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)
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