Instructions for printer-friendly copy.


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(click on pic for entertaining source)

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    Macroevolution: The Origin of Species

    Macroevolution is the genesis of reproductively isolated populations from an ancestral population.

    • New species evolve from pre-existing species.
    • New species remain nested in the ancestry of their predecessors.
      • Birds give rise to birds.
      • Frogs give rise to frogs.
      • Yeast give rise to yeast.
      • Parents do not give rise to offspring drastically different from themselves.

    • Speciation is a temporal process.
    • Populations exist in various stages of speciation at any given time.
    • Extant populations are even now undergoing microevolutionary changes
      x that may eventually give rise to new species.

    • Incipient species are on the verge of becoming separate species.
    • Sister species (or taxa) have already diverged from a common ancestor.

    What are the natural processes that cause one species to become two?

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(click on pic for source)

    Concepts of Speciation

    Darwin thought that species arose via anagenesis.

    Anagenesis (= phyletic evolution) is the conversion of an entire species to a morphologically different form, generating a new species.

    • The evolutionary line does not branch.
    • There is no net increase in species diversity.
    • The ancestral form gives rise to only one new form.

    A more modern view is that species arise via cladogenesis.

    Cladogenesis (= diversifying evolution) is the divergence of an ancestral species/taxon into two sister species/taxa.

    • The evolutionary line branches.
    • There is a net increase in species diversity.
    • The ancestral form gives rise to two new forms.

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    Anagenesis Doesn't Count

    Anagenesis is not a quantifiable aspect of macroevolution.

    Fossils are endpoints in a clade, just as extant species are.

    All ancestors are hypothetical because

    • They are dead and gone
    • They cannot be directly observed
    • Ancestor status cannot be assigned to a
      particular fossil with any degree of certainty.

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    Illustrating Anagenesis vs. Cladogenesis:
    Evolution of the Modern Horse

    Anagenesis is the gradual transformation
    of an ancestral lineage into successive,
    morphologically distinct descendant lineages.

    Traditional depictions of horse evolution often displayed
    in natural history museums imply anagenesis.

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    If Horses Had Undergone Anagenesis...

    An anagenetic view of horse evolution would hold that
    • Hyracotherium evolved into
      • Mesohippus and [Miohippus ---> extinct].
      • Mesohippus evolved into
          • Merychippus and [Parahippus ---> extinct] .
          • Merychippus evolved into
            • Pliohippus and [Hipparion ---> extinct].
            • Pliohippus evolved into Equus

    In this scenario, Hyracotherium is considered a direct ancestor of Equus.

    Is this position scientifically defensible?

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(click on pic for source)

    Cladogenesis in the Horse Lineage

    Cladogenesis is the sequential, side by side divergence
    of ancestral taxa into two sister taxa over evolutionary time.

    Any speciation event one could explain
    by invoking anagenesis can be explained
    more accurately by invoking cladogenesis and extinction.

    Remember: by definition, an ancestral taxon becomes
    extinct when it splits into two daughter taxa.

    All ancestors are hypothetical.
    Their descendants are the endpoints on the phylogenetic tree.

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Miohippus, Parahippus, and Hipparion have not been included here only for simplicity. If they had been, each would be placed on the tree in an appropriate spot, branching from its ancestor along with its sister taxa.

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(a three-pointer!)

    Modern Equus Arose via Cladogenesis

    The cladistic view of horse evolution would be:
    • An original ancestor gave rise to (1) Hyracotherium and (2) Ancestor x.
      xAncestor x gave rise to (1) Mesohippus and (2) Ancestor y
      xxAncestor y gave rise to (1) Merychippus and (2) Ancestor z
      xxxAncestor z gave rise to (1) Pliohippus and (2) Equus

    Thus, Equus did not evolve from Hyracotherium.

    All five horse genera share the same common ancestor.
    All but Equus (consisting of seven extant species) went extinct.

    Equus is the only surviving genus of the original ancestor.

