1. THE ORIGIN OF SPECIES
CHAPTER 24

2. Microevolution vs Macroevolution
Microevolution – changes in allele frequencies in a given population
Macroevolution – broad pattern of evolution above species level (origins of new groups of organisms through a series of speciation events

3. Biological Species Concept
Species - A group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring – but do not produce viable fertile offspring with members of other such groups
The members of a biological species are united by being potentially reproductively compatible
Speciation – process by which one species splits into two or more species
Biological Species Concept emphasizes the separateness of species from one another due to reproductive barriers

4. Other Definitions of Species
Definitions Based on Unity of Species
Morphological Species Concept – characterizes species by body shape and other structural features; can be applied to sexual and asexual organisms
Ecological Species Concept – views a species in terms of how members of the species interact with the biotic and abiotic factors of their environment
Phylogenetic Species Concept – defines species as smallest group of individuals that share a common ancestor

5. Reproductive Barriers: Pre-zygotic
Pre-zygotic – barriers that prevent fertilization
Habitat isolation – populations cannot get together
Behavior isolation – different mating behavior
Temporal isolation – breed or flower at different times
Mechanical isolation – different anatomical structures
Gamete isolation – gametes fail to fuse; do not recognize each other

6. Reproductive Barriers: Post-zygotic
Post-zygotic – barriers that prevent zygote from developing into fertile offspring
Reduced hybrid viability – zygote either fails to develop or to reach sexual maturity
Reduced hybrid fertility – hybrid is sterile
Hybrid breakdown – hybrid produces offspring by offspring are either not viable or not fertile

7. Modes of Speciation
Allopatric – gene flow is interrupted when a population is segregated by a geographic barrier (ocean, mountain range, etc.)
Favoring conditions:
Small population at fringe of larger one
Better change is gene pool already somewhat different
Different selection factors in same population
Adaptive Radiation – emergence of numerous species from a common ancestor introduced into a new environment

8. Modes of Speciation
Sympatric – new species arise within the parent population
Reproductive isolation without a geographic barrier
Polyploidy – accident during cell division that results in extra sets of chromosomes (~80% of plants today are the result of this)
Habitat differentiation – subpopulations exploit a habitat or resource not utilized by the entire population
Sexual selection – mate choice based on different appearance of mates of same population

9. Genetic Change of Subpopulations
Adaptive Divergence
2 parts of population adapt to different environments
Each accumulates different gene pools
Reproductive Barriers Develop
Usually secondary to change in gene pool
May be the side effect of sexual selection

10. TEMPO OF SPECIATION Gradualism
One species gradually evolves into a new species
Represents microevolution
Big changes occur through the accumulation of small changes

11. TEMPO OF SPECIATION Punctuated Equilibrium
Long periods of stasis (equilibrium) punctuated by episodes of speciation
Species undergo most change when they branch from parent species; then change very little after
Species develop in spurts of rapid change
Neither slow nor gradual; end of age of dinosaurs, beginning of age of mammals