Chapter 18

• Chapter 18 The Evolution of Populations

Introductory Questions #11) Define what a gene pool is. 2) What are the three aspects in a population we

examine in order to understand how evolution is occurring in a population.

3) If a population had 2500 individuals that are diploid, how many total alleles would be present?

4) In a population of 1000 humans, 840 possess the ability to roll their tongues (dominant trait) and 160 cannot. Determine the frequency of the dominant and recessive alleles in the population.

5) What is happening if the population is in “genetic equilibrium”

6) What is the significance of the Hardy Weinberg principle?

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• 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

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)

Introductory Questions #11) Define what a gene pool is. 2) What are the three aspects in a population we

examine in order to understand how evolution is occurring in a population.

3) If a population had 2500 individuals that are diploid, how many total alleles would be present?

4) In a population of 1000 humans, 840 possess the ability to roll their tongues (dominant trait) and 160 cannot. Determine the frequency of the dominant and recessive alleles in the population.

5) What is happening if the population is in “genetic equilibrium”

6) What is the significance of the Hardy Weinberg principle?

Introductory Questions #21) How can allele frequencies change in a population and

increase variation? Give three examples. What do we call this when this is happening?

2) Does natural selection operate directly on the phenotype or genotype of organisms? Briefly explain your choice.

3) Name the three modes of selection. Explain how each mode is different and draw a graph representing each mode.

4) Define what genetic polymorphism is and why balanced polymorphism is unique. Give the two mechanisms observed for balanced polymorphism.

Microevolution

• Involves small or minor changes in the allele frequencies within a population

• Five processes have been identified:– Nonrandom mating (inbreeding & assortative mating)– Gene flow (migration between populations) – Genetic drift (bottleneck 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)

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

Microevolution

• Founder Effect: a cause of genetic drift attributable to colonization by a limited number of individuals from a parent population

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

frequencies

Microevolution

Nonrandom mating: inbreeding and assortative mating (both shift frequencies of different genotypes)

Mates are chosen according to desired charachterisitics

Microevolution

• Natural Selection: differential success in reproduction; only form of microevolution that adapts a population to its environment

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

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• Neutral Variations: offers no selective advantage or

disadvantage Examples ???• Geographical variations and Clines (Clinal variation) 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