17.1 Genes and Variation17.2 Evolution as Genetic Change in

Populations

Page 483

17.1 Big Idea• After Mendel’s work was rediscovered in

1910, genetics took off! Researchers discovered that heritable traits are controlled by genes that are carried on chromosomes.

• This fit perfectly into evolutionary theory. Variation is the raw material for natural selection.

• Now scientists could understand why variation occurs.

Remember Genotype/Phenotype?• Some individuals have phenotypes that are

better suited to their environment than others. • Genetic Variations & Evolution are both studied

in populations.• Gene Pool: common group of genes and alleles

shared in a population. (Carpool means share)• Remember Alleles?• Relative Frequency: # of times an allele is in a

gene pool compared to total alleles in the gene pool. (Frequency means how often)

Example

Single Gene Traits

• The number of phenotypes produced for a trait depends on how many genes control the trait.

• Single- Gene Trait: Trait controlled by only 1 gene. (single means one)

• Example: snail shell bandsControlled by 1 gene 2 alleles• Page 485

Polygenic Traits

• Polygenic Traits – controlled by 2 or more genes. (poly means many)

• Each gene has 2 or more alleles.• Example: Human Height• Pg 486

17.2 Big Idea

• Evolutionary fitness is success in passing genes to the next generation.

• Evolutionary adaptation is any genetically controlled trait that increases an individual’s ability to pass along its alleles.

• Natural selection on single-gene traits can lead to changes in allele frequencies and thus to changes in phenotype frequencies.

Initial Population Generation 10 Generation 20 Generation 30

90% 80% 70% 40%

10% 20% 30% 60%

Natural Selection on Polygenic Traits

• Natural Selection on polygenic traits can affect the relative fitness of phenotypes and thereby produce one of 3 types of selection.

Genetic Drift

• In small populations, individuals that carry a particular allele may leave more descendants that other individuals leave, just by chance.

• Over time, a series of chance occurrences can cause an allele to become more or less common in a population.

• Genetic Drift – Random change in allele frequency. (Drifting = random change)

Drift• Bottleneck effect: a change in allele frequency

following a dramatic reduction in the population size. (bottle necks are reduced/small) ex: disease

• Founder effect: When allele frequencies change as a result of the migration of a small subgroup of a population. (Haven’t found way back, so new group somewhere else). EX fruit flies on islands

Hardy-Wineberg Principle• Describes conditions under which evolution does

NOT occur. - Genetic Equilibrium• Hardy-Wineberg Principle: Allele frequencies in a

population remain constant unless 1 or more factors cause those frequencies to change. (it is hard for evolution to occur)

• 1. large population (drift on small pop.)• 2. No mutations• 3. Random Mating • 4. No immigration/emigration• 5. No natural selection (no advantages)

Example of Equation• p2 + 2 pq + q2 = 1 Note: p+q=1

• P = A (frequency of dominate allele)• P2 = AA frequency of homozygous dominate• Q = a (Frequency of recessive allele) • q2 =aa (frequency of homo. Recessive)• 2PQ = Aa (frequency of heterozygous)