Lecture #3 – Origin of Species
Cartoon – gentleman and ape
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Key Concepts:
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Species concepts
Development of reproductive isolation
Patterns of speciation
Macroevolution
Human evolution
Evolution continues…..
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My pet
peeve
is….
specie
“Species” is both singular and plural
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Major Species Concepts
• Biological
• Morphological
• Phylogenetic
Diagram –
variation in
beaks between
species
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Biological species – the basic standard
for separating species (Ernst Mayr, 1942)
• Species are defined by natural
reproductive isolation
Individuals that can produce successful
offspring are considered the same species
Image – Sarracenia rubra
Image – Sarracenia flava
≠
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Critical Thinking
• Biological species are defined by natural
reproductive isolation
Individuals that can produce successful offspring
are considered the same species
• Definition doesn't always work – why not???
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Critical Thinking
• Biological species are defined by natural
reproductive isolation
Individuals that can produce successful
offspring are considered the same species
• Definition doesn't always work
Speciation often occurs as the gradual
divergence of multiple populations
Fuzzy boundaries during divergence
• Also, can’t be used to classify extinct
species
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Morphological species – the first way to
separate species (Linnaeus, ~1750 & others)
• Species are defined by differences in form
Individuals with the same morphology and/or
anatomy are considered the same species
Image – Hymenocallis coronaria
Image – Hymenocallis floridana
≠
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Critical Thinking
• Morphological species are defined by
differences in form
Individuals with the same morphology and/or
anatomy are considered the same species
• Definition doesn't always work – why not???
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Critical Thinking
• Morphological species are defined by
differences in form
Individuals with the same morphology and/or
anatomy are considered the same species
• Definition doesn't always work
Some species have a lot of natural phenotypic
variation
• But, the only way to classify extinct species
and species that lack sexual reproduction
Also important in describing new species
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Phylogenetic species – the new
standard for separating species???
• Species are defined based on evolutionary
history
Species defined by the smallest monophyletic
group in an evolutionary tree
Monophyletic = lineage is derived from a
common ancestor
• Definition doesn't always work
Don’t have good phylogenies for all species or
groups
Also, imperfect agreement on interpretations
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Development And Maintenance Of
Reproductive Isolation: the
essence of speciation
It is generally accepted that natural
reproductive isolation defines and
preserves separate species in sexually
reproducing organisms
• What constitutes a barrier to reproduction?
• How do reproductive barriers develop?
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Pre-zygotic Barriers
• Remember, the zygote is the fertilized egg cell
 The first cell of the new offspring
• Pre-zygotic barriers prevent the formation of the
zygote
• Natural, evolved incompatibilities prevent
successful fertilization
 Habitat isolation
 Behavioral isolation
 Temporal isolation
 Structural isolation
 Chemical isolation
Image – blue-footed boobies
mating behavior
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Critical Thinking
• Natural, evolved incompatibilities prevent
successful fertilization
• Think of some examples of:
Habitat isolation
Behavioral isolation
Temporal isolation
Structural isolation
Chemical isolation
Euphorbia in very xeric
habitat
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Critical Thinking
• Habitat isolation – different ecological
niches
• Behavioral isolation – changes in mating
behaviors….
• Temporal isolation – the timing of
reproductive events
• Structural isolation – mutations that change
morphology of reproductive structures
• Chemical isolation – gametes must be
compatible, pollen must “match”
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Post-zygotic Barriers
• Post-zygotic barriers prevent successful
development of offspring
 Hybrids don’t develop properly
 Hybrids don’t reach sexual maturity
 Hybrids don’t produce viable gametes
 Hybrid lineages fail over time
• Natural genetic incompatibilities prevent
successful long-term reproduction
Horse x Donkey = robust
but sterile Mule
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Critical Thinking
The Darwinian fitness of an individual is
measured by
a. its ability to reproduce.
b. how long it lives.
c. the number of mates it attracts.
d. the number of its offspring that survive
to reproduce.
e. its physical strength.
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Critical Thinking
The Darwinian fitness of an individual is
measured by
a. its ability to reproduce.
b. how long it lives.
c. the number of mates it attracts.
d.the number of its offspring that survive
to reproduce.
e. its physical strength.
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Patterns of Speciation
Barriers result from separations that persist long
enough that eventually new species have developed
Diagram – different species of fish in separated ponds
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Patterns of Speciation
• Pattern depends on the mechanism of gene
flow interruption
 Allopatric speciation occurs when populations are
separated by a geographical barrier
 Sympatric speciation occurs in the absence of a
geographic barrier
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Critical Thinking
• Allopatric speciation occurs when populations are
separated by a geographical barrier
• Such as????
• How could such barriers form???
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Critical Thinking
• Allopatric speciation occurs when populations are
separated by a geographical barrier
• Such as rivers, canyons, mountains, oceans,
glaciers…..
• How could such barriers form???
