f09 Lec14 Speciation

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Speciation

I. What is a species?

A. Biological species: groups of

populations whose membershave the potential to

interbreed in their natural

environmentand produce viable offspring;maintained by reproductive

isolation


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Speciation

I. A. 1. This doesnt work for extinctpopulations, or

2. Asexually reproducingorganisms

because reproductive

isolation cannot be

evaluated


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Speciation

I. B. Morphospecies: similarities inmorphology with the

underlying assumptionthat distinctive morphologieswould arise in isolated,independent

populations, i.e.,no gene flow


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Speciation

I. C. Phylogenetic species: a clusterof organisms that represents a

genealogy or a lineage ofdescent from a singleancestor


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Speciation

II. How does it happen? A portion ofan ancestral species becomes

reproductively isolated; there aretwo major mechanisms


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Speciation

II. A. Allopatric speciation:geographic isolation

leads to genetic drift and/ornatural selection to such anextent that the derivedspecies can no

longer mate successfully withmembers of its ancestral

species; this is the major

mechanism of speciation


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Speciation

II. A. 1. Dispersal


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Speciation

II. A. 2. Vicariance


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Speciation

II. B. Sympatric speciation: a newspecies arises within the midst

of the ancestral population1. Common in plants


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Speciation

II. B. 2. Polyploidy

a. Autoploidy

b. Alloploidy

3. Also observed in variousanimal species


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Speciation

III. Maintenance of species: twotypes of reproductive isolation

(prezygotic barriers andpostzygotic barriers)


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Speciation

III. A. Prezygotic barriers: preventfertilization

1. Temporal isolation: lifecycle timing differences


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Speciation

III. A. 2. Habitat isolation: physicalseparation


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Speciation

III. A. 3. Behavioral (or, sexual)isolation: no attraction

between male and female


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Speciation

III. A. 4. Mechanical isolation:incompatible

reproductivestructures


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Speciation

III. A. 5. Gametic isolation: chemicalincompatibility of gametes


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Speciation

III. B. Postzygotic barriers: reductionof the viability or fertility of a

hybrid


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Speciation

III. B. 1. Hybrid inviability:embryonic death of a

hybrid


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Speciation

III. B. 2. Hybrid breakdown:offspring of hybrid

unable to reproduce


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Speciation

III. B. 3. Hybrid sterility: hybridcannot reproduce as an

adulta. Mule: female horse +

male donkey

b. Hinny: female donkey +male horse


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Speciation

IV. Possible outcomes of post-speciation contact

A. Reinforcement

B. Hybrid Zones

C. Hybridization

1. New species2. Species extinction

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f09 Lec14 Speciation