The Origin of Species

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5

Concept 24.3: Hybrid zones provide opportunities to study factors that cause reproductive isolation

A hybrid zone is a region in which members of different species mate and produce hybrids.

A hybrid zone can occur in a single band where adjacent species meet.

Hybrids often have reduced fitness compared with parent species.

The distribution of hybrid zones can be more complex if parent species are found in multiple habitats within the same region.

Slide 30

Hybrid Zones

EUROPE

Fire-bellied

toad range

Hybrid zone

Yellow-bellied

toad range

Yellow-bellied toad,

Bombina variegata

Fire-bellied toad,

Bombina bombina

Allele frequency (log scale)

Distance from hybrid zone center (km)

40

30

20

20

10

10

0

0.01

0.1

0.5

0.9

0.99

Slide 31

Hybrid Zones over Time

When closely related species meet in a hybrid zone, there are three possible outcomes:

Reinforcement -- Strengthening of reproductive barriers reducing gene flow.

Fusion -- Weakening of reproductive barriers with eventual fusion into one species.

Stabilizing -- Continued formation of hybrid individuals.

Slide 32

Hybrid Zones Over Time

Gene flow

Population

(five individuals

are shown)

Barrier to

gene flow

Isolated population

diverges

Hybrid

zone

Hybrid

Possible

outcomes:

Reinforcement

of gene flow barrier.

OR

OR

Fusion into

one species.

Stability:

Continued formation

of hybrid individuals.

Slide 33

The Time Course of Speciation

Broad patterns in speciation can be studied using the fossil record, morphological data, or molecular data.

The fossil record includes examples of species that appear suddenly, persist essentially unchanged for some time, and then apparently disappear

Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibrium to describe periods of apparent stasis (no change) punctuated by brief periods of rapid change.

The punctuated equilibrium model contrasts with a Darwinian model of gradualism: slow continuous change over time in a species’ existence.

Slide 34

Patterns in Speciation

Punctuated Equilibrium

pattern

Gradualism pattern

Change / Time

Slide 35

Studying the Genetics of Speciation

The explosion of genomics is enabling researchers to identify specific genes involved in some cases of speciation.