Fig. 24.3

Time of Day

Time of Year

Courtship

Sounds/Songs

Flowers

Snails

Plants

Broadcast Spawners

Bullfrog x

Leopard Frog

Horse (2n=64) x

Donkey (2n=62) Mule (2n=63)

I. Reproductive Isolation

C. Limitations of Biological Species Concept• Mayr’s definition emphasizes reproductive

isolation; may not work in all situations• Ex: Classifying fossil organisms• Ex: Species that reproduce asexually [prokaryotes,

some protists, fungi, plants (e.g. bananas), animals (e.g. fishes, lizards)]

• Ex: Multiple species are inter-fertile but remain distinct (e.g. orchids)

II. Speciation

• Occurs when a population becomes reproductively isolated from rest of species

• May be allopatric or sympatric

A. Allopatric Speciation• Population becomes geographically separated• Over time, mutation, genetic drift, natural

selection genetic divergence• Thought to be responsible for development of

most new animal species• How do populations become isolated?

Fig. 24.5

II. Speciation

A. Allopatric Speciation1. Geographical barriers

a. Land bridges form, separating aquatic populations (e.g. Isthmus of Panama)

b. Land masses separate or split off from continents (e.g. South America & Africa)

c. Mountain ranges formd. Water levels in water bodies become lower, creating

multiple smaller poolse. Rivers change course (Ex: oxbow lakes)f. Glaciation occursg. Islands form and are colonized (e.g. Galàpagos, Hawaii,

Madagascar)• Note: Geographic barriers for some species aren’t barriers

for others• Ex: Birds and many insects can fly between isolated

patches of habitat• Ex: Some fishes can swim long distances• Ex: Airborne pollen and drifting gametes in the ocean

can be transported long distances

II. Speciation

A. Allopatric Speciation2. Conditions Favoring Allopatric Speciation

• Typically occurs at edges of parent population’s range• Splinter population (peripheral isolate) may be good

candidate for speciation because:

a. Gene pool different from parent population• Likely to represent extreme of genotypic range• Speciation more likely if founder population small• Ex: Harris’ and white-tailed antelope squirrels on rims

of Grand Canyon

b. Genetic drift within peripheral isolate• Can lead to rapid divergence from parent population

c. Natural selection• Diversifying or directional selection under conditions

at extremes tolerated by parent population

Fig. 24.6

Fig. 24.10

II. Speciation

B. Sympatric Speciation• Population becomes reproductively isolated

without geographic separation• May be common in plants; importance in

animals less clear

1. Plantsa. Autopolyploidy

• Results from error in mitosis

II. Speciation

B. Sympatric Speciation1. Plants

a. Autopolyploidy• Results from error in mitosis

b. Allopolyploidy• Error in meiosis + hybridization

Fig. 24.11

II. Speciation

B. Sympatric Speciation1. Plants

b. Allopolyploidy• Allopolyploids typically can’t produce fertile

offspring with either parent (incompatible chromosome numbers)

• If population of allopolyploids becomes established, typically one of three outcomes:

1) New species unable to compete successfully; goes extinct

2) New species competes successfully; coexists with parent species

3) New species competes very successfully; causes extinction of one or both parent species

II. Speciation

B. Sympatric Speciation1. Plants

b. Allopolyploidy• May be very common in plants• Up to 80% of flowering plant species are

polyploid• May account for 25-50% of plant species• Mechanism for very rapid speciation (single

generation)• May account for rapid radiation of plants in fossil

record and high diversity of flowering plants (>290,000 species)

II. Speciation

B. Sympatric Speciation2. Animals

• Mechanisms of sympatric speciation less well understood than in plants

• Polyploidy usually lethal

• Habitat differentiation• Ex: North American apple maggot fly (article)

• Mutation short-term isolation reinforced by non-random mating (sexual selection)

• Ex: African cichlids

Fig. 24.12

II. Speciation

C. Allopatric vs. Sympatric Speciation• Animals (usu. allopatric)

• Isolating mechanisms?

• Plants (usu. sympatric)• Isolating mechanisms?

Fig. 24.3

Time of Day

Time of Year

Courtship

Sounds/Songs

Flowers

Snails

Plants

Broadcast Spawners

Bullfrog x

Leopard Frog

Horse (2n=64) x

Donkey (2n=62) Mule (2n=63)

II. Speciation

D. Adaptive Radiation• Evolution of many diversely adapted species

from common ancestor• Island chains offer unutilized habitat and open

ecological niches• Ex: Colonization of Hawaii by honeycreepers• Ex: Silversword alliance in Hawaii

Fig. 25.20

II. Speciation

C. Adaptive Radiation• Occurs when niche space is available

• Ex: Radiation of mammals after K/T extinction

• Radiation events often are associated with the appearance of novel features

• Why?• Ex: Shells & skeletons first appeared at beginning of

Paleozoic (may have facilitated radiation)