AP Biology

The Origin of Species

Mom, Dad…There’s something you need to know…

I’m a MAMMAL!

AP Biology

“That mystery of mysteries…”Darwin never actually tackled how new species arose…

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.

How did this flightless bird & tortoise come to live on the isolated Galápagos Islands?

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Evolutionary theory must explain how new species originate and how populations evolve:§ Microevolution - consists of changes in allele

frequency in a population over time. § Macroevolution - refers to broad patterns of

evolutionary change above the species level, e.g., the origin of new groups of organism such as mammals or flowering plants, through a series of speciation events. Results in a new sp.

§ Speciation explores the “bridge” between micro and macroevolution and the mechanisms by which species originate

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So…what is a species?§ Biological species concept; gene flow between sp.

u defined by Ernst Mayru population whose members can interbreed

& produce viable, fertile offspringu reproductively compatibleu Gene flow between populations holds a

species together genetically

Western MeadowlarkEastern Meadowlark

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

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Grizzly bear (U. arctos)�

� Polar bear (U. maritimus)

Hybrid“grolar bear”�

Limitations of the Biological Species

Concept:§ The biological species

concept cannot be applied to fossils or asexual organisms (including all prokaryotes)

§ The biological species concept emphasizes absence of gene flow

§ However, gene flow can occur between distinct speciesu For example, grizzly

bears and polar bears can mate to produce �grolar bears�

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How and why do new species originate?§ Species are created by a series of evolutionary

processes; an absence of gene flowu populations become isolated

§ geographically isolated § reproductively isolated (pre & post)

u isolated populations evolve independently

§ Isolationu Allopatric “different country”

§ geographic separationu Sympatric “same country”

§ still live in same area§ some gene flow can occur

SympatricAllopatricA population forms a new sp. While geographically isolated from its parent population.

A subset of a population forms a new sp. without geographic isolation.Less common.

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Exploring reproductive barriers:Prezygotic barriers Postzygotic barriers

Habitatisolation

Temporalisolation

Behavioralisolation

Mechanicalisolation

Gameticisolation

Reducedhybrid

viability

Reducedhybridfertility

Hybridbreakdown

Individuals of different

speciesMATINGATTEMP

T

FERTILI-ZATION

VIABLE,FERTILE

OFF-SPRING

(l)(i)

(j)

(h)(g)(f)(e)(c)(a)

(b)

(d)

(k)

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§ Obstacle to mating or to fertilization if mating occursPre-reproductive/prezygotic barriers:

behavioral isolation

geographic isolation habitat isolation temporal isolation

mechanical isolation gametic isolation

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Geographic isolation:

§ Species occur in different areas

u physical barrier

u allopatric speciation

§ “other country”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

Ammospermophilus leucurusAmmospermophilus harrisii

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Isolation is Needed for Speciation:e.g. The Harris and Antelope Squirrels of the N. and S. rims of

the Grand Canyon are closely related. Were likely once the same sp. but diverged when they became separated

by the canyon. Overtime, speciation occurred.

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Habitat isolation:§ Species occur in same region, but occupy

different habitats so rarely encounter each otheru reproductively isolated

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

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

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Temporal isolation:§ Species that breed during different times of day,

different seasons, or different years cannot mix gametesu reproductive isolationu sympatric speciation

§ “same country”

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

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Behavioral isolation:§ Unique behavioral patterns & rituals isolate species

u identifies members of species u attract mates of same species

§ courtship rituals, mating calls§ reproductive isolation

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

sympatric speciation?

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firefly courtship displays

courtship display of Gray-Crowned Cranes, Kenya

courtship songs of sympatricspecies of lacewings

Recognizing your own species:

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Mechanical isolation:§ Morphological differences can prevent successful mating

u reproductive isolationEven 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.

Plants

sympatric speciation?

Animals

Their shell’s spiral in different directions. As a result, their genital openings don’t line up & mating can’t be complete.

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Mechanical isolation:

§ For many insects, male &

female sex organs of

closely related species do

not fit together, preventing

sperm transfer

u lack of “fit” between sexual organs:

hard to imagine for us… but a big issue for insects with

different shaped genitals!

Damsel fly penises

Animals

I can’t even imagine!

The selection is intense because it

directly affects offspring production --

it is affecting sex itself.

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Gametic isolation:§ Sperm of one species may not be able to fertilize

eggs of another speciesu mechanisms

§ biochemical barrier so sperm cannot penetrate eggw receptor recognition: lock & key between egg & sperm

§ chemical incompatibilityw 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.

sympatric speciation?

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Why are cats and dogs reproductively isolated?