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?