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Lecture #3 – Origin of Species

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Key Concepts:

• Species concepts

• Development of reproductive isolation

• Patterns of speciation

• Macroevolution

• Human evolution

• Evolution continues…..

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specie

“Species” is both singular and plural

My pet peeve is….

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Diagram – variation in beaks between species

Major Species Concepts

• Biological

• Morphological

• Phylogenetic

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Image – Sarracenia flava Image – Sarracenia rubra

Biological species – the basic standard for separating species (Ernst Mayr, 1942)

• Species are defined by natural reproductive isolationIndividuals that can produce successful

offspring are considered the same species

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Critical Thinking

• Biological species are defined by natural reproductive isolationIndividuals 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 isolationIndividuals that can produce successful

offspring are considered the same species

• Definition doesn't always work

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Image – Hymenocallis floridanaImage – Hymenocallis coronaria

Morphological species – the first way to separate species (Linnaeus, ~1750 & others)

• Species are defined by differences in formIndividuals with the same morphology and/or

anatomy are considered the same species

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Critical Thinking

• Morphological species are defined by differences in formIndividuals 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 formIndividuals with the same morphology and/or

anatomy are considered the same species

• Definition doesn't always work

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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 treeMonophyletic = lineage is derived from a

common ancestor

• Definition doesn't always workDon’t have good phylogenies for all species or

groupsAlso, imperfect agreement on interpretations

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Development And Maintenance Of Reproductive Isolation

• What constitutes a barrier to reproduction?

• How do reproductive barriers develop?

It is generally accepted that natural reproductive isolation defines and

preserves separate species in sexually reproducing organisms

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Image – blue-footed boobies mating behavior

Pre-zygotic Barriers

• Remember, the zygote is the fertilized egg cellThe first cell of the new offspring

• Pre-zygotic barriers prevent the formation of the zygote

• Natural, evolved incompatibilities prevent successful fertilizationHabitat isolationBehavioral isolationTemporal isolationStructural isolationChemical isolation

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Critical Thinking

• Natural, evolved incompatibilities prevent successful fertilization

• Think of some examples of:Habitat isolationBehavioral isolationTemporal isolationStructural isolationChemical isolation

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Critical Thinking

• Habitat isolation –

• Behavioral isolation –

• Temporal isolation –

• Structural isolation –

• Chemical isolation –

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Post-zygotic Barriers

• Post-zygotic barriers prevent successful development of offspringHybrids don’t develop properlyHybrids don’t reach sexual maturityHybrids don’t produce viable gametesHybrid 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

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.

The Darwinian fitness of an individual is measured by

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Critical Thinking

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.

The Darwinian fitness of an individual is measured by

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Diagram – different species of fish in separated ponds

Patterns of SpeciationBarriers result from separations that persist long

enough that eventually new species have developed

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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 barrierSympatric 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

• How could such barriers form???

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Critical Thinking

• Allopatric speciation occurs when populations are separated by a geographical barrier

• How could such barriers form???

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Critical Thinking

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Allopatric Speciation

• Once populations are physically isolated, speciation may occur due to all the evolutionary processes we talked about earlier SelectionDriftSelective matingMutation

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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???

• 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???

• What if the isolated population is from edge of the range of the original population???

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Images – different species of chipmunk on either side of the Grand Canyon

Allopatric Speciation due to geographic separation

Pla

nts?

??? B

irds???

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Diagram – sympatric allopatric either sympatric again or not.

Speciation may, or may not, occur…

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Diagram – sympatric speciation in a forest environment

Sympatric Speciation

• Occurs when a population becomes reproductively isolated without geographic barriersMutations 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 (mostly plants)

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Diagram – meiosis errors

Sympatric Speciation

• Occurs when a population becomes reproductively isolated without geographic barriersMutations or selection pressures that

lead to changes in behavior, habitat, food source, phenology….

