Chapter 16: Evolution of Populations

HOMEWORK

• Chapter 16 Section Assessments: Due Fri. 5/2

• Chapter 16.1 SA: p. 396 (1-5)• Chapter 16.2 SA: p. 402 (1-5)• Chapter 16.3 SA: p. 410 (1-2)• Chapter 16 Assessment: p. 413 (1-10, 12,

17, 18, 19)

VOCABULARY (DUE FRI. 5/2)Provide the glossary definition and etymology for the following terms: • Gene pool• Relative Frequency• Single-gene trait• Polygenic trait• Directional selection• Stabilizing selection• Disruptive selection• Genetic drift• Founder effect• Hardy-Weinberg

principle• Genetic equilibrium• Speciation• Reproductive

isolation• Behavioral isolation• Geographic isolation• Temporal isolation

16.1: Genes and Variation• A weakness in Darwin’s theory

stemmed from his lack of understanding in genetic variation.• In the 1930s, evolutionary biologists

combined Mendel’s work with that of Darwin’s to support evolutionary change, specifically the process of natural selection.• How do you think Mendel would have

felt about that?

16.1 Variation and Gene Pools• Population: a group of individuals of the same

species that interbreed.

• Gene pool: consists of all genes, including all the different alleles, that are present in a population.

• Relative frequency: the number of times that the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur. (Fig. 16-2)

• Key Concept: In genetic terms, evolution is any change in the relative frequency of alleles in a population.

16.1 Sources of Genetic Variation• Key Concept: The two main sources

of genetic variation are mutations and the genetic shuffling that results from sexual reproduction. • Remember mutations are changes in

the genetic code. Some are expressed, some remain silent. Some mutations can affect an organism’s fitness, while others have not effect. • Crossing over and independent

assortment during gamete formation leads to genetic variation. (23 pairs of chromosomes can have 8.4 million different combinations of genes!)

16.2 Evolution as Genetic Change

16.2 Natural Selection on Single-Gene Traits

• Key Concept: Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. (Fig. 16-5)• Remember: Evolution is change over

time in the relative frequencies of alleles in a population, therefore it is populations that can evolve, not individual organisms.

16.2 Natural Selection on Polygenic Traits

• Key Concept: Natural selection can affect the distributions of phenotypes in any of three ways: • Directional Selection: when individuals

at one end of the curve have higher fitness than individuals in the middle or at the other end. (Fig. 16-6)• Stabilizing Selection: center curve has

higher fitness (Fig. 16-7)• Disruptive Selection: upper and lower

ends have high fitness (Fig. 16-8)

Natural Selection and Normal Distribution

Series1

Directional Selection

Beak Size

# o

f bir

ds

Series1

Stabilizing Selection

Birth Mass

# o

f bir

ds

Series1

Disruptive Selection

Beak Size

# o

f bir

ds

One extreme favored Average favored

Both extremes favored

16.2 Genetic Drift• Key Concept: In small populations,

individuals that carry a particular allele may leave more descendants that other individuals, just by chance. Over time, a series of chance occurrences of this type can cause an allele to become common in a population. (Fig. 16-9)• Genetic Drift: Random change in

allele frequencies that occurs in small populations• Founder Effect: A situation in which

allele frequencies change as a result of the migration of a small subgroup of a population

16.2 Hardy-Weinberg and Genetic Equilibrium

• Genetic Equilibrium: A situation in which allele frequencies remain constant.• Key Concept: Five conditions are required to maintain genetic equilibrium from generation to generation:1. Random mating2. Large population size3. No movement into or out of the

population4. No mutations5. No natural selection

16.2 Hardy-Weinberg Equation (p. 401-402)

• p + q = 1• p = dominant allele frequency

(A)• q = recessive allele frequency (a)

• p2 + 2pq + q2 = 1• p2: frequency of AA homozygous• 2pq: frequency of Aa heterozygous• q2: frequency of aa homozygous• 1: sum of frequencies for all

genotypes (100%)

• Extra Credit Assignment!! Due Fri. 5/2

Bio Warm-Up April 28, 2014 (Do Not Copy)

• List and describe three types of natural selection that occur in a population. Draw a distribution curve for the population before and after selection.

• The inability to taste PTC paper (t) is recessive to being able to taste it (T). At UC Academy, 135 out of the 450 students are unable to taste PTC paper. Calculate the frequency for the following:• homozygous dominant individuals• heterozygous individuals• homozygous recessive individuals• frequencies of the T and t alleles in our school population.

Bio Warm-Up April 29, 2014 (Do Not Copy)

1. Having mid-digital hair (hair on the skin of the second bone in your finger) is dominant to not having hair there. At Chaffey College, 7350 out of the 15,000 students have no mid-digital hair. Calculate the frequency of the following individuals who are: • homozygous recessive• heterozygous• homozygous dominant• dominant mid-digital hair allele (H) • recessive no mid-digital hair allele (h).

2. When individuals at only one end of a bell-shaped curve of phenotype frequencies have high fitness, the result is ___________ selection.

16.3 The Process of Speciation

16.3 Isolating Mechanisms• Speciation: formation of new species• Key Concept: As new species evolve,

populations become reproductively isolated from each other.• Reproductive Isolation: When

members of two populations cannot interbreed and produce fertile offspring. They now have separate gene pools.

16.3 Isolating Mechanisms

• Behavioral Isolation: capable of interbreeding, but have different courtship rituals or other reproductive behaviors.• Geographic Isolation: separated by physical barriers (i.e. rivers, mountains, etc…)• Temporal Isolation: reproduce at different times.Is the definition of species

constant? “Can vs. Will”

Causes of reproductive isolation:

16.3 Testing Natural Selection in Nature

• Peter and Rosemary Grant have spent more than 20 years observing collecting data on Galapagos finches.•They found variation in physical characteristics.•They observed natural selection occurring during drought seasons.

16.3 Speciation in Darwin’s FinchesKey Concept: Speciation