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Population Genetics
Evolution by Natural Selection
Evolution is ______________________________ (through several generations).
 Darwin
Those individuals with ____________________ characteristics survive
and pass on their traits to the next ____________________
Population gradually includes more individuals with
__________________ characteristics.
Gene Variation in Nature
 A gene with more than one allele is _______________________.
Natural populations tend to have more polymorphic genes than can be
accounted for by ______________________.
Before Hardy-Weinberg
Population Genetics - Study of properties of genes in populations.
 _______________________ inheritance was widely accepted.
New genetic variants would quickly be ______________________.
 ______________________ had not yet been discovered.
 Why don’t recessive traits ______________________after a while?
Hardy-Weinberg Principle
Hardy-Weinberg – Two scientists came up with an explanation at the same time
Original proportions of genotypes in a population will remain _______________from
generation to generation
Five Assumptions:
Population is ___________________________
Mating is __________________________
No ___________________
No ______________________
No __________________ occurring
Calculate Genotype Frequencies
To find allele frequencies,let:
T=frequency of _________________ allele
t=frequency of __________________ allele
T + t = 1 (100% of alleles in population)
Frequency of homozygous recessive individuals (______) in the population is:
t x t = t2
Frequency of ________________ allele (t) in the population is:
t = √ t2
Frequency of ____________________ allele (T) in the population is:
Agents of Evolutionary Change
1. Mutation
 Mutation rates are generally so ________________ that they have
_____________ effect on Hardy-Weinberg proportions of common alleles.
Ultimate source of genetic __________________.
2. Gene Flow
 ____________________ of alleles from one population to another.
Tend to ___________________ allele frequencies.
3. Genetic Drift
 Frequencies of particular alleles may change by _____________________.
Important in _______________ populations.
 __________________ Effect - Few individuals found (start) new
population (small allelic pool).
 __________________ Effect - Drastic reduction in population, and
gene pool size.
4. Selection
 Artificial - Breeders exert ___________________.
 Natural - ___________________ exerts selection.
Variation must exist among individuals.
Variation must result in differences in numbers of viable offspring
Variation must be genetically inherited.
 Natural Selection is a process, and Evolution is an outcome.
Selection Pressures:
 Avoiding Predators
 Matching Climatic Condition
 Pesticide Resistance
Evolutionary Forces Maintaining Polymorphism
Adaptive Selection Theory
 Heterogeneous environments produce many alleles.
Neutral Theory
 Balance between mutation and genetic drift.
Gene Flow versus Natural Selection
 Genetic drift may decrease frequency of an allele favored by selection.
Forms of Selection
Disruptive Selection
 Selection eliminates intermediate types.
Directional Selection
 Selection eliminates one extreme from a phenotypic array.
Stabilizing Selection
 Selection acts to eliminate both extremes.
Fitness - Number of surviving offspring passed to the next generation.