Chapter 24

Occurs between micro- & macroevolution

• • Microevolution
• – Changes in allele frequency in a population
• – Short time frame (generations)
• – Ex. Mutation, selection, drift, gene flow

• • Macroevolution
• – Broad patterns over long time spans
• – Origins of entire new groups of organisms
• – Ex. Origin of mammals, flowering plants etc.

The Formation of new species

Species = populations that can interbreed and produce viable and fertile offspring

• – New species form with reproductive isolation
• • Different species may appear similar
• • One species may have great diversity 

• Prevents two individuals from producing viable, fertile offspring
• – Maintains existing species boundaries
• – Can lead to the formation of new species

• • Reasons for isolation:
• – Prezygotic barriers
• • Before formation of the zygote
• – Postzygotic barriers
• • After formation of the zygote

• Two species rarely encounter each other
• – Occupy different habitats

• – Ex. Two species of snakes
• • One species aquatic, one species terrestrial
• • Unlikely to be in the same place

• Species that breed at different times
• • Timeofday,differentseasons,ordifferentyears

• – Ex. Spotted skunks
• • One species mates in winter, one mates in summer
• • Unlikely to be sexually active during same period

Behavioral isolation

– Courtship rituals and other behaviors unique to a species are effective barriers

– Ex. Blue-footed boobies courtship display

Mating attempted, but morphological differences prevent completion

– Ex. Snail genital openings don’t align

Sperm of one species cannot fertilize egg of another species

• • Sperm cannot survive in reproductive tract
• • Sperm cannot penetrate the egg

– Ex. Sea urchin egg/sperm proteins won’t fuse

Incompatible genes from parents interact and impair development and survival

– Ex. Salamander hybrids that do not fully develop or are frail

Reduced Hybrid Fertility

– Vigorous hybrids produced, but are sterile

• • Often due to chromosomal differences
• – Chromosome #, chromosome structure

–Ex. Donkey+HorseMule(sterile) 

Some first generation hybrids are viable and virile, but next generation are weak or sterile

• • Often accumulate recessive disorder alleles
• – Ex. Many plant strains 

• Geographic barriers prevent gene flow
• – Rivers, oceans, mountains, canyons etc. etc.
• Ex. 24.6 Antelope squirrels & G. Canyon
• Ex. 24.8 Shrimp species separated by isthmus
• Ex. 24.9 Salamander reproductive isolation increases with distance 

• Occurs in populations with range overlap
• Can arise from:

• – Polyploidy
• • Subpopulation with an extra set of chromosomes
• – Habitat differentiation
• • Subpopulation can exploit a new microhabitat
• – Sexual selection (24.12)
• • Ex. Subpopulation with different morphology
• • New subpopulations no longer interbreed 

• More common in plants
• – Autopolyploidy

• • Errors in cell division create new subpopulation within a single original species
• – Allopolyploidy
• • Hybridization between two species creates new subpopulation with different chromosome # 

• Can occur at different rates
• – Evidence of this in the fossil record
• • Punctuated equilibrium
• – Describes long periods of species stability followed by sudden change (rapid speciation)
• • Gradual change over time also possible