structures such as the eustachian tubes (between
    the ear and mouth).
  • Therefore, the early embryo of a higher animal is
    never like a lower animal, but only like its
    early embryo.
  • Human embryos never pass through a stage
    equivalent to an adult fish or bird.
  • Rather, human embryos initially share
    characteristics in common with fish and avian
  • Later, the mammalian and other embryos diverge,
    none of them passing through the stages of the

Comparative Embryology Darwin
  • Darwin did some comparative embryology himself.
  • Darwins drawing (left) compares human and cat
  • Photo (right) compares human and dog for
    comparison to confirm Darwins accuracy

Ernst Haeckel
  • Recall Haeckel who developed the controversial
    recapitulation theory ("ontogeny recapitulates
    phylogeny") claiming that development, or
    ontogeny, parallels and summarizes its species'
    entire evolutionary development, or phylogeny,
    based on continuing embryological studies.

Comparative Embryology
  • Structure unfolds during development in a
    conservative fashion.
  • Structural innovations usually appear at the end
    of development proceses, not the beginning.
  • This minimizes the disruption of potentially
    ancient and well established regulatory
  • New character traits form by remodeling rather
    than by new construction without a foundation.

beak limb buds
Homology and Analogy
  • Darwins contemporary, the comparative anatomist
    and paleontologist, Richard Owen, coined the
    terms homology and analogy (and Dinosauria).
  • To Owen, homologies indicated that organisms were
    created following a common (Divine/Vitalist) plan
    or archetype.

Homology and Analogy
  • Homology Any similarity between phenotypic
    characters that is due to their shared ancestry.
    Homologous structures may retain the function
    they served in the common ancestor or they may
    evolve to fulfill different functions.
  • Analogy Any phenotypic characters that perform
    the same or similar function by a similar
    mechanism but evolved separately.

Developmental Homology
  • Developmental Homology A related concept
    meaning that structures arose from the same
    tissue in embryonal development the ovaries and
    clitoris of female humans and the testes and
    penis of male humans are homologous in this

Developmental Homologies
All exhibit external head, tail, limb buds,
pharyngeal pouches, as well as internal
Homologies are Everywhere!
  • The leaves of vascular plants (oak, gingko, pine)
    are homologous.

Homologies are Everywhere!
  • The wings of insects (dragonfly, butterfly,
    cockroach) are homologous.

Classic Structural Homologies
Determining that Phenotypic Traits are
Homologous Allows Scientists to Build Phylogenies
Classic Structural Homologies
The vertebrate forelimbs have the same basic bone
design one long bone attached to two other long
bones attached to a forefoot with many carpals
attached to five or fewer digits. The vertebrate
hindlimb offers similar evidence of homology.
Different Functions but Similar Underlying
Forelimb Anatomy
  • Homology of 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.

Classic Structural Homologies
  • Several animals, including pigs, cattle, deer,
    and dogs have reduced, nonfunctional digits,
    referred to as dewclaws.
  • The foot of the pig has lost digit 1 completely,
    digits 2 and 5 have been greatly reduced, and
    only digits 3 and 4 support the body.

Classic Structural Homologies
  • These are best explained if there was a common
    ancestor that at one time had more functional
  • Thus pigs, cattle, deer, and dogs can be thought
    to have a common ancestor and are homologous to
    each other (although distantly)

Classic Structural Homologies
  • The dog retains four functional digits and digit
    1 has become a dewclaw

Classic Structural Homologies
  • The number of digits was variable in the earliest
    fossil amphibians but pentadactyly became the
    standard for all later vertebrates, though there
    have been many reductions in number and a much
    fewer increases in certain finned reptiles.

Diversity of Type, Unity of Pattern
  • Parts may be similar in ancestry, function,
    and/or appearance.
  • Respectively, these are defined as homology,
    analogy, or homoplasy.
  • Although the vertebrate species differ, the
    underlying pattern of the forelimb is
    fundamentally the same.

