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(a) Similarity between different species (b) Diversity within a species

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Prezygotic barriers Postzygotic barriers

Habitatisolation

Temporalisolation

Behavioralisolation

Mechanicalisolation

Gameticisolation

Reducedhybrid

viability

Reducedhybridfertility

Hybridbreakdown

MATINGATTEMPT

FERTILI-ZATION

VIABLE,FERTILE

OFF-SPRING

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

(j)

(k)

(d)

(b)

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Grizzly bear (U. arctos)

Polar bear (U. maritimus)

Hybrid “grolar bear”

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Allopatric speciation: formsa new species whilegeographically isolated.

(a) (b) Sympatric speciation: a subsetforms a new species withoutgeographic separation.

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Under low predation:body shape that favorslong, steady swimming

(b)Under high predation:body shape that enablesrapid bursts of speed

(a)

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A. formosus A. nuttingi

ATLANTICOCEAN

PACIFICOCEAN

A. panamensis A. millsae

Isthmus of Panama

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Experiment

Mating experimentsafter 40 generations

Some flies raisedon starch medium

Results

Some flies raisedon maltose medium

Initial populationof fruit flies(Drosophila

pseudoobscura)

Female Female

Starch MaltoseStarch

population 1Starch

population 2

22

8 20

9 18 15

1512

Mal

e

Mal

eM

alto

seS

tarc

h

Sta

rch

po

pu

lati

on

1S

tarc

hp

op

ula

tio

n 2

Number of matingsin experimental group

Number of matingsin control group

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Celldivisionerror

Tetraploid cell4n 12

New species(4n)

2n 6

2n

Gametes producedby tetraploids

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Species A2n 6

Species B2n 4

Normalgameten 3

Normalgameten 3

Hybrid with7 chromosomes

Unreduced gametewith 7 chromosomes

Unreduced gametewith 4 chromosomes

New species:viable fertile hybrid(allopolyploid)2n 10

Meiotic error; chromosomenumber not reduced from 2n to n

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Normal lightMonochromatic

orange light

Experiment

P. pundamilia

P. nyererei

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Fire-belliedtoad range Fire-bellied toad, Bombina

bombina

Yellow-belliedtoad range

Hybrid zone

Hybridzone

Fire-belliedtoad range

Yellow-belliedtoad range

Distance from hybrid zone center (km)

2010010203040

0.99

0.9

0.1

0.01

Fre

qu

enc

y o

fB

. va

rieg

ata-

spec

ific

all

ele

Yellow-bellied toad, Bombinavariegata

0.5

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Isolatedpopulationdiverges.

Possibleoutcomes:

Reinforcement

Fusion

Stability

Hybridzone

Hybridindividual

Barrier togene flow

Gene flow

Population

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Pundamilia nyererei Pundamilia pundamilia

Pundamilia “turbid water,”hybrid offspring from alocation with turbid water

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(a) Punctuated model

(b) Gradual model

Time

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(a) Mimulus lewisii M. lewisii withM. cardinalis allele

(b)

(c) Mimulus cardinalis M. cardinalis withM. lewisii allele

(d)

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