Chapter 23 – Population GeneticsThe Evolution of Populations

(Outline)• The smallest unit of evolution

• It is the population, not the individual, that evolves.

23.6.Caribou populations in the Yukon. One species, two populations.

These two populations are not totally isolated.Yet, individuals of population I are more likely to breed with members of their own population than with members of population II and are thus more closely related to one another than to members of the other population.

Chapter 23 – Population GeneticsThe Evolution of Populations

(Outline)• The smallest unit of evolution

• It is the population, not the individual, that evolves.

• Gene pool, allele frequencies, Hardy-Weinberg equilibrium

• Conditions that must be met for a population to remain in Hardy-Weinberg equilibrium.

• A closer look at natural selection.

• Directional, disruptive and stabilizing selection

Can new alleles be formed?

• Mutation: A change in the nucleotide sequence of an organism’s DNA.

• Only changes in cells that produce gametes can be passed on to offspring.

• Even a change in one base pair (a “point” mutation) can impact the phenotype of an organism (f.e. sickle-cell anemia)

• Mutations are rare (about 1 mutation in 100,000 genes per generation) and are commonly lethal to the organism.

• Mutations and sexual reproduction produce genetic variation.

How do we measure genetic variation? Computing allele frequencies.Population with N individuals (thus 2N allelles) of red-flowered (AA, Aa) and white flowered (aa) plants.

# of AA = X# of Aa = Y X + Y + Z = N# of aa = Z

Each AA individual has 2 A allelesEach Aa individual has 1 A allele total # of A alleles = 2X + Y

Similarly, the total number of a alleles = 2Z + Y

Since the total number of alleles = 2N,

the frequency of the A allele = p = (2X + Y)/ 2Nthe frequency of the a allele = q = (2Z + Y)/ 2N

If the allele frequencies are the same in two populations, are the genotypic and phenotypic frequencies also the same?Two populations with each 200 individuals

Pop 1: 90AA, 40Aa, 70aaPop 2: 45AA, 130Aa, 25aa

the frequency of the A allele = p = (2X + Y)/ 2Nthe frequency of the a allele = q = (2Z + Y)/ 2N

Population 1: p = (2X + Y)/ 2N = (180 + 40)/400 = 0.55q = (2Z + Y)/ 2N = (140 + 40)/400 = 0.45

Population 2: p = (90 + 130)/400 = 0.55q = (50 + 130)/400 = 0.45

Allele frequencies are the same (same gene pool), but alleles are distributed differently among genotypes (different genotypic and phenotypic structure)

Mendelian inheritance preserves genetic variation from one generation to the next.

(see book page 473 for a discussion of a similar example).No matter how many generations of alleles are segregated (by meiosis) and combined (by fertilization) the allele frequency in the gene pool will remain constant unless acted upon by “selective” forces.

23.8.Hardy-Weinberg principle.

The gene pool remains constant from one generation to the next. Mendelian processes alone do not alter allele frequencies

The probability of generating an CRCR genotype isp2=0.8 X 0.8 = 0.64 The probability of generating an CWCW genotype isq2=0.2 X 0.2 = 0.04 The probability of generating an CRCW genotype is2pq= 2 X 0.8 X 0.2 = 0.32 P2 + 2pq + q2 = 1

How does this apply to the human condition?(see also book page 474)

For example: Calculate % of human population that carries an allele for a particular inherited disease(i.e., is heterozygous for this allele)Phenylketon uria (PKU), inherited disorder that may lead to mental retardation caused by improper metabolic processing of phenylanaline (essential aminoacid not metabolized due to a missing enzyme). One in 10,000 babies are born with PKUDue to a recessive allele so frequency of individuals in the U.S. born with PKU = q2.

q2 = 0.0001 q = √0.0001 = 0.01. Therefore p = 0.99 Frequency of carriers (that may pass the allele on to offspring) = 2pq = 2 x 0.99 x 0.01 = 0.0198 orapprox 2% of the U.S. population.

Hardy-Weinberg Assumptions

• 1.     No mutations (because they alter the gene pool)

• 2.     Population is isolated, i.e. no migration of individuals into or out of the population (no gene flow)

• 3.     Population must be very large and made up of sexually reproducing diploid individuals (small populations show genetic drift)

• 4.     Mating is random (gametes mix randomly)

• 5.     All individuals must survive and reproduce equally well (no natural selection)

A population that followsthe Hardy-Weinberg rule is

non-evolving!

In a Hardy‑Weinberg population, the frequency of the a allele is 0.4.What is the frequency of individuals with Aa genotype?

A. 0.20

B. 0.48

C. 0.60

D. 0.16

E. approximately 1.0

Hardy-Weinberg Assumptions

• 1.     No mutations (because they alter the gene pool)

• 2.     Population is isolated, i.e. no migration of individuals into or out of the population (no gene flow)

• 3.     Population must be very large and made up of sexually reproducing diploid individuals (small populations show genetic drift)

23.9. Genetic drift

23.8. Genetic drift

23.8. Genetic drift

23.10.The bottleneck effect: an analogy

Cheetahs, the bottleneck effect

23.11. Bottleneck effect and reduction of genetic variation.

The Illinois population of prairie chickens dropped from millions of birds in the 1800s to just 50 birds in 1993 (habitat loss due to the conversion of native tallgrass prairies to cropland).

Consequently, as a result of genetic drift, both the number of alleles per locus (mean across six loci studied) and the percentage of eggs that hatched decreased.

Hardy-Weinberg Assumptions

• 1.     No mutations (because they alter the gene pool)

• 2.     Population is isolated, i.e. no migration of individuals into or out of the population (no gene flow)

• 3.     Population must be very large and made up of sexually reproducing diploid individuals (small populations show gene drift)

• 4.     Mating is random (gametes mix randomly)

• 5.     All individuals must survive and reproduce equally well (no natural selection)

A nonheritable difference within a population

Spring

Late Summer

European Map Butterflies

Seasonal differences in coloration are due to hormones and not to genetic differences.

Polymorphism: Genetic variation within populations. (somewhat stable frequencies of two or more discrete phenotypes in a population, maintained by selection)

Genetic variation within a population may also be maintainedby geographical variation.Cline: Gradual changes in some trait over a geographical area.

23.5. A cline determined by temperature (individuals with the Ldh-Bb allele can swim faster in cold water than can individuals with other alleles.The Ldh-Bb allele codes for the production of an enzyme that is an excellent metabolic catalyst in cold waters.

Balanced polymorphism: Genetic variation within a population may also be maintained when the heterozygote is at an advantage.

Normal and sickled cells: Natural selection maintains two or more alleles at the same locus

23.17. Mapping malaria and the sickle-cell allele

23.13. Modes of selection

Stabilizing selection for beak size in a Galápagos population of the medium ground finch (Grant and Grant)

Disruptive selection in a finch population

Smaller-billed birds feed more efficiently on small seeds.

Larger-billed birds can crack hard seeds.

Chapter 23 Review (p.485-486)

• 23.1. Mutation and sexual reproduction produce the genetic variation that makes evolution possible.

• 23.2. The Hardy-Weinberg equation can be used to test whether a population is evolving.

• 23.3. Natural selection, genetic drift, and gene flow can alter allele frequencies in a population.

• 23.4. Natural selection is the only mechanism that consistently causes adaptive evolution.

In a West African finch species, birds with large and small bills survive better than birds with intermediate‑sized bills. The type of natural selection operating on these bird populations is

A. directional selection.

B. deme selection.

C. stabililizing selection.

D. nonrandom selection.

E. disruptive selection.