Diversity of Type, Unity of Pattern
  • Although the vertebrate species differ, the
    underlying pattern of the forelimb is
    fundamentally the same.
  • Note again that the major changes come in the
    number of phalanges and thelength and number of
    bones making up the phalanges

  • Humans and all other apes have chests that are
    broader than they are deep, with the shoulder
    blades flat in back, with associated differences
    in shoulder muscle attachments, an adaptation to
  • Monkeys and most other tetrapods walk
    quadrupedally and have narrow, deep chests with
    shoulder blades on the sides.
  • This homology, the anatomical and behavioral
    adaptations for brachiation, is a shared derived
    character for the apes.

  • Homologous structures may be adapted for
    different roles in which case their structure may
    vary considerably. Consider mammalian incisors
    and canines.

Structural Homologies
orchid flower structure
From Darwin (1862)
Structural Homologies
  • The Eukaryotic Cell
  • Nucleus
  • mitosis
  • meiosis
  • DNA supercoiling with histones and higher levels
  • Cytoplasm
  • all cellular organelles
  • most central cellular biochemical pathways
  • Plasma membrane
  • phospholipid bilayer
  • cholesterol present

A Deep Homology The Genetic Code
This is one of the most powerful of
all homologies, as it links all living
organisms on Earth to a common ancestor!
A few microbes have a few differences.
Protein Homologies
  • The gene that codes for the protein hormone
    insulin is homologous in humans and other
    mammals the similar DNA sequences descended from
    a common ancestral gene sequence.
  • Human insulin (right) and pig insulin (left),
    showing their one amino acid difference

Behavioral Homologies
  • Birds, crocodiles, and mammals provide parental
    care to their offspring
  • This behavior was inherited from a common
    ancestor and is certainly homologous for birds
    and crocodilians
  • It is less certain their common ancestor with the
    mammals exhibited a similar degree of parental

Homology A Genetic Flaw That Humans Share With
Charcot-Marie-Tooth Disease
PMP-22 peripheral myelin protein 22
  • Analogy Any phenotypic characters present in
    different taxa which perform the same or a
    similar function by a similar mechanism but which
    evolved separately.
  • The structural carbohydrate chitin in fungal cell
    wall and in arthropod exoskeletons

Traits their common ancestor lacked.
shark killer whale
hydrodynamic body forms
  • Thylacosmilus is a marsupial sabre-tooth cat
  • Smilodon is a placental saber-tooth cat.

Camera-type Eyes
Analogy Oxygen-Binding Proteins
  • Hemoglobin combines iron heme groups and globin
    proteins, red pigment found in vertebrate
  • Myoglobin combines iron heme groups and globin
    proteins, red pigment found in vertebrate muscle
  • Hemocyanin combines copper prosthetic groups and
    globin proteins, blue pigment found in arthropod
    and mollusc blood plasma
  • Hemerythrin combines iron-containing non-heme
    globin proteins, pink/violet vs colorless pigment
    found in some marine invertebrate blood plasma
  • Chlorocruorin combines iron heme groups and
    globin proteins, red vs green pigment found in
    many annelids
  • Vanabins (aka vanadium chromagens) combine the
    rare metal vanadium in prosthetic groups and
    globin proteins, colorless molecules found in sea
    squirts blood plasma
  • Erythrocruorin is a giant free-floating blood
    protein containing many dozens possibly
    hundreds of iron- and heme-bearing protein
    subunits bound together into a single protein
    complex with a molecular mass greater than 3.5
    million daltons Found in many annelids, including
  • Pinnaglobin is a brown manganese-based porphyrin
    protein only found in the mollusc Pinna squamosa.
  • Leghemoglobin combines iron heme groups and
    globin proteins, colorless compound found in
    leguminous plants, such as alfalfa or soybeans.

  • Homoplasy is the logical opposite of homology.
  • Homoplasy is the structural similarity between
    two traits in two species without phyletic
  • Even though the traits are similar, the common
    ancestor of species A and B did not present the
  • There are three different types of homoplasy
    convergence, parallelism, and reversal.

  • Convergence is the evolution of similar traits in
    response to similar adaptive pressures, but not
    to similar genes and developmental processes
  • An example of convergence is found in the
    electroreception of mormyrids (African weak
    electric fish) and gymnotoids (South American
    electric eels) while the organs responsible for
    this perceptual capacity are similar, they are
    not derived from a common ancestor.

Convergence of Design
Groups of animals often adapt to habitats that
differ from those of most other members of their
group. 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
  • 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.

