# Chapter 23: The Evolution of Populations

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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 allele f (q) = frequency of the recessive allele

Calculating allele frequency:

Genotype # of Individuals MM 1787 MN 3039 NN 1303 Total 6129
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

(the more offspring that you have that survive = more fit you are)
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)