Speciation
What is Speciation? How does speciation occur?The formation of a species; when two or
more species are created from an ancestral group
Occurs when a population can no longer interbreed with any other population
Reproductive Barriers
Reproductive barriers prevent organisms from two different species from producing viable, fertile offspring
Prezygotic BarriersPrevent mating between speciesDo not allow the egg to be fertilized if members of
different species attempt to mate
Postzygotic BarriersPrevent the zygote from developing into a viable, fertile
adult if the egg does become fertilized
Prezygotic Barriers
Habitat IsolationSpecies that live in
different habitats within the same area will rarely (if ever) encounter one another
Behavioral IsolationSpecial signals that
attract mates are species-specific
Ie. courtship rituals
Prezygotic Barriers
Temporal IsolationSpecies that breed during different times of the day,
different seasons, or different years cannot mix their gametes
Mechanical IsolationAnatomically incompatible species
Gametic IsolationSperm of one species can’t survive in the female
reproductive tract of another speciesOva may only recognize sperm of their own species
Postzygotic Barriers
Reduced Hybrid ViabilityOffspring dies during embryonic development
Reduced Hybrid FertilityEven if hybrids survive, they are usually sterileMule – horse & donkey
Hybrid BreakdownEven if the first generation is fertile and viable,
offspring of the first generation is infertile
What are the two ways in which speciation can occur?
1. Allopatric Speciation
2. Sympatric Speciation
Modes of Speciation
Figure 24.5
(a) (b)Allopatric speciation.A population forms anew species whilegeographically isolatedfrom its parent population.
Sympatric speciation.A subset of a populationforms a new specieswithout geographicseparation.
Allopatric SpeciationHappens due to physical
separation – dispersal to new habitat, physical barrier
Example: squirrel species on opposite rims of the Grand
Canyon
Figure 24.8
A. formosus
Atlantic Ocean
A. nuttingi
Isthmus of Panama
Pacific Ocean
A. panamensis A. millsae
Example: 15 sibling species of shrimp at the isthmus of Panama
Sympatric Speciation
Sympatric speciation occurs when new species arise within the same area of the ancestral population
Reproductive isolation evolves without geographical isolation
What are some factors that can result in sympatric speciation taking place?Occupation of a new ecological niche
(food source, breeding grounds, habitat location)
PolyploidySexual Selection
Polyploidy in plant species
Ex: strawberries, wheat, potatoes
Sexual Selection in Cichlid species
Females or males choose mates based on certain characteristics which can cause reproductive isolation
Ex: Cichlid species in lake
victoria
Cichlids in Lake Victoria!
What can be learned?
What can be learned from this experiment?Species grown separately with separate
food sources led to a reproductive barrier in fruit flies -> allopatric speciation!
How quickly do new species evolve?
Two theories:GradualismPunctuated Equilibrium
Figure 24.17
(a) Punctuatedpattern
Time
(b) Gradualpattern
Represent these two models in graphic form
Gradualism
Species are the product of slowly accumulating, small evolutionary changes
Punctuated Equilibrium
Species undergo long periods of very little change, followed by rapid, large evolutionary changes
Adaptive Radiation
What is Adaptive Radiation?When one species
evolves into many species that occupy a variety of available niches
Common after Mass extinctions or when organisms are isolated on islands
Ex: mammals, galapagos finches
1. Bishop pines and Monterrey pines release their pollen at different times of year
Pre or post reproductive barrier?
Specific type?Allopatric or
Sympatric?
2. To attract male fireflies, female fireflies give a species-specific sequence of flashes
Pre or Post?Specific Type?Allopatric or
Sympatric?
6. Eastern meadowlarks and western meadowlarks are almost identical morphologically, but hybrid offspring are largely infertile
Pre or Post?Specific Type?Allopatric or
Sympatric?