Convergence Ecological Equivalents
Ecological Equivalentswithout Convergence
Pileated Woodpecker ? And Woodpecker Finch ?
Madagascar Aye-Aye
  • Pollination in the Bee orchid (Ophrys apifera) is
    enhanced by reproductive mimicry and the
    pheremones that attract specific male bumblebees.

An extinct South American Litoptern
  • Parallelism occurs in closely related taxa, and
    is defined as the independent development of a
    descendant character that is not present on a
    common ancestor.
  • Parallelism occurs when two taxa develop the same
    character after evolutionary divergence since
    the trait is absent in a common ancestor, but
    present in both descendant species, it is
    probable that the developmental genetics that
    produces the structures in the different taxa is
    the same, which means it was inherited from the
    common ancestor.
  • Thus, there is homology between the developmental
    and genetic materials, but not on the final
  • (Also ecological equivalents.)

Snake eyes
  • Reversals are instances of homoplasy in which a
    character appears, subsequently disappears, and
    later reappears among the descendants in one

blind lizard
Homoplasy (convergence)
The Pandas 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.

Homology and Model Organisms
Human (Intelligent) Design
  • Machine functions vary and machine designs vary
  • Unlike biological organisms, human designed and
    built machines show no correspondence of parts
    from planes (flight), to mining machines
    (burrowing), or to boats (swimming).

Morphological Series
  • Comparative evidence allows systematists to
    collect taxa into nested clusters
  • Introduction of new shared derived characters can
    help establish both grades and clades
  • Availability of fossil specimens improves the
    understanding of common ancestors
  • Related taxa can be organized into morphological
    series for explanatory purposes (grades and

Trilobite Phylogeny
Morphological SeriesEvolution of Jaws
  • Vertebrate jaws evolved from the front set of
    gill arches of jawless ancestors.

Morphological SeriesEvolution of Limbs from Fins
  • Note that the bones of the fish hip and shoulder
    correspond to bones in this early tetrapod
  • Here, the morphological series carries from one
    type of vertebrate (fish) to another (amphibian),
    and from one environment (water) to another

Evolution of Horses
O.C. Marsh 1879
Evolution of Horses
Bruce McFadden 2005
J.W. Gidley 1907
Evolution of Horses
Another Modern Horse Phylogeny
This morphological series illustrates the
correspondence between parts (feet, teeth, skull)
and their modifications.
Transitional Fossils Documenting the Evolution of
Birds from Dinosaurs
Dromaeosaurus (with feathers?)
Transitional Fossils Documenting the Evolution of
Whales from Legged Ancestors
Reported 2001
Reported 1994
Reported 1990
Hominoid Morphological Series
Well save our discussion of the morphological
series leading to Homo sapiens until Chapters 14
Vestigial Features
  • The term Vestigial Feature refers to behaviors,
    anatomical structures, physiological processes,
    biochemical pathways or gene sequences
    (pseudogenes and junk DNA), which have lost most
    or all of their original adaptive function, an
    adaptive function which may still be observed in
    the corresponding homologous feature in some
    related species or other taxon.
  • Some of these vestigial features disappear early
    in embryonic development, but others are retained
    in adulthood.
  • Vestigial Features typically appear to be in a
    degenerate, atrophied, or rudimentary condition,
    and tend to be much more variable than similar
  • Although largely or entirely functionless,
    vestigial features may retain lesser functions or
    develop minor new ones.

Developmental Vestigial Features
Developmental Vestigial Features
Chick embryo
fifth digit
No remnant persists in the adult
Vestigial Features
  • Whales evolved from terrestrial tetrapods. But in
    whales, the hips and hindlimbs are reduced to
    small bones with no function.
  • Snakes evolved from lizards with four legs. But
    in primitive snakes (e.g., pythons), the remnants
    of hindlimbs persist (forelimbs are absent).

