Chapter 23-Microevolution

Population genetics• Population:

a localized group of individuals belonging to the same species

• Species: a group of populations whose

individuals have the potential to interbreed and produce fertile offspring

• Gene pool: the total aggregate of genes in a

population at any one time• Population genetics:

the study of genetic changes in populations

• Modern synthesis/neo-Darwinism• “Individuals are selected, but populations

evolve.”

Hardy-Weinberg Principle• States: frequencies of alleles & genotypes in a population’s

gene pool remains constant from generation to generation.• Model proposed in 1908• Represents an ideal situation• Seldom occurs in nature• Mathematical model is used to compare populations• Allows biologists to calculate allele frequencies in a

population• Serves as a model for the genetic structure of a non-evolving population (equilibrium)Represents “genetic equilibrium”If the allele frequencies deviate from the predicted values of HW then

the population is said to be evolving.

Hardy-Weinberg Theorem

5 conditions for Equilibrium

-Very large population size

- No migration

- No net mutations

- Random mating

- No natural selection

**when all these are met then a population is not evolving

Hardy-Weinberg Equation

• p=frequency of one allele (A); q=frequency of the other allele (a)

• p+q=1.0 • (p=1-q & q=1-p)• P2=frequency of AA genotype • 2pq=frequency of Aa• q2=frequency of aa genotype;

p2 + 2pq + q2 = 1.0**Operates like a deck of cards: No matter

how many times you shuffle the deck the deck itself remains the same.

Solving & Analyzing HW Principle• Problem: If you had 90 individuals that possessed the recessive

condition in a population of 1000 individuals, determine the frequency of dominant and recessive alleles present in the population as well as the genotypic and phenotypic frequencies.

(1) Always start with the # of homozygous recessive alleles - aa = 90 and q2 = 90/1000 which is 0.09 - a = square root of 0.09 which is 0.3 - A = (1 – 0.3) which is 0.7 - AA = (0.7) 2 which is 0.49

- Aa = ???

**Remember that p2 + 2pq + q2 = 1 (AA) (Aa) (aa)

Ch. 23 Microevolution (Video)1. How are Mosquito populations affected by genetic and

environmental conditions2. What affect did the insecticide have on the insects?3. Which forms of isolation are presented in the last

segment?4. What are some examples of species that are endemic to

Catalina Island? How is inbreeding prevented for these species?

***Write the title for each segment and FIVE statements for each segment.

Microevolution

Two situations increase the impact of Genetic drift:

**Bottleneck & Founder Effect

Microevolution• Involves small or minor changes in the allele frequencies

within a population• Five processes have been identified that change allele

frequencies: (see pg. 458)– Nonrandom mating (sexual selection)

– Gene flow (migration between populations)

– Genetic drift (bottleneck effect & Founder effect)

– Mutations (unpredictable change in DNA)

– Natural selection (differential reproduction)

**certain alleles are favored over others in nature

Microevolution

A change in the gene pool of a population over a succession of generations

Genetic drift: changes in the gene pool of a small population due to chance

-Usually reduces genetic variability (losses of alleles)

-Caused by Bottleneck Effect & Founder Effect

Microevolution

• The Bottleneck Effect: type of genetic drift resulting from a reduction in population (natural disaster) such that the surviving population is no longer genetically representative of the original population

• (Fires, Floods, etc.)• Caused by Chance Events• Humans can have an impact

Bottleneck Effect

• Sudden change in the environment causing a shift in allele frequencies.

• Present Population does not reflect the original population

• Certain alleles become over represented

Ex. Elephant Seal Hunting (1890’s) were limited to just 20 individuals.

**Genetic Variation is reduced

Microevolution

• Founder Effect: 2nd cause of genetic drift attributable to

• Colonization by a limited number of individuals breaking off and becoming isolated from the parent population.

Ex. Retinitis pigmentosa occurrence in

Tritan da Cunha

Microevolution

Gene Flow:

genetic exchange due to the migration of fertile individuals or gametes between populations (reduces differences between populations)

Microevolution

Mutations: A Change in the DNA - source of new alleles - genetic variation - “raw materials of natural selection -unpredictable in nature -Doesn’t determine the direction of

evolution -causes small changes in allele

frequenciesApprox. Mutation rate: One in every

100,000 genes per generation

Microevolution

Nonrandom mating: sexual selection

Mates are chosen according to desired characteristics

(Phenotypic traits)

Microevolution

• Natural Selection: – Adapts a population to its environment – Accumulates and maintains FAVORABLE

genotypes in a population– differential success in reproduction-only form of microevolution that adapts a

population to its environment**recognizing Friends in a crowd.

Natural selection

• Fitness: refers to the contribution an individual makes to the gene pool of the next generation

3 types of Selection:• A. Directional• B. Diversifying• C. Stabilizing

MicroevolutionMicroevolution• SmallSmall genetic changes in a genetic changes in a

populationpopulation• Change in frequency of a Change in frequency of a single single

alleleallele due to due to selectionselection

Three Types of Selection

Three modes of Selection• Stabilizing Selection:

-well adapted to the environment-observed in many plants -selection eliminates extreme phenotypes-intermediate form is favored

• Directional Selection:

-one phenotype extreme is favored

-bell shaped curve is shifted (genetic drift)-Examples: Darwin’s Finches & Peppered moth

• Disruptive Selection: -causes divergence; splitting apart of the extreme phenotypes-extreme traits are favored-intermediate traits become elimanated

Natural Selection in a Population

• Selects only favorable phenotypic traits• Unfavorable alleles are eliminated• Can maintain genetic diversity

-heterozygous advantage (sickle cell anemia) Pg. 399-frequency-dependent selection: rarer phenotypes are

maintained, most common phenotypes eliminated and decrease in number. (Observed in cichlids)• Neutral Variations: offers no selective advantage or

disadvantage Examples ???• Geographical variations and Clines (Clinal variation) **Observed in the common yarrow wildflower in the

Sierra Nevada Mtns. (Pg. 401)

Population Variation

• Polymorphism: coexistence of 2 or more distinct forms of individuals (morphs) within the same population

• Geographical variation: differences in genetic structure between populations (cline)

Preserving Variations in a Population

Prevention of natural selection’s reduction of variation

Diploidy 2nd set of chromosomes hides variation in the heterozygote

Balanced Polymorphism - heterozygote advantage (hybrid vigor; i.e., malaria/sickle-cell anemia); - frequency dependent selection (survival & reproduction of any 1 morph declines if it becomes too common; i.e., parasite/host)

Sexual selection

• Sexual dimorphism: secondary sex characteristic distinction

• Sexual selection: selection towards secondary sex characteristics that leads to sexual dimorphism

Balanced Polymorphism

Two or more alleles persist in a population over many generations.

Preserved by:

• Heterozygote advantage

• Frequency-dependent selection

Ch. 23 Microevolution (Video )1. How are Mosquito populations affected by genetic and

environmental conditions2. What affect did the insecticide have on the insects?3. Which forms of isolation are presented in the last

segment?4. What are some examples of species that are endemic to

Catalina Island? How is inbreeding prevented for these species?

***Write the title for each segment and FIVE statements for each segment.