Ch. 24 The Origin of Species

Objective:Show the different ways in which

speciation can occur.

Overview• Speciation is the process of the

creation of a new species.– Explains differences and

similarities among species.• Anagenesis: 1 species changes into

another• Cladogenesis: 1 species changes

into 2

– Connects micro- to macroevolution (evolution above species level)

(b) Cladogenesis(a) Anagenesis

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Animation: Macroevolution Right-click slide / select “Play”

24.1 Reproductive Isolation

Biological Species Concept• A species is individuals that can mate and

have viable offspring– Reproductively isolated (no gene flow)

• Barriers:– Prezygotic: prevents fertilization– Postzygotic: zygote won’t be fertile

Prezygotic BarriersHabitat Isolation• 2 species that occupy different habitats within

the same area may only encounter each other rarely if ever.

Temporal Isolation• Species that breed during different times of

the day, seasons, or year.

Leopard Frogs

Behavioral Isolation• Courtship rituals that attract mates and other

behaviors unique to a species are effective reproductive barrier.

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Video: Albatross Courtship Ritual

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Video: Giraffe Courtship Ritual

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Video: Blue-footed Boobies Courtship Ritual

Mechanical Isolation• Morphological differencesGametic Isolation• Gametes can’t fertilize each other.

Postzygotic Isolations

Reduced Hybrid Viability• The genes of different parent species may

interact and impair the hybrid’s development.

Reduced Hybrid Fertility• Hybrids created but, they may be sterile. If chromosomes

of the parents differ in number or structure, meiosis in the hybrids won’t produce normal gametes.

Hybrid Breakdown• Some first-generation hybrids are viable and fertile, but

when they mate with each other the next offspring are feeble or sterile.

24.2 Speciation Can Take Place With or Without Geographic SeparationAllopatric (“Other Country”) Speciation• Physical separation of 1 population into >1.• Founder’s Effect: Galapagos finches• Separation leads to reproductive isolation by:

– Different mutations– Sexual selection differs– Genetic drift

Sympatric (“Same Country”) Speciation• Speciation in overlapping populations due to

reduced gene flow.• Polyploidy

– Autopolyploidy: individual with more than two sets of chromosomes.

– Plants would be able to self pollinate to make correct gametes creating isolation from other plants nearby.

2n = 64n = 12

2n

4n

• Polyploidy– Allopolyploid: different species make a hybrid

offspring which should be sterile except for asexually reproducing organisms.

• Ex: type of weeds brought from Europe, oats, cotton, potatoes, tobacco, and wheat.

Figure 24.9

Meiotic error;chromosomenumber notreduced from2n to n

Unreduced gametewith 4 chromosomes

Hybrid with7 chromosomes

Unreduced gametewith 7 chromosomes Viable fertile hybrid

(allopolyploid)

Normal gameten = 3

Normal gameten = 3

Species A 2n = 4

Species B 2n = 6

2n = 10

• Habitat Differentiation and Sexual Selection– Apple flies: original fed on hawthorn trees but

started using apples, which mature more quickly. (temporal)

– Nonrandom mating: cichlids coloration in Africa differs. (behavioral)

P. nyererei

Normal lightMonochromatic

orange light

P. pundamilia

24.3 Hybrid Zones Reveal Factors That Causes Reproductive Isolation

• Hybrid zones are regions in which members of nearly different species meet and mate creating a hybrid.

EUROPE

Yellow-belliedtoad, Bombinavariegata

Fire-belliedtoad range

Hybrid zone

Yellow-belliedtoad range

Fire-bellied toad, Bombina bombina

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B. v

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Yellow-belliedtoad range

Hybridzone

Fire-belliedtoad range

Distance from hybrid zone center (km)40

0.99

0.9

0.5

0.1

0.0130 20 10 0 10 20

24.4 Speciation Can Occur Rapidly or Slowly and Can Result From Changes in Few or Many Genes

The Tempo of Speciation•Punctuated Equilibrium

– Periods of no change followed by sudden change according to fossil record.

•Gradualism– Organisms are constantly changing

(but not necessarily in bone structure or a way that shows up in fossils).

•Once initiated, speciation completes quickly

Time

Studying the Genetics of Speciation• Speciation can be traced at the genetic level.

– Ex: flower plants differ in their pollinators by having different genes for flower color and type of nectar.