Vestigial Features
Eyes Wings
Hind limbs
Vestigial Features
Hoatzin chick wing claws adaptive
Emu adult wing claws vestigial
Vestigial Features
telson (tail)
Vestigial Features
Many Taraxacum species produce seeds asexually by
apomixis, where the seeds are produced without
Vestigial Features
Hieracium aurantiacum (orange hawkweed)
Antennaria dioica (mountain everlasting)
Vestigial Wings?
Galapagos Cormorant yes
Ostrich no
Kiwi yes
Penguin no
Vestigial Features
  • 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.

rabbit cecum
Human Vestigial Features
(15,000 year old woman)
vermiform appendix
wisdom teeth 3rd molars
muscles to move the external ear
arrector pili muscle
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 toesIV, II, Iwere lost and the
    central toeIIIemphasized. (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.

Atavistic Features
  • In 2006, a bottlenose dolphin with palm-sized
    fins near its tail was found in Japan.
  • The extra set of fins are atavistic hind limbs.

Atavistic Features
  • Supernumerary nipples. (Third and fourth nipple
    of male scandinavian).
  • A - regular birthmark.
  • B - regular nipple.
  • C - supernumerary nipple

Another male
Atavistic Features
  • A radiogram of the sacral region of a six-year
    old girl with an atavistic tail.
  • The tail was perfectly midline and protruded form
    the lower back as a soft appendage.
  • The five normal sacral vertebrae are indicated in
    light blue and numbered the three coccygeal tail
    vertebrae are indicated in light yellow.
  • The entire coccyx (usually three or four tiny
    fused vertebrae) is normally the same size as the
    fifth sacral vertebrae.

Atavistic Features
  • A well-developed tail is characteristic of the
    human embryo in the second month.
  • Ususally during the third month the tail
    regresses and disappears as an anatomic external
  • Occasionally the tail persists and grows with the
    rest of the body.
  • Tails as long as 23 cm have been reported.

From Homology to Phylogeny
  • Homologous traits can be grouped into series
  • Homologous traits can include adaptive traits,
    maladaptive traits, vestigial features and
  • All homologous traits can be of potential use in
    deriving phylogenetic trees
  • However, care must be taken to avoid using
    analogous traits by mistake
  • Finally, homologous traits are excellent evidence
    that evolution has, in fact, occurred!

The Geological Record Evidence for Evolution
Weve already looked at the Fossil Record as
Evidence that Evolution Has Occurred
Distributional Evidence for Evolution
  • "The most wonderful mystery of life may well be
    the means by which it created so much diversity
    from so little physical matter.  The biosphere,
    all organisms combined, makes up only about one
    part in ten billion of the earths mass.  It is
    sparsely distributed through a kilometer of soil,
    water, and air Yet life has divided into
    millions of species, the fundamental units, each
    playing a unique role in relationship to the
    whole."  - E.O.Wilson The Diversity of Life

Distributional Evidence for Evolution
  • Biogeography is the study of the spatial
    distribution of species (and higher taxa) and
    their ecological communities through time.
  • Biogeography organizes information about where
    organisms live, how they arrived there, and their
    evolution and ecology.
  • The patterns of species distribution are usually
    explained through a combination of principles
  • evolution (acaptation, speciation, extinction,
    ecology, etc.)
  • geoclimate (continental drift, glaciation,
    fluctuations in sea level, weather, wind and
    water movements, etc.)
  • geography (surface topography, soil composition,
    energy and resource availability, etc.)

Alfred Russel Wallaceis the Father of
  • As a young man, Wallace studied the distributions
    of living organisms in the Malay Archipeligo
    where he lived for several years.
  • His classic works, the two-volume work, The
    Geographical Distribution of Animals (1876) and
    Island Life (1880 ) would serve as the definitive
    texts on biogeography for the next 80 years.

Wallaces Line
  • Wallace propssed six biogeographic realms for the
    earth, based primarily on animal distributions.
  • He was most familiar with South East Asia where
    we still honor him with Wallaces Line which
    marks the boundary between Australian and Asian

Biogeographic Realms
  • Below is a modern version of Wallaces six
    biogeographic realms based on the number of
    mammalian families.

