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Natural selection




Slide 34

Reproductive Barriers

Reproductive Barriers

Any mechanism that impedes two species from producing fertile and/or viable hybrid offspring.

Two barriers:

1. Pre-zygotic barriers

2. Post-zygotic barriers

Slide 35

1. Pre-zygotic Barriers

1. Pre-zygotic Barriers

a. Temporal isolation:

Breeding occurs at different times for different species.

b. Habitat isolation:

Species breed in different habitats.

c. Behavioral isolation:

Little or no sexual attraction between species.

Slide 36

1. Pre-zygotic Barriers

1. Pre-zygotic Barriers

d. Mechanical isolation:

Structural differences prevent gamete exchange.

e. Gametic isolation:

Gametes die before uniting with gametes of other species, or gametes fail to unite.

Slide 37

2. Post-zygotic Barriers

2. Post-zygotic Barriers

a. Hybrid inviability:

Hybrid zygotes fail to develop or fail to reach sexual maturity.

b. Hybrid sterility:

Hybrid fails to produce functional gametes.

c. Hybrid breakdown:

Offspring of hybrids are weak or infertile.

Slide 38

Allopatric Speciation

Allopatric Speciation

Induced when the ancestral population becomes separated by a geographical barrier.


Grand Canyon and ground squirrels

Slide 39

Adaptive Radiation

Adaptive Radiation

Emergence of numerous species from a common ancestor introduced to new and diverse environments.


Darwinís Finches

Slide 40

Sympatric Speciation

Sympatric Speciation

Result of a radical change in the genome that produces a reproductively isolated sub-population within the parent population (rare).

Example: Plant evolution - polyploid

A species doubles itís chromosome # to become tetraploid.

Slide 41

Interpretations of Speciation

Interpretations of Speciation

Two theories:

1. Gradualist Model (Neo-Darwinian):

Slow changes in species overtime.

2. Punctuated Equilibrium:

Evolution occurs in spurts of relatively rapid change.

Slide 42

Convergent Evolution

Convergent Evolution

Species from different evolutionary branches may come to resemble one another if they live in very similar environments.


1. Ostrich (Africa) and Emu (Australia).

2. Sidewinder (Mojave Desert) and

Horned Viper (Middle East Desert)

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