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Modes of Speciation

Physical, ecological, and genetic factors often interact to drive populations to evolve and undergo cladogenesis.

Mechanisms that drive speciation can be

Four general modes of speciation can be defined,
though differences between some of them can be subtle.

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By Andrew Z. Colvin - Own work

    Allopatric Speciation

    Allopatric (Greek: allos, "other"; patri, "Fatherland") species occupy different geographic regions.

    Allopatric speciation is the evolution of a new species from an ancestral species due to geographic division of the ancestral population that prevents gene flow between them.

    Allopatric speciation is also known as geographic speciation.

    This may be the most common mode of speciation.
    There are numerous examples throughout the world.
    Here are just two of the many.

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    Allopatric Speciation: Continental Drift

    As Pangaea split into Laurasia (north) and Gondwanaland (south),
    and then into the continents of today, species living on split with them.

    Differences in

    • mutation
    • climate
    • natural selection
    ... drove their divergence from their common ancestral forms.

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    Allopatric Speciation: Isthmus of Panama

    The Isthmus of Panama formed 2.8 million years ago, separating the Atlantic and Pacific Oceans and generating the Gulf Stream.

    Populations all species living on either side of the isthmus were separated, and proceeded to evolve in isolation from one another.

    Snapping Shrimp on either side of the Isthmus speciated.
    Alphaeus nuttingi and A. millsae are sister species.
    A. formosus and A. panamensis are sister species.

    Porkfish on either side of the Isthmus speciated.
    Anisotremus taeniatus abnd A. virginicus are sister species.

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By Andrew Z. Colvin - Own work

    Peripatric Speciation

    Peripatric (Greek: peri, "around"; patri, "Fatherland") species occupy overlapping ranges.

    Peripatric speciation is the evolution of a new species when an isolated peripheral deme/population undergoes reproductive isolation from its ancestral population due to reduced gene flow.

    Peripatric speciation is sometimes considered a special case of allopatric speciation.

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    Peripatric Speciation: California Channel Islands

    The California Channel Islands form an archipelago of eight islands along the coast of southern to central California. Like the mainland, the habitat is primarily coastal chapparal.

    An endemic species is defined as one that is native and restricted to
    a particular geographic location.

    In the California Channel Islands, several unique, endemic species and subspecies share common ancestors with their mainland sister taxa.

    • Torrey Pine (Pinus torreyana insularis)
    • Island Tree Mallow (Malva assurgentiflora )
      • closest relative is a Mexican Tree Mallow
    • Island Night Lizard (Xantusia riversiana)
      • sister species: Bolson Night Lizard (Xantusia bolson) (Mexico)
    • Island Scrub Jay (Aphelocoma insularis)
      • sister species: California Scrub Jay (Aphelocoma californica)
    • San Clemente Loggerhead Shrike (Lanius ludovicianus mearnsi)
    • San Clemente Sage Sparrow (Artemisiospiza belli clementeae)
    • Channel Islands Spotted Skunk (Spilogale gracilis amphiala)
    • Island Fox (Urocyon littoralis)
      • sister species: Gray Fox (Urocyon cinereoargenteus)

      Species (and incipient species) evolving from small founder mainland populations arise via peripatric speciation.

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    Paripatric Speciation:
    Grass Tolerance to Heavy Metals

    Populations of Vernal Sweet Grass, Anthoxanthum odoratum, living around strip mines have undergone selection for tolerance to heavy metals in the soil.
    • Original population is not tolerant of heavy metals.
    • Incipient daughter species is tolerant of heavy metals.
    • Natural selection removes metal-intolerant hybrids from contaminated soil.
    • Over generations, further divergence is likely to occur.

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By Andrew Z. Colvin - Own work

    Parapatric Speciation

    Parapatric (Greek: para, "beside"; patri, "Fatherland") refers to a geographic distribution that is continuous, but characterized by differences in habitat.

    Parapatric speciation is the evolution of a new species from an ancestral species without extrinsic geographic barriers to gene flow.

