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)