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The Origen of Species
Ch 24
• Evolution above the species level that
produces novelties like vertebrae or feathers
and contributes to speciation
– Speciation- the process by which new species
– Species: capable of reproducing
and making fertile offspring
Reproductive Isolation
• Prevents different species from making fertile
• May be “prezygotic” or “postzygotic”
Prezygotic barriers
1. Habitat isolation
2. Behavioral Isolation
3. Temporal(time) isolation
4. Mechanical isolation
5. Gametic isolation
Postzygotic barriers
• 1. Reduced hybrid viability- zygote doesn’t
• 2. Reduced hybrid fertility- offspring is sterile
• 3. Hybrid breakdown- fertile hybrids mate but
their offspring are weak/sterile
Allopatric Speciation
• “Different country” speciation due to
geographic separation
– Mountains forming, rivers separating, continental
Sympatric Speciation
• “Same country” speciationSome barrier prevents mating
(Temporal, behavioral…)
– Common in plants that easily
become polyploid instead of
--may result from Habitat
differentiation and
sexual selection
Adaptive radiation
(divergent evolution)
• -many species arise from a common ancestor which
moved into a new area or niche
Rates of Speciation
• Gradualism- gradual accumulation of
• Punctuated equilibrium- stable periods with
occasional bursts of rapid changes in the fossil
Evolution of Genes that Control
• “Evo-devo” is the branch of study where
evolution and developmental biology meet.
-Slight changes can make major differences
in morphology difference between species.
Homeotic Genes
• Master control genes that determine the
location and organization of body parts
– Hox genes determine positional information in
animal embryos that prompt cells to develop
structures in a particular place
– Changes can create drastic morphological effects
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