1. The Origin of Species
Chapter 24

2. Macroevolution vs. Microevolution
When changes in allele frequency in a population occur over time
Can be supported through direct observation
Macroevolution
A broad pattern of evolution over long time spans. Involves the formation of new species from old and how all of life evolved from a single celled organism
Supported by the fossil record

3. Speciation
The process by which one species splits into 2 or more new species
Members of a species often resemble each other because their populations are connected by gene flow
Biological species are defined in terms of reproductive compatibility, the formation of a new species must involve reproductive isolation (no gene flow)
Relies on the existence of barriers to prevent 2 species from producing fertile offspring by preventing gene flow

4. Reproductive Isolation Barriers I
Prezygotic Barriers
Blocks fertilization from happening; occurs before the zygote
Act by…
Impeding members of different species from attempting to mate
Preventing an attempted mating from being completed successfully
Hindering fertilization if mating is completed
Postzygotic Barriers
Contribute to reproductive isolation after the hybrid zygote is formed
Embryo may not survive due to developmental errors or may be infertile preventing gene flow

5. Reproductive Isolation Barriers II
Prezygotic Barriers
Habitat isolation
Temporal isolation
Behavioral isolation
Mechanical isolation
Gametic isolation
Postzygotic Barriers
Reduced hybrid viability
Reduced hybrid fertility
Hybrid breakdown

6. 24.2 Allopatric vs. Sympatric Speciation
When gene flow is interrupted because a population is divided into geographically isolated subpopulations
Once separated, rely on mutations, natural selection, and genetic drift to change allele frequencies
Sympatric
When speciation occurs in populations that live in the same geographic area.
Gene flow is reduced by factors such as polyploidy, habitat differentiation, and sexual selection
Polyploidy occurs when there is an error during cell division such as a cell not dividing after it has replicated its chromosomes; 2n = 4n

7. Allopatric vs. Sympatric Speciation II

8. 24.3 Hybrid Zones
A region in which members of different species mate, producing at least some offspring of mixed ancestry

9. 24.4 The Time Course of Speciation
Speciation can occur rapidly or gradually
The fossil record includes many episodes in which new species appear suddenly in geologic stratum, persist relatively unchanged through several strata, and then disappear
Stephen Jay Gould and Niles Eldredge coined the term punctuated equilibria to describe this pattern
Other species do not show a punctuated pattern; instead, they change more gradually over long periods of time

10. Studying the Genetics of Speciation
By identifying the genes that control the traits that cause reproductive isolation, scientists can explore how many genes actually change when a new species forms
Sometimes it is just the mutation of one gene
Ex) the gene that controls the direction in which a snail shell spirals induces a mechanical barrier to reproduction