Lecture #10 Date ________

• Chapter 23~ The Evolution of Populations

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 Theorem• Serves as a model for the

genetic structure of a nonevolving population (equilibrium)

• 5 conditions:• 1- Very large population

size;• 2- No migration;• 3- No net mutations;• 4- Random mating;• 5- No natural selection

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 plus aA genotype; q2=frequency of aa genotype; p2 + 2pq + q2 = 1.0

Problem 1 The allele for black coat is recessive. We can use the Hardy-Weinberg equation to determine the percent of the pig population that is heterozygous for white coat.1. Calculate q2 :Count the individuals that

are homozygous recessive in the illustration above. Calculate the percent of the total population they represent. This is q2.

2. Find q. Take the square root of q2 to obtain q, the frequency of the recessive allele.

3. Find p. The sum of the frequencies of both alleles = 100%, p + q = l. You know q, so what is p?

4. Find 2pq. The frequency of the heterozygotes is represented by 2pq. This gives you the percent of the population that is heterozygous for white coat:

Sample Problem 2

In a certain population of 1000 fruit flies, 640 have red eyes while the remainder have sepia eyes. The sepia eye trait is recessive to red eyes. How many individuals would you expect to be homozygous for red eye color?

Answer:160

Sample Problem 3• If 9% of an African population is

born with a severe form of sickle-cell anemia (ss), what percentage of the population will be more resistant to malaria because they are heterozygous(Ss) for the sickle-cell gene?

• 42% are heterozygous

Microevolution, I• A change in the gene

pool of a population over a succession of generations

• 1- Genetic drift: changes in the gene pool of a small population due to chance (usually reduces genetic variability)

Microevolution, II• 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, III

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

Microevolution, IV• 2- Gene Flow:

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

Microevolution, V• 3- Mutations:

a change in an organism’s DNA (gametes; many generations); original source of genetic variation (raw material for natural selection)

Microevolution, VI• 4- Nonrandom

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

Microevolution, VII• 5- Natural

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

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)

Variation preservation• Prevention of natural

selection’s reduction of variation

• Diploidy 2nd set of chromosomes hides variation in the heterozygote

• Balanced polymorphism 1- heterozygote advantage (hybrid vigor; i.e., malaria/sickle-cell anemia); 2- frequency dependent selection (survival & reproduction of any 1 morph declines if it becomes too common; i.e., parasite/host)

Natural selection• Fitness:

contribution an individual makes to the gene pool of the next generation

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

Sexual selection• Sexual dimorphism:

secondary sex characteristic distinction

• Intersexual and Intrasexual selection

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