    • The population is continuous.
    • The population does not mate randomly.
    • Individuals are more likely to mate with neighbors than distant individuals.
    • This creates a cline of genotypes.
    • Individuals at opposite ends of the range are reproductively isolated.
    • It is sometimes considered a form of ecological speciationoug.

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Siberian Greenish Warblers exhibit a ring pattern generated by parapatric speciation.

    Parapatric Speciation: Ring Species

    A ring species is a geographically contiguuous series of neighbouring demes, each of which can interbreed with neighboring demes, but for which there exist at least two "terminal" populations too distantly related to interbreed.

    Salamanders in the genus Ensatina spp. form a ring complex.

    The Euphorbia tithymaloides species complex, with populations distributed around the Caribbean, is the only plant system for which molecular data supports a ring-species pattern. (Cacho Lab, UC Davis)

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By Andrew Z. Colvin - Own work

    Sympatric Speciation

    Sympatric (Greek: patri, "Fatherland"; sym, "together") species occupy the same geographic region.
    They exhibit sympatry.

    Sympatric speciation is the evolution of a new species from an ancestral species while both continue to inhabit the same geographic region.

    Sympatric speciation can result from

    • a sudden genetic event (e.g., polyploidy)
    • differential resource utilization (ecological)

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(Click on pic for source)

    Sympatric Speciation:
    Hawthorne Flies and Apple Maggot Flies

    Hawthorne flies (Rhagoletis pomonella) are native to the U.S.

    Before European settlers arrived, the flies laid their eggs
    on hawthorne fruit, which are related to apples.

    When European settlers brought and planted apples,
    a subset of R. pomonella began laying eggs on apples.

    The flies will lay eggs only on fruit type
    in which they spent their maggothood.

    Not surprisingly, there is now a R. pomonella population
    reproducing only on apples.

    Gene flow between this new population
    and the original population is highly reduced.

    Genetic divergence between the incipient species can already be seen.

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Autopolyploidy.


Allopolyploidy.

    Sympatric Speciation: Polyploidy

    Unlike most animals, plants can generate polyploid zygotes that grow into viable individuals.
    • Usually larger and more robust than their their parents.
    • Usually cannot back-cross with their parents due to chromosomal differences.
    • They can often self-pollinate or cross-pollinate with polyploid siblings.
    • Instant reproductive isolation!

    In autopolyploidy, chromosomes come from one parent species.

    In allopolyploidy, chromosomes come from two related species.

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Gilia minor


Gilia clokeyi

    Allopolyploidy in a Desert Flower
    (Gilia, Polemonaceae)

    Gilia minor, Gilia clokeyi and Gilia transmontana
    are all native to the Mojave Desert in the southwestern U.S.

    Chromosomal analysis reveals that Gilia transmontana is the product of hybridization between Gilia minor and Gilia clokeyi.

    Gilia transmontana has two sets of chromosomes from each parent species, making it allotetraploid.

    It is reproductively isolated from its two parent species.

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Gilia transmontana by Steve Matson

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<--required video)

    Side by side, the modes of speciation can be visualized this way.

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    Ecological Aspects of Speciation

    Ecological speciation is the process by which natural selection operates on ecological characters, resulting in reproductive barriers between populations.

    The modes of speciation we have discussed so far and ecological speciation are by no means mutually exclusive.

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    The Ecological Niche

    The ecological niche of any given species includes
    • its role and position in its environment
    • the ways it acquires food and shelter
    • its survival mechanisms
    • its reproductive strategies and natural history
    • all of its interactions with biotic and abiotic factors in its environment

    Ecological interactions between species and their environment
    drive natural selection and speciation.

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    Adaptive Radiation

    Living populations migrate and diversify.
    Over time, such populations undergo repeated cladogenesis
    as lineages evolve and "radiate" into new ecological niches.

    This diversification can be driven by

    • mutation
    • migration
    • assortative mating
    • genetic isolation
    • natural selection

    The end result is adaptive radiation.