Endemic families are those found in only the one
location (realm).
Eight Modern Biogeographic Realms
Wallaces six and
Eighteen biomes are mapped within realms in this
modern global map.
Darwin and Biogeography
  • In The Origin of Species Darwin wrote of centers
    of creation, which later came to be known as
    centers of origin for various taxa.
  • A center of origin is a proposed location, based
    on fossil or modern distribution evidence, for
    the home of the oldest common ancestor of a
    taxonomic group
  • A related concept is center of diversity, a
    region where the greatest proportion of members
    of the taxanomic group are present.
  • A center of origin and a center of diversity are
    often the same location, or not far from one

Centers of Origin and Dispersal
  • These centers remain useful concepts.
  • However, from the 1860s to the 1960s they
    sometimes confused issues because scientists
    assumed the continents had always been in the
    same locations throughout geologic time.
  • If so, then terrestrial organisms were
    distributed across the globe in ways that were
    difficult to explain.
  • Hypotheses for methods of long distance dispersal
    were required.

Examples of Centers of Origin
Land Bridges
  • Connections between geographic areas allow for or
    restrict dispersal of plants and animals. (a)
    Corridors allow for the relatively uninterrupted
    spread of organisms.
  • The doubleheaded arrows indicate such open
    expanses across Eurasia and North America.

Land Bridges
  • Connections between geographic areas allow for or
    restrict dispersal of plants and animals. (a)
    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.

filter bridge
filter bridge
Land Bridges
  • (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.

filter bridge
Land Bridges
  • If continents do not move, however, then how do
    we explain the bridge to disperse marsupials
    between Australia and the New World or alligators
    and dogwoods between the Southeast United States
    and Southeast China?

alligators and dogwoods
There were dozens, if not hundreds, of equally
mystifying distributions known!
Methods of Long Distance Dispersal
  • You can read a classic paper on the subject
  • Mammals and Land Bridges by George Gaylord
    Simpson (1940)
  • Recall that Simpson was the paleontologist among
    the founders of The Modern Synthesis of Evolution

Continental Drift
  • Alfred Wegener (1880 1930), a German
    meteorologist, proposed continental drift in 1912
    based on fossil and mineral distributions and
    continental coast lines
  • He proposed that all the continents were once
    joined in a single landmass, which he called
  • Plate Tectonics was not accepted as the
    explanatory theory until the 1960s.

Continental Drift
Laurasia Gondwana
  • Changing continental positions through most of
    the Phanerozoic era. Time, in millions of years,
    is approximate.

Plate Tectonics
  • Plate tectonics describes the large scale motions
    of Earth's lithosphere.
  • The theory encompasses the older concepts of
    continental drift, developed during the first
    half of the 20th century, and seafloor spreading,
    understood during the 1960s.

Plate Tectonics
  • The outermost part of the Earth's interior is
    made up of two layers above is the lithosphere,
    comprising the crust and the rigid uppermost part
    of the mantle.
  • Below the lithosphere lies the asthenosphere.
  • Although solid, the asthenosphere has relatively
    low viscosity and can flow like a liquid on
    geological time scales.
  • The deeper mantle below the asthenosphere is more
    rigid again due to the higher pressure.

Plate Tectonics
  • The lithosphere is broken up into what are called
    tectonic plates  in the case of Earth, there are
    seven major and many minor plates.
  • The lithospheric plates ride on the
  • Earthquakes, volcanic activity,
    mountain-building, and oceanic trench formation
    occur along plate boundaries.

Tectonic Plates
Continental Drift
  • Léon Croizat (1894 1982), an Italian biologist,
    whose career included time in the US (1936-47)
    and later in Venezuela, also proposed continental
    drift, more or less independently, based on
    distribution of communities of living organisms
  • Croizat lived to see Plate Tectonics accepted as
    a Theory in the 1960s
  • He was still writing scientific papers when he
    died at age 88!

Léon Croizat
  • Based on the metaphor that "life and earth evolve
    together" -which means that geographic barriers
    and biotas co-evolve ? Croizat developed a new
    biogeographic methodology, which he named
  • This method was basically to plot distributions
    of organisms on maps and connect the disjunct
    distribution areas or collection localities
    together with lines called tracks.
  • Croizat found that individual tracks for
    unrelated groups of organisms were repetitive,
    and considered the resulting summary lines as
    generalized tracks which indicated the
    preexistence of ancestral biotas, subsequently
    fragmented by tectonic and/or climatic changes.

