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Lecture #3 – Origin of Species

Cartoon – gentleman and ape

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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

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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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

Some species have a lot of natural phenotypic variation

• But, the only way to classify extinct species and species that lack sexual reproductionAlso 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 treeMonophyletic = 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

• 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 cell

The 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

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 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 • 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 buildingRiver erosionGlaciationTectonic eventsCave 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 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???Speciation is likely to occur more rapidlyMore 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 rapidlyMore 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 divergeProbably already adapting to frontier or edge

conditionsAlso, probably more likely to migrate

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Allopatric Speciation due to geographic separation

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

lant

s???

? Birds????

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Diagram – sympatric allopatric either sympatric again or not, as a population separates around a mountain range and then re-unites

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 or multiple events (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 (some plants can be self-fertile, vegetative reproduction)

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

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Polyploidy – one mechanism for sympatric speciation

Diagram – errors in meiosis can lead to polyploids

Some plants can self-pollinate, 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 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 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

Diagram showing adaptive radiation

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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???Highly unlikelyThere 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 unlikelyThere will just be a new set of species

• Can we destroy the planet???Highly unlikelyThe 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 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-opted

Feathers 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 Thinking

• No, there are lots of lineages that are now extinct

• Evolution may look directed, but it’s not....

Was 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 – chordates

Sub-phylum – vertebrates• Class – mammals• Order – primates• Family – hominoids• Genus – Homo• Specific epithet – sapiens

Image of human fossil

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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 Sub-phylum: 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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Diagram – phylogeny of humans

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?• 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

6-10myaComplete 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 toolsMost 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 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

Diagram – phylogeny of humans

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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

DNA evidence supports this

pathway

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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???

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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