Chapter 23:The Evolution of Populations
Population Genetics
• microevolution – change in genetic makeup of a population from generation to generation
• macroevolution – evolutionary change above the species level
population – group of individuals of the same species living in the same
area
gene pool – all the genes in a given population at a given time
allele frequency – proportion of an allele in a gene pool
• p = dominant allele• q = recessive allele
f (p) = frequency of the dominant allelef (q) = frequency of the recessive allele
Calculating allele frequency:
Genotype # of Individuals Genotypic frequencies
MM 1787 MM = 1787/6129 = 29%
MN 3039 MN = 3039/6129 = 50%
NN 1303 NN = 1303/6129 = 21%
Total 6129
Hardy-Weinberg Theorem
• helps measure changes in allele frequencies over time
• provides an “ideal” population to use as a basis of comparison
Conditions for Hardy-Weinberg Equilibrium:
• Large population• No gene flow• No mutations• Random mating• No natural selection
– hypothetical population that is not evolving– rarely met in nature
Mutation and sexual recombination only sources of new variations
• mutation – changes in nucleotide sequence in DNA
• point mutations – change in one nucleotide
• gene duplication – duplication of a chromosome segment
sexual recombination – crossing over, shuffling of genes during meiosis
Genetic Drift – change in allele frequencies due to chance
• usually in smaller populations• reduces genetic variation
bottleneck effect –when a population has been dramatically reduced, and the gene pool is no longer reflective of the
original population’s
Human actions can create a genetic bottleneck
founder effect – when a small number of individuals colonize a new area;
new gene pool not reflective of original population
The Fugate family Kentucky's Troublesome Creek
gene flow – when a population gains or loses alleles
• a movement of fertile individuals leaving/arriving
• – a reduces differences between populations
genetic variation – heritable variations in a population
discrete characteristics – are all one discrete variety
quantitative characteristics – vary along a continuum, usually due to
influence of two or more genes
average heterozygosity – measure of polymorphism in a population
geographic variation – difference in variation between population subgroups in different areas
• cline – a graded change in a trait along a geographic axis
Evolutionary Fitness
• fitness – contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals
(the more offspring that you have that survive = more fit you are)
• relative fitness – fitness of a particular genotype
Types of selection
• directional selection – shift toward a favorable variation
• disruptive selection• – favors the extremes
• stabilizing selection• – favors the mean
Heterozygous Advantage – when individuals heterozygous
• Recessive allele is maintained in the population
Example: sickle-cell anemia
prevelence of malaria sickle-cell disease
Sexual selection– a natural selection for mating
success
Sexual dimorphism – differences between the sexes in secondary sexual
characteristics
• Not necessarily better adaptations; example – mane on lion very hot, feathers on peacock very “expensive” to make
Common misconceptions:
• Natural selection acts on phenotype, not genotype!
• Natural selection does not create more perfect organisms! (what is perfect in one environment may not be perfect in another)
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