CH 24
• Speciation: origin of new species
• Microevolution: changes in allele frequencies
• Macroevolution: changes that result in
formation of different species
I. The Biological Species Concept
• Biological species: group of populations
whose members can breed and produce
viable, fertile offspring
A. Reproductive Isolation
• biological barriers that prevent formation of
viable fertile offspring
• Can be prezygotic or postzygotic
• Prezygotic barriers:Prevent the formation of
a zygote (fertilized egg)
– Habitat Isolation: if 2 populations occupy
different habitats, they don’t encounter each
other to mate
– Behavioral Isolation: Differences in behavior
prevent mating
– Temporal Isolation: Different breeding times
prevent mating
– Mechanical Isolation: Genitalia are incompatible
– Gamete isolation: sperm doesn’t fertilize egg
• Postzygotic Barriers: Prevents the formation
of normal fertile offspring
– Embryo formed is not viable
– Offspring produced are weak and die
– Offspring produced are sterile
II. Mechanisms of Speciation
• Allopatric Speciation
– Results from lack of gene flow between
populations that have been geographically
– Populations evolve independently due to
mutation, natural selection, genetic drift
– Reproductive isolation can result from genetic
• Sympatric speciation
– Evolution of different species WITHOUT
geographic isolation
– Polyploidy in plants is the doubling of
chromosomes from one generation to the next
resulting in reproductive isolation
– Appearance of new ecological niches in an area
can result in sympatric speciation
III. Hybrid Zones
• Regions where two species with incomplete
reproductive barriers meet and produce
hybrid offspring
• Usually a band where 2 species meet
• 3 possible outcomes in the hybrid zone:
– Reinforcement of reproductive barriers: hybrid
offspring are less fit so rate of hybridization
– Fusion: if hybrids are as fit as parents there could
be enuf mating and gene flow between the
populations that they fuse into 1 species
– Stability: extensive gene flow prevents the
continued selection of reproductive barriers