Errors in meiosis that lead to polyploidy (mostly plants)

Hybrids that develop into fertile populations through vegetative reproduction (mostly plants)

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Diagram – errors in meiosis can lead to polyploids

Polyploidy – one mechanism for sympatric speciation

Some plants can be self-fertile, or vegetative reproduction can produce multiple fertile individuals

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Sympatric Speciation• Occurs when a population

becomes reproductively isolated without geographic barriersMutations 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 (mostly plants)

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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 provide 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 placeVolcanic island chains often contain many endemic

speciesNo biota until they were colonized by a few individuals

(founder effect)

• These small populations then evolved into new speciesAllopatric 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 islandsSympatric speciationNo geographic barriersAdaptive radiation into new habitats

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Diagrams – adaptive radiation in birds

Adaptive Radiation

Galapagos finches and Hawaiian honeycreepers

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Diagram – diversification of mammals after extinction of the dinosaurs

Diagram – mass extinctions over the past 2.5 billion years

Adaptive Radiation is a common theme – both between and within lineages

MammalsMass Extinction Events

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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???

• 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???

• Can we destroy the planet???

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Speciation is a Constant

• When migration, isolation or other selection pressures force divergence, reproductive isolation can eventually lead to speciationSpeciation 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-optedFeathers for thermoregulation feathers for

flight

• Large phenotypic changes can result from small changes in regulatory genesControl over the timing and length of

developmental events, or the spatial organization of body parts

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Diagram – phylogeny of the modern horse

Critical Thinking

Was the evolution of the modern

horse a series of directed events????

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Critical ThinkingWas the evolution of the modern horse a

series of directed events???

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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

Platanthera ciliaris

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Humans can also be classified!

• Domain – eukarya

• Kingdom – animal

• Phylum – chordatesSub-phylum – vertebrates

• Class – mammals

• Order – primates

• Family – hominoids

• Genus – Homo

• Specific epithet – sapiens

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Diagram – from this slide to slide #65 – phylogenies of the animal kingdom, showing the classification of humans through the taxonomic hierarchy from the phyla to the families in the primate order

Phyla in the Animal Kingdom:

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Phyla in the Animal Kingdom:Chordates

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Sub-phyla in the Chordate Phylum:

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Sub-phyla in the Chordate Phylum: Vertebrates

Sub-phylum

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Classes in the Vertebrate Sub-phylum:

Sub-phylum

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Classes in the Vertebrate Sub-phylum: Mammals

Sub-phylum

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Close-up: Classes in

the Vertebrate

Sub-phylum

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Orders in the Mammal Class:

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Orders in the Mammal Class:

Primates

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Families in the Primate

Order:

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Families in the Primate

Order: Hominoids

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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

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Critical Thinking

• Is your uncle a monkey???

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Critical Thinking

• Is your uncle a monkey???

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Diagram – phylogeny of humans, same diagram on slide #73

Two key steps – bi-pedalism and large brain

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Critical Thinking

• Why is bi-pedalism so important?

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Critical Thinking

• Why is bi-pedalism so important?

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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?

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The fossil record shows changes in our species over time

• The path of human evolution is not ladder-like

• We are currently a mono-specific family, but….

• Human phylogeny reveals many extinct lineagesWe are animalsWe are subject to natural selectionThere is a record!

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All but one lineage of hominids are extinct

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Diagram – multi-regional vs. “out of Africa” hypotheses for human migration patterns; same diagram on following 2 slides

Out of Africa – Human Migration

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Critical Thinking

How would you test these alternate hypotheses???

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Critical Thinking

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Evolution is a Constant

• Constant supply of genetic variation + constant application of selection pressuresAll 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 processA slice through the crown of a multi-

dimensional tree

• Evolution is NOT finished!

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….as the tree grows, so grows the tree of life…

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….as the tree grows, so grows the tree of life…

• Species concepts• Development of

reproductive isolation• Patterns of speciation• Macroevolution• Human evolution• Evolution continues…..

Key Concepts:

Questions???