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(Click on pic for more!)

    Adaptive Radiation: Anolis spp.

  • Anolis distichus group in the West Indies share a common ancestor.
  • The ancestor arrived on the islands millions of years ago.
  • The ancestral population gave rise to at least 16 different species.
  • Each species occupies a distinct ecological niche.
    • Body morphology depends on the specific habitat the species occupies.
    • Dewlap color displacement results from competition for mates.
  • Both sexual selection and natural selection have modified Anolis characters.

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    Adaptive Radiation: Poison Dart Frogs

  • A single species, the Strawberry Poison Dart Frog (Dendrobates pumilio)
    exhibits different color morphs in different regions of Panama.
  • The more brightly colored the frog, the more toxic it is.
  • Females significantly prefer individuals of their own color morph.
  • Frogs mate assortatively, based on coloration.
  • This reinforces the ecological signal that warns predators of toxicity.

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MacArthur (1958) reported evolutionary divergence of five closely related warbler species that underwent adaptive radiation. Each species now uses a different part of their evergreen forest habitat for foraging.

    Competitive Exclusion

    The Competitive Exclusion Principle (Gause's Law):
    In a stable environment, two species cannot coexist
    if they occupy exactly the same ecological niche.

    Species competing for the same resources must
    evolutionarily adapt to exploit different resources.

    This leads to resource partitioning: dividing a common resource
    so that each competing species uses only a portion of that resource.

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    Character Displacement

    Changes in the competing species' morphology
    associated with resource partioning are
    often accompanied by character displacement:

    • accentuation of differences among species when they are sympatric
    • minimization (or loss) of differences among species when they are not sympatric

    Character displacement is a result of competition leading to resource partitioning.

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    Character Displacement:
    Hawaiian Honeycreepers

    • An ancestral finch colonized Hawaii millions of years ago.

    • From this ancestor, 51 species of Hawaiian Honeycreepers evolved.

    • Each species is evolutionarily adapted for a specific foraging strategy.

    • More than 30% are now extinct (Sadly, 15 since Europeans arrived).

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    Character Displacement: Darwin's Finches

    • An ancestral finch colonized the Galapagos millions of years ago.

    • From this ancestor, 16 different species comprising five genera evolved.

    • The bill size and shape of each species reflects a specific foraging strategy.

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    Extinction

    Extinction is the endpoint of evolution.

    It is a natural process.
    Most species that ever existed on earth are now extinct.

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    Life is Ancient

    Life on earth originated at least 4 billion years ago.
    It has been diversifying and expanding ever since.

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    But as new species arise,
    existing species may go extinct.

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(click on pic for source)

    What Drives Extinctions?

    Past extinctions have been driven by
    • species interactions
    • local ecosystem changes (habitat loss)
    • natural disasters
    • climate change
      • glaciation
      • changes in sea level

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(click for source)

    Mass Extinctions

    The Phanaerozoic (Greek phaneros, "visible" and zo, "animal") is the eon encompassing all time including

    • the Cambrian (~ 540-->485 mya)

    • to present time

    The fossil record contains evidence of at least five past global extinction cycles.

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    Evolution of the Oxygen-rich Atmosphere

    Photosynthesis is the chemical process by which
    plants and some bacteria

    • sequester atmospheric carbon dioxide
    • use it to build carbohydrates.

    The waste product of photosynthesis is oxygen.

    Oxygen is highly reactive and can be deadly
    if you're not evolutionarily equipped to use it.

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    When Bacterial F*rts Nearly Wiped Out Life on Earth

    Before photosynthesis evolved, earth's atmosphere contained little free O2.

    About 2.8 billion years ago, photoautotrophic bacteria appear in the fossil record.
    They contained chlorophyll a, the most primitive form of chlorophyll.
    Photosynthesis began producing oxygen.

    For 1.2 billion years, photosynthetically produced oxygen reacted with
    exposed iron in the ocean and earth's crust, forming iron oxide (rust).

