Chapter 24 ~ Chapter 24 ~ The Origin of The Origin of SpeciesSpecies

Macroevolution: Macroevolution: evolutionary change above the evolutionary change above the species level:species level:

• Speciation: Speciation: the origin of new speciesthe origin of new species– Results in diversity of life formsResults in diversity of life forms

– Can be slow(gradualism) or occur in Can be slow(gradualism) or occur in bursts (punctuated equilibrium). bursts (punctuated equilibrium).

Speciation and extinction: Speciation and extinction:

• Mass extinctions are often Mass extinctions are often followed by adaptive radiationfollowed by adaptive radiation– Example: radial expansion of Example: radial expansion of

mammals after the extinction of mammals after the extinction of dinosaursdinosaurs

Two forms of speciation:Two forms of speciation:

• 1-1- Anagenesis Anagenesis (phyletic evolution): (phyletic evolution): accumulation of heritable changesaccumulation of heritable changes– Transforms a species into a species with Transforms a species into a species with

different characteristics different characteristics

Two forms of speciation: Two forms of speciation:

• Cladogenesis: Cladogenesis: – branching evolution branching evolution

– The splitting of the gene pool into The splitting of the gene pool into two or more separate pools, which two or more separate pools, which give rise to one or more new speciesgive rise to one or more new species

Anagenesis vs. cladogenesisAnagenesis vs. cladogenesis

• Which form of Which form of speciation is the speciation is the basis for basis for biological biological diversity?diversity?

How does the environment How does the environment influence speciation? influence speciation?

(physiological responses)(physiological responses)

• Examples: Examples: – Phototropism(plants and planaria) the Phototropism(plants and planaria) the

response to the presence of lightresponse to the presence of light

– Photoperiodism(flowering plants, Photoperiodism(flowering plants, chickens and eggs) response in change chickens and eggs) response in change in the length of the nightin the length of the night

More examples: More examples: • Circadian rhythms: physiological Circadian rhythms: physiological

cycle of about 24 hours cycle of about 24 hours – Present in all eukaryotesPresent in all eukaryotes

– Can persist in the absence of external Can persist in the absence of external cuescues

• Diurnal/nocturnalDiurnal/nocturnal

• Seasonal responses- hibernation, Seasonal responses- hibernation, estavation and migrationestavation and migration

More examples: More examples: • Quorum sensing in bacteria: Quorum sensing in bacteria:

regulation of gene expression in regulation of gene expression in response to cell population and response to cell population and densitydensity

What is a species?What is a species?

• Biological species concept (Mayr):Biological species concept (Mayr): a a population or group of populations population or group of populations whose members have the potential whose members have the potential to interbreed and produce viable, to interbreed and produce viable, fertile offspring (genetic exchange fertile offspring (genetic exchange is possible and that is genetically is possible and that is genetically isolated from other populations)isolated from other populations)

Reproductive Isolation (isolation of Reproductive Isolation (isolation of gene pools)gene pools)

• New species arising when two New species arising when two populations diverge from a populations diverge from a common ancestor and become common ancestor and become reproductively isolatedreproductively isolated– Must be maintained for species to Must be maintained for species to

remain distinctremain distinct

Reproductive Isolation (isolation of Reproductive Isolation (isolation of gene pools)gene pools)

• Prezygotic barriers:Prezygotic barriers: impede mating impede mating between species or hinder the fertilization between species or hinder the fertilization of the ovaof the ova

• Habitat (snakes; water/terrestrial)Habitat (snakes; water/terrestrial)• Behavioral (fireflies; mate signaling)Behavioral (fireflies; mate signaling)• Temporal (salmon; seasonal mating)Temporal (salmon; seasonal mating)• Mechanical (flowers; pollination Mechanical (flowers; pollination

anatomy)anatomy)• Gametic (frogs; egg coat receptors)Gametic (frogs; egg coat receptors)

Reproductive Isolation (cont.)Reproductive Isolation (cont.)• Postzygotic barriers: Postzygotic barriers: fertilization occurs, fertilization occurs,

but the hybrid zygote does not develop into but the hybrid zygote does not develop into a viable, fertile adulta viable, fertile adult

• Reduced hybrid viability (frogs; zygotes fail to Reduced hybrid viability (frogs; zygotes fail to develop or reach sexual maturity)develop or reach sexual maturity)

• Reduced hybrid fertility (mule; horse x Reduced hybrid fertility (mule; horse x donkey; cannot backbreed)donkey; cannot backbreed)

• Hybrid breakdown (cotton; 2nd generation Hybrid breakdown (cotton; 2nd generation hybrids are sterile)hybrids are sterile)

Other definitions of species: Other definitions of species: emphasize unity within a species: emphasize unity within a species: • 1.1. morphological species concept:

– characterizes a species by its body shape, size, and other structural features.

•

• 2. paleontological species concept:– focuses on morphologically discrete

species known only from the fossil record

Other definitions of species: Other definitions of species: emphasize unity within a species: emphasize unity within a species:

• 3. 3. ecological species concept:– views a species in terms of its

ecological niche, its role in a biological community.

Other definitions of species: Other definitions of species: emphasize unity within a species: emphasize unity within a species:

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Other definitions of species: Other definitions of species: emphasize unity within a species: emphasize unity within a species:

• 4. 4. phylogenetic species concept:– defines a species as a set of

organisms with a unique genetic history- a branch on a tree of life

Other definitions of species: Other definitions of species: emphasize unity within a species: emphasize unity within a species:

Modes of speciation:Modes of speciation:(based on (based on how gene flow is interrupted)how gene flow is interrupted)

• Allopatric:Allopatric: populations segregated by a populations segregated by a geographical barriergeographical barrier– Separated from its parent populationSeparated from its parent population

– can result in adaptive radiation (island can result in adaptive radiation (island species)species)

– Occurs when potential to interbreed and Occurs when potential to interbreed and produce fertile offspring no longer existsproduce fertile offspring no longer exists

– Examples: flycatchersExamples: flycatchers

Modes of speciationModes of speciation(based on (based on how gene flow is interrupted)how gene flow is interrupted)

SympatricSympatric:: reproductively isolated reproductively isolated subpopulation in the midst of its parent subpopulation in the midst of its parent populationpopulation

*no geographic isolation but *no geographic isolation but reproductive barriersreproductive barriers

*(change in genome); polyploidy in *(change in genome); polyploidy in plants; cichlid fishesplants; cichlid fishes

Modes of speciationModes of speciation(based on how (based on how gene flow is interrupted)gene flow is interrupted)

Tempo of Speciation: Gradualism Tempo of Speciation: Gradualism vs. Punctuated Equilibriavs. Punctuated Equilibria

• gradualism: gradualism: – Gradual divergence from a common Gradual divergence from a common

ancestorancestor

– Change in morphology over a long Change in morphology over a long time as unique adaptations are time as unique adaptations are acquiredacquired

Tempo of Speciation: Gradualism Tempo of Speciation: Gradualism vs. Punctuated Equilibriavs. Punctuated Equilibria

• Punctuated equilibrium: Punctuated equilibrium: – Sudden changeSudden change

– Evolution in short burstsEvolution in short bursts

– New species bud from parent species New species bud from parent species and then changes little for the rest of and then changes little for the rest of its existenceits existence