Diagram
showing
development
of a canyon
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Critical Thinking
• Allopatric speciation occurs when populations are
separated by a geographical barrier
• Such as rivers, canyons, mountains, oceans,
glaciers…..
• How could such barriers form???
• Geological processes
 Mountain building
 River erosion
 Glaciation
 Tectonic events
 Cave formation
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Critical Thinking
• Also….
• Climate changes that cause large lakes to
dry up or form smaller, isolated lakes
• Colonization events that separate a group
from the rest of the population (founder
effect)
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Allopatric Speciation
• Once populations are physically isolated,
speciation may occur due to all the
evolutionary processes we talked about
earlier
Selection
Drift
Selective mating
Mutation
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Critical Thinking
• What if the isolated population is small???
• What if the isolated population is from
edge of the range of the original
population???
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Critical Thinking
• What if the isolated population is small???
Speciation is likely to occur more rapidly
More genetic drift, less gene flow
• What if the isolated population is from
edge of the range of the original
population???
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Critical Thinking
• What if the isolated population is small???
Speciation is likely to occur more rapidly
More genetic drift, less gene flow
• What if the isolated population is from
edge of the range of the original
population???
It may be even more likely to diverge
Probably already adapting to frontier or edge
conditions
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Also, probably more likely to migrate
Birds????
Plants????
Allopatric Speciation due to
geographic separation
Images – different species of chipmunk
on either side of the Grand Canyon
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Speciation may, or may not, occur…
Diagram – sympatric  allopatric  either sympatric again or
not, as a population separates around a mountain range and
then re-unites
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Sympatric Speciation
• Occurs when a population
becomes reproductively isolated
without geographic barriers
 Mutations or selection pressures that
lead to changes in behavior, habitat,
food source, phenology….
 Errors in meiosis that lead to
polyploidy (some plants can be selffertile, vegetative reproduction)
 Hybrids that develop into fertile
populations through vegetative
reproduction or multiple events
(mostly plants)
Diagram –
sympatric
speciation in a
forest
environment
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Sympatric Speciation
• Occurs when a population
becomes reproductively isolated
without geographic barriers
 Mutations or selection pressures that
lead to changes in behavior, habitat,
food source, phenology….
Diagram – meiosis errors
 Errors in meiosis that lead to
polyploidy (some plants can be selffertile, vegetative reproduction)
 Hybrids that develop into fertile
populations through vegetative
reproduction or multiple events
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(mostly plants)
Polyploidy – one mechanism for
sympatric speciation
Diagram – errors in meiosis can lead to polyploids
Some plants can self-pollinate, or vegetative
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reproduction can produce multiple fertile individuals
Sympatric Speciation
• Occurs when a population
becomes reproductively
isolated without geographic
barriers
 Mutations or selection pressures
that lead to changes in behavior,
habitat, food source,
phenology….
 Errors in meiosis that lead to
polyploidy (some plants can be
self-fertile, vegetative
reproduction)
 Hybrids that develop into fertile
populations through vegetative
reproduction or multiple events
(mostly plants)
Image
showing
hybrid asters
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Speciation is NOT a Given
• Must have an interruption to gene flow
PLUS
• Must have enough change in the separated
populations to produce a barrier to
reproduction
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Endemic Species and Adaptive Radiation
• Endemic species = restricted in distribution to a
particular place, generally because they evolved
in place
 Volcanic island chains often contain many endemic
species
 No biota until they were colonized by a few individuals
(founder effect)
• These small populations then evolved into new
species
 Allopatric speciation due to the geographic barrier from
the founder effect
• But also……
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Endemic Species and Adaptive Radiation
• Many new species develop that are adapted to
the diverse new habitats found in such islands
 Sympatric speciation
 No geographic barriers
 Adaptive radiation into new habitats
Diagram showing adaptive radiation
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Adaptive Radiation
Galapagos finches and
Hawaiian honeycreepers
Diagrams – adaptive radiation in birds
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Adaptive Radiation is a common theme –
both between and within lineages
Diagram – mass
extinctions over the
past 2.5 billion years
Diagram – diversification of
mammals after extinction of
the dinosaurs
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Mass Extinction Events
Mammals
Critical Thinking
• Humans have initiated a mass extinction
event
• Will life cease to exist on the planet???
• Can we destroy the planet???
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Critical Thinking
• Humans have initiated a mass extinction
event
• Will life cease to exist on the planet???
Highly unlikely
There will just be a new set of species
• Can we destroy the planet???
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Critical Thinking
• Humans have initiated a mass extinction
event
• Will life cease to exist on the planet???
Highly unlikely
There will just be a new set of species
• Can we destroy the planet???
Highly unlikely
The earth has survived for at least 4 billion
years!
Species come and go….