Croizats Panbiogeographic Tracks
Similar taxa or communities of taxa are linked
along the panbiogeographic tracks.
Dispersal Versus Vicariance
  • Darwin, Wallace, Simpson, and most other
    biogeographers, before the 1960s, looked for
    dispersal events to explain distributions.

Croizat and his disciples offer an alternative
Dinosaur Distribution Vicariance
  • During the middle of the Mesozoic era, the
    dinosaur Allosaurus occupied the large, single
    continent of Pangaea.
  • 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 red

Cladistic Biogeography
  • Upper figure shows a cladogram based on geologic
    evidence for breakup of Gondwanan continents
  • Lower figure uses the distributions of some
    shared taxa (hylid frogs, ratite birds, and
    xylontine fishes) to estimate breakup of Pangean

Other Biogeographical Concepts
  • dispersal barrier or ecological barrier An area
    of unfavourable habitat that separates two areas
    of favourable habitat
  • oceans or rivers for terrestrial organisms
  • desert or grassland for of woodland organisms.
  • waterfalls for river/stream organisms
  • etc.

Other Biogeographical Concepts
  • sweepstakes dispersal An unlikely chance event
    which carries a few individuals to a new and
    distant piece of favorable habitat

Other Biogeographical Concepts
  • founder effect the loss of genetic variation
    when a new colony is established by a very small
    number of individuals from a larger population.
  • It was first fully outlined by Ernst Mayr in
  • As a result of the loss of genetic variation, the
    new population may be distinctively different,
    both genetically and phenotypically, from the
    parent population from which it is derived.
  • In extreme cases, the founder effect may lead to
    the speciation and subsequent evolution of new

Other Biogeographical Concepts
Zhejiang province, Eastern China
  • bottleneck effect an evolutionary event in which
    a significant percentage of a population or
    species is killed or otherwise prevented from
    reproducing, and the population is reduced by 50
    or more, often by several orders of magnitude.
  • Population bottlenecks increase genetic drift, as
    the rate of drift is inversely proportional to
    the population size.
  • They also increase inbreeding due to the reduced
    pool of possible mates.

Summary Main Sources of Evidence for Evolution
  • Comparative Studies
  • Demonstration of Homologies
  • Demonstration of Shared Derived Characters
  • The Geological and Paleontological Record
  • Biogeographical Distribution Patterns

Is There Necessarily a ConflictBetween Science
and Religion?
  • Methodological Naturalism The only hypotheses
    researchers propose to account for natural
    phenomena, and the only explanations they accept,
    are hypotheses and explanations that involve
    strictly natural causes.
  • Ontological Naturalism The natural world, the
    physical material universe, is all there is.
  • There are scientists in both camps.

Further Reading
  • The theme of this chapter is that the scientific
    evidence for the occurrence of evolution is now
    overwhelming and no educated person should doubt
    the fact of evolution.
  • There can be no contradiction whatsoever between
    believing in evolution and believing in a God or
    creative force behind the universe.
  • However, there is plenty of evidence to
    contradict the concept of an Intelligent
  • But the conflict goes on . . .

End Chapter 6
  • "Even more peculiar is the course of the
    recurrent laryngeal nerve, which corrects the
    brain to the larynx and allows us to speak. In
    mammals, this nerve avoids the direct route
    between brain and throat and instead descends
    into the chest, loops around the aorta near the
    heart, then returns to the larynx. That makes it
    seven times longer than it needs to be! For an
    animal like the giraffe, it traverses the entire
    neck twice, so it is fifteen feet long (fourteen
    feet of which are unnecessary). Not only is this
    design wasteful, but it also makes an animal more
    susceptible to injury. Of course, the bizzare
    pathway of this makes perfect sense in
    evolutionary terms. In fish and early mammal
    embryos, the precursor of the recurrent laryngeal
    nerve attached to the sixth gill arch, deep in
    the neck and body region. Fish still retain this
    pattern, but during late human embryology, the
    gill arches are modified into the tissues of our
    throat region and pharynx. Parts of the old
    fish-like circulatory system were rearranged, so
    the aorta (also part of the sixth gill arch)
    moved back into the chest, taking the recurrent
    laryngeal nerve (looped around it) backward as
    well." (Prothero, Donal R. "Evolution What the
    Fossils Say and Why It Matters", 2007, pp.