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(16-1) Population Genetics
• Study of evolution from a genetic point of view
• Population: individuals of the same species that interbreed
How to get Variations
• Genetic factors– Mutations– Recombination (crossing-over & indep.
assortment)– Random fusion of gametes (fertilization)
• Environmental factors
Definitions
• Gene pool: total genetic info in a pop.
• Allele frequency: how often a certain allele occurs in the gene pool– # of certain alleles /
total # of alleles in pop.
Definitions (cont.)
• p = freq. of dominant allele
• q = freq. of recessive allele
• Phenotype frequency: # of individuals w/ a particular phenotype / total # of individuals
Hardy-Weinberg Equation
• p2 + 2pq + q2 = 1
• p2 = ho/go dominant
• 2pq = he/go
• q2 = ho/go recessive
Hardy-Weinberg Equilibrium
• Allele frequencies remain constant over time (no evolution)
• Pop. in H-W equil. when certain assumptions are held…
H-W Assumptions
1. Random mating
2. No selective advantage of genotypes (no natural selection)
3. No mutations
4. No migration of individuals
5. Large pop. size
(16-2) Causes of Equilibrium Disruptions
1. Mutation
2. Migration
3. Genetic drift
4. Nonrandom mating
5. Natural selection
Mutation
• Occur at low rates
• Mutagen: mutation-causing agent– Radiation– Chemicals
• Can produce new alleles in a pop.– Most are harmful
Migration
• Immigration: movement into a pop.
• Emigration: movement out of a pop.
• Gene flow: genes moving from 1
pop. to another
Genetic Drift
• Allele freq. in a pop. change as a result of random events or chance– Very significant in small pops.
• Ex: old-order Amish & genetic disorders
Nonrandom Mating
• Mate selection based upon:– Geographic area– Physical characteristics
• Assortive mating
Sexual Selection
• Choosing a mate based on certain traits– In order to leave offspring male must be
selected by female
• Genes of successful reproducers, rather than those of successful survivors are amplified by natural selection
Directional Selection
• Individuals that display a more extreme form of a trait have the highest fitness
Disruptive Selection
• Individuals w/ either extreme variation of a trait have the greater fitness than the avg. of the trait
(16-3) Formation of Species
• Speciation: species formation
• Morphology: internal & external structure & appearance of an organism– Used for classification
Biological Species Concept
• A pop. of organisms can successfully interbreed but cannot breed w/ other groups
• Modern definition of species includes both morphology & biological species concept
Isolating Mechanisms
• Speciation begins w/ isolation
• 2 important types:– Geographic isolation– Reproductive “
Geographic Isolation
• Physical separation of members of a pop.– Ex: canyon develops through habitat
• Leads to allopatric speciation
Reproductive Isolation
• Results from barriers (not physical) to successful breeding b/w pop. groups in the same area
• 2 types:– Prezygotic: before fertilization
• Difference in mating times
– Postzygotic: after fertilization• Offspring may be unhealthy or infertile
Rates of Speciation
• Gradualism– Species develop by consistent & slow
evolution
• Punctuated equilibrium– Stops & starts in evolution in response to
dramatic environmental changes cause speciation