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Speciation is a Constant
• When migration, isolation or other
selection pressures force divergence,
reproductive isolation can eventually lead
to speciation
Speciation might be gradual or abrupt
(punctuated equilibrium)
Transitions (either gradual or abrupt) may or
may not be captured in the fossil record
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Macroevolution: larger-scale
changes in organisms
Also contributes to speciation
• Small, population-scale changes can
accumulate
• Exaptations – traits can be co-opted
Feathers for thermoregulation  feathers for
flight
• Large phenotypic changes can result from
small changes in regulatory genes
Control over the timing and length of
developmental events, or the spatial organization
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of body parts
Critical Thinking
Was the
evolution of
the modern
horse a
series of
directed
events
????
Diagram – phylogeny of the modern horse
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Critical Thinking
Was the evolution of the modern horse a
series of directed events???
• No, there are lots
of lineages that
are now extinct
• Evolution may
look directed, but
it’s not....
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Selection is a series of gates!!!
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A Preview of the Taxonomic Hierarchy:
this is how we classify diversity
Taxonomic Category
Example (taxon)
Domain
Eukarya = all eukaryotic organisms
Kingdom
Plantae, also Metaphyta = all plants
Division (phylum)
Magnoliophyta = all angiosperms
Class
Liliopsida = all monocots
Order
Asparagales = related families (Orchidaceae,
Iridaceae, etc)
Family
Orchidaceae = related genera (Platanthera,
Spiranthes, etc)
Genus
Platanthera = related species (P. ciliaris, P. integra,
etc)
Specific name/epithet
ciliaris = one species
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Images – the yellow fringed orchid
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Platanthera ciliaris
Humans can also be classified!
• Domain – eukarya
• Kingdom – animal
• Phylum – chordates
Image of human fossil
Sub-phylum – vertebrates
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Class – mammals
Order – primates
Family – hominoids
Genus – Homo
Specific epithet – sapiens
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Phyla in the Animal Kingdom:
Chordates
This and next 6 slides show the phylogenetic placement of
humans in the animal kingdom
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Sub-phyla in the Chordate Phylum:
Vertebrates
Sub-phylum
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Classes in the Vertebrate Subphylum: Mammals
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Close-up:
Classes in
the
Vertebrate
Sub-phylum
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Orders in the
Mammal Class:
Primates
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Families in
the Primate
Order:
Hominoids –
a monotypic
family
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Some key steps
in the evolution
of primates –
note that our
last common
ancestor with
other modern
primates was 6
to 10 MILLION
years ago
Loss of dinosaurs,
Rise of mammals
Diagram showing the
different orders of primates
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Critical Thinking
• Is your uncle a monkey???
Cartoon showing gentleman and ape
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Critical Thinking
• Is your uncle a monkey???
• Of course not!!!
• Humans are NOT evolved from monkeys!
Diagram showing phylogenetic relationships between primates
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Monkeys, apes and humans share a common
ancestor, but have followed different
evolutionary pathways for > 6 million years!
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Two key steps – bi-pedalism and large brain
Diagram – phylogeny of humans
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Critical Thinking
• Why is bi-pedalism so important?
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Critical Thinking
• Why is bi-pedalism so important?
• Bipedalism functionally separates the limbs
• Legs for energy efficient walking and
elevation of the eyes
• Arms for hunting, gathering, tool use, caring
for offspring, artistic endeavors, etc….
Current evidence is that this divergence began
6-10mya
Complete conversion in hominids by about
2mya
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Images – human fossil and fossil footprints
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Critical Thinking
• Why is a large brain so important?
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Critical Thinking
• Why is a large brain so important?
• Large brain allows for complex thought,
abstract reasoning, spirituality, creativity,
language, complex tools
Most of the traits that we consider uniquely
human…
Larger brain began emerging about 2mya,
stable for about 200,000 years
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The fossil record shows changes in
our species over time
• The path of human evolution is not ladderlike
• We are currently a mono-specific family,
but….
• Human phylogeny reveals many extinct
lineages
We are animals
We are subject to natural selection
There is a record!
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All but one lineage of hominids are extinct
Diagram – phylogeny of humans
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Out of Africa – Human Migration
Diagram – multi-regional vs. “out of Africa” hypotheses for
human migration patterns; same diagram on following 2 slides
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Critical Thinking
How would you test
these alternate
hypotheses???
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Critical Thinking
DNA evidence
supports this
pathway
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Evolution is a Constant
• Constant supply of genetic variation +
constant application of selection pressures
All species are in some degree of flux
• New species are constantly diverging
….and going extinct
• At any given time, we are just looking at a
cross section of the process
A slice through the crown of a multidimensional tree
• Evolution is NOT finished!
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….as the tree grows, so
grows the tree of life…
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Questions???
….as the tree grows, so
grows the tree of life…
Key Concepts:
• Species concepts
• Development of
reproductive isolation
• Patterns of speciation
• Macroevolution
• Human evolution
• Evolution continues…..
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Hands On
• We’ll be starting with plants next time
• Bring in samples of plants to examine
microscopically and macroscopically
• Anything that interests you – from nature,
your kitchen, garden…..
• Parts or whole plants
• Save reproductive parts for later in the
week
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