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BIOA01H3 Chapter Notes -Allele Frequency, Heterozygote Advantage, Population Bottleneck

Biological Sciences
Course Code
Ivana Stehlik

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Ch 17 Microevolution: Genetic changes
within populations
17.1 Variation in Natural Populations (Pg 374)
Microevolution: heritable change in the genetics of a population
o change in the genetic makeup of a population from generation to generation due to
natural Selection
o leads to variation in genetic makeup of population over time
Population: group of individuals belonging to the same species living at the same time and in
the same area (individuals can interact)
a new era of biology began in 1859 when Charles Darwin published On the Origin of Species by
Means of Natural Selection
o focused biologists’ attention on great diversity of organisms
o Darwin made two major points:
presented evidence that present-day species are descendants of ancestral
species that were different
o proposed a mechanism for evolutionary processes = Natural Selection
Differential Survival in Nature (= Natural Selection):
o populations in nature do not grow exponentially indefinitely
o reach a stable level (carrying capacity) determined by limiting resources
o leads to struggle for existence
o only a portion of offspring survive and reproduce
o most offspring die (80% is typical in wild populations)
Variation: differences between individuals in a population
Heredity: - some variations are passed on to offspring
o individuals with some variations survive better than others with none
o traits that increase survival are passed on to offspring
Result: - differential ability to survive and reproduce will lead to gradual change in population
with accumulation of favourable traits selected over generations (by nature)
Favourable traits that survive lead to genetic variation among individuals in populations -
contribute to evolution of species
common misconception about evolution is that individual organisms evolve during their
Natural Selection acts on individuals, but populations evolve
Phenotypic variation: differences in appearance or function that are passed from generation to
evolutionary biologists describe and quantify phenotypic variation due to:
o heritable variation in appearance and/or function

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genetic cause - mutations, recombination through crossing over, independent
assortment and random fertilization)
o phenotypic plasticity (environmental cause): the ability of an organism to change its
phenotype in response to changes in the environment
Quantitative variation: individuals differ in small, incremental ways.
o Characteristics with a range of variation
o Controlled by multiple genes
Qualitative variation: they exist in two or more discrete states and intermediate forms are often
o Characteristics with distinct states
o Polymorphism: distinct variants of character
The occurrence of something in different forms, in particular.
o Describe by percentage or frequency of each
Causes of phenotypic variation
o Genetic differences between individuals
o Diff in env. Factors that individuals experience
o Interaction between genetics and the environment
Genetic and phenotypic variations may not be perfectly correlated.
Diff genotypes can have the same phenotype
Same genotype can have diff phenotype. Eg: acidity of soil changes flower color
Only genetically based variation is subject to evolutionary change

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Phenotype is product of interaction btween genotype and its env.
o Test for env cause of variation: same genotype, change one env variable and measure
o Breeding experiments can demonstrate the genetic basis of phenotypic variation
Traits that vary quantitatively will respond to artificial selection only if variation
has genetic basis.
o Genetic causes of variation can also be identified by analyzing genealogical pedigrees.
Genealogical: Of or relating to the study or tracing of lines of family descent
What generates genetic variation
o 2 potential sources
Production of new alleles
Arise from small scale mutations in DNA
Rearrangement of existing alleles
Arise from large scale changes in chromosome structure or number
several forms of genetic recombination
o crossing over between homologous chromosomes during
o independent assortment of nonhomologous chromosomes
during meiosis
o random fertilizations between genetically different sperm and
o More than 10600 combinations of alleles are possible in human gametes.
1960s, gel electrophoresis of proteins.
Identification of protein polymorphism allows researchers to infer genetic variation at the locus
coding for that protein.
o Problems: same size but diff amino acids can’t distinguish.
o Every locus exhibits some variability in its nucleotide sequence.
17.2 Population Genetics (Pg 377)
study of how populations change genetically over time
reconciled ideas of Darwin and Mendel
focuses on populations as units of evolution
for a population to evolve, its members must have heritable genetic variation (= raw material
on which agents of evolution act)
Phenotype = the physical / physiological expressions of all of the genes of an individual
Genotype = genetic make-up of all of the genes of an individual organism
A population evolves when individuals with different genotypes survive or reproduce at
different rates.
heritable variation determined in genotype
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