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The Origin of Species

Chapter 22

“Species” must account for 2 things:1) distinctiveness of sp. (populations)

together at a single locale

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The Nature of Species

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The Nature of Species

2) connection that exists among diff. pop. belonging to the same sp.• Subspecies

–ind. in sp. that occur in diff. areas and are distinct from one another

Biological species concept:– defined by Ernst Mayr– population whose members can interbreed &

produce viable, fertile offspring– reproductively compatible

Western MeadowlarkEastern Meadowlark

Distinct species:songs & behaviors are different enough to prevent interbreeding

The Nature of Species

Song Comparisons

Eastern Meadowlarkhttp://www.youtube.com/watch?v=Th4T4YrtR2o

Western Meadowlarkhttp://www.youtube.com/watch?v=lvAUgFb1cLY

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Darwin never actually tackled Darwin never actually tackled how new species arose…how new species arose…

The Nature of Species

“Both in space and time, we seem to be brought somewhat near to that great fact—that mystery of mysteries—the first appearance of new beings on this Earth.”

-Charles Darwin

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Speciation• the process by which new sp. arise by:

– transformation of 1 sp. into another (anagenesis)

– splitting of 1 ancestral sp. into 2 descendant sp. (cladogenesis)

The Nature of Species

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Reproductive isolation – pop. whose members don’t mate with each other

or who can’t produce fertile offspring– isolating mechanisms:

• Geographic• Ecological • Behavioral• Temporal• Mechanical• Gamete fusion• Hybridization

Reproductive Isolation

Isolating Mech. Poster

• Name mechanism• Is mechanism prezygotic or

postzygotic?• Define/Describe how mechanism works• Give 3 examples of mechanism

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Prezygotic Isolation

1. Geographic Isolation– sp. occur in different areas– physical barrier

Harris’s antelope squirrel inhabits the canyon’s south rim (L). Just a few miles away on the north rim (R) lives the closely related white-tailed antelope squirrel

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Two different types of geographic isolation:1. Allopatric speciation

• result of geo. isolation

2. Sympatric speciation • occurs without geo. iso. (in the same

geo.)

Geography Isolation

2. Ecological Isolation– Species occur in same region, but occupy different

habitats so rarely encounter each other

2 species of garter snake, Thamnophis, occur in same area, but one lives in water & other is terrestrial

lions & tigers could hybridize, but they live in different habitats: lions in grasslands tigers in rainforest

Prezygotic Isolation

Tiglon: Cross between a male tiger and a female lion

3. Behavioral Isolation– Unique behavioral patterns & rituals isolate species– identifies members of species – attract mates of same species

• courtship rituals, mating calls

Blue footed boobies mate only after a courtship display unique to their species

Prezygotic Isolation

http://www.youtube.com/watch?v=LLdC-8nqPog&feature=related

4. Temporal Isolation– Species that breed during different times of day, different

seasons, or different years cannot mix gametes

Eastern spotted skunk (L) & western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer

Prezygotic Isolation

5. Mechanical Isolation– Morphological differences can prevent successful

mating

Even in closely related species of plants, the flowers often have distinct appearances that attract different pollinators. These 2 species of monkey flower differ greatly in shape & color, therefore cross-pollination does not happen.

PlantsPrezygotic Isolation

– For many insects, male & female sex organs of closely related species do not fit together, preventing sperm transfer

• lack of “fit” between sexual organs: hard to imagine for us… but a big issue for insects with different shaped genitals!

Damsel fly penises

AnimalsPrezygotic Isolation

6. Gametic isolation– Sperm of one species may not be able to fertilize eggs of another

species– Mechanisms:

• biochemical barrier so sperm cannot penetrate egg– Sperm cannot penetrate egg

• chemical incompatibility – sperm cannot survive in female reproductive tract

Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes. Gametes of different species— red & purple —are unable to fuse.

Prezygotic Isolation

Hybridization• Hybrid offspring don’t develop into a

viable, fertile adult– reduced hybrid viability

Post-zygotic Isolation

zebroid

Species of salamander genus, Ensatina, may interbreed, but most hybrids do not complete development & those that do are frail.

– reduced hybrid fertility• Even if hybrids are vigorous they may be sterile• chromosomes of parents differ in number or structure &

meiosis in hybrids fail to produce normal gametes

Mules are vigorous, but sterile

Donkeys have 62 chromosomes(31 pairs)

Horses have 64 chromosomes(32 pairs)

Mules have 63 chromosomes!

Post-zygotic Isolation

– hybrid breakdown• Hybrids may be fertile & viable in first generation, but

when they mate offspring are feeble or sterile

In strains of cultivated rice, hybrids are vigorous but plants in next generation are small & sterile.On path to separate species.

Post-zygotic Isolation

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Adaptive RadiationAdaptive radiation

– closely related sp. ev. from a com. ancestor by adapting to different parts of the env.

– caused by OR results in geographic isolation

• Does speciation happen gradually or rapidly?

Charles DarwinCharles Lyell

Stephen Jay GouldNiles Eldredge

Gradualism Punctuated Equilibrium

rapid bursts of change mixed with

long periods of little or no change

gradual accumulation of small changes over long time

The Pace of Speciation

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The Pace of Evolution

Evolution most likely includes both!

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Speciation and ExtinctionSpeciation, through time, has surpassed

extinction– 5 mass extinctions have occurred– not all groups of organisms are affected equally

during extinctions

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Consequence of extinction– previously dom. groups may perish, changing

the course of ev.

Speciation and Extinction

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6th extinction is underway– Estimates:

• 1/4th of all species will become extinct in the near future

• rebound in sp. diversity may be slower than following previous mass extinction events

• large proportion of the world’s resources will be taken up by human activities

Speciation and Extinction

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The Future of EvolutionHuman influences on the environment affect

the evolutionary processes• changing patterns of NS• global climate change• decreased pop. sizes increase the likelihood

of genetic drift• geographic isolation removes effect of gene

flow• chemicals and radiation could increase

mutation rate• introduction of sp. into areas they aren’t

naturally found

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Tigers now exist in geographically isolated populations

The Future of Evolution