    Once the rust "sink" was filled, oxygen began to enter the atmosphere.

    The result was a mass extinction sometimes known as The Great Oxygen Catastrophe.

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    Saved By the Plants

    By about 600 million years ago, atmospheric O2 levels were about
    1% of Present Atmospheric Level (PAL) .

    At this point:

    • Cellular Respiration replaced fermentation as the most common metabolic pathway.

    • An ozone (O3) layer started to form in the upper atmosphere

    • Ozone shielded microorganisms from lethal ultraviolet (UV) and gamma radiation.

    • Oceanic shallows and wet terestrial areas could now be colonized.

    • By about 400 million years ago, the earliest plants were established on land.

    • Atmospheric oxygen levels were about 10% PAL, and continuing to creep upwards.

    • Today's atmosphere is about 21% oxygen, thanks to photosynthesis.

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    Late Ordivician Extinction (~440 mya)

    Biodiversity expanded tremendously in the Ordovician's relatively mild climate.
    The northern hemisphere was mostly ocean, with shallow water conducive to life.

    The Ordovician (485 - 444 mya) fossil record reveals
    a rich marine invertebrate fauna, early fish, and macroalgae.

    By the late Ordovician, Gondwanaland (the largest land mass)
    occupied the south pole.
    Massive glaciation and sea level drop caused shallow seas to dry up,
    with devastating ecological effects.

    About 60% of all marine invertebrate genera
    and 25% of all marine invertebrate families went extinct.

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    Late Devonian Extinction (365 mya)

    The main casualties of this event were benthic invertebrates
    jawless fishes, and placoderm fishes living in shallow water ecosystems,
    primarily in the tropics.

    The causes are still controversial, with candidates including
  • asteroid impact
  • global anoxia
  • plate tectonics
  • sea level change
  • climate change
  • expansion of tropical, terrestrial plants
  • Massive shale deposits from this period indicate large-scale organic detritus buildup.
    Terrestrial plant matter may have caused eutrophication and anoxia in tropical shallow waters.

    About 20% of all animal families and 70-80% of all animal species died off.

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    Permian-Triassic Extinction (252 mya)

    This began after a series of massive volcanic eruptions in what is now Siberia.
    • Lava and ash directly caused die-offs.
    • Excess CO2 caused global temperatures to rise ~ 6oF
    • More uniform temperature across the globe reduced ocean water cycling, reducing oxygen content (ocean anoxia)
    • Anaerobic bacterial overgrowth in oceans may have increased concentrations of toxic, ozone-destroying hydrogen sulfide (H2S)
    • Ozone depletion may have contributed to extinctions.

    Over the course of about 60,000 years,
    approximately 96% of all animal and plant species died off.

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    Late Triassic Extinction (201 mya)

    The cause of this extinction is also controversial, but include
    • gradual climate change
    • asteroid impact
    • massive volcanic eruptions resulting in
      • breakup of Pangaea
      • severe climate change
      • ecological disruptions

    About 30% of marine genera and 42% of
    all terrestrial tetrapods disappeared.

    This extinction may have contributed to the rise of the dinosaurs.

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    Cretaceous-Tertiary (K-T) Extinction (65.5 mya)

    (In paleontology, K is the abbreviation used for Cretaceous.
    C is used as an abbreviation for Cambrian.)

    This event defined the end of the Cretaceous and the beginning of the Tertiary.

    This extinction was most likely caused by impact of the Chixulub Comet,
    whose crater lies at the bottom of the ocean near Mexico's Yucatan peninsula.

    About 75% of terrestrial plant and animal species disappeared,
    including the non-avian dinosaurs.
    More than half of all marine species also died off.

    A relatively recent theory about the immediate effects of the
    Chixulub comet is entertainingly presented here by the folks at Radiolab.
    <-- Watch: Dinopocalypse.

    Science as Performance Art.

    Also, you will never sleep again.

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    Extinction Happens

    Which brings us to the sixth Mass Extinction, occurring now.

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Click on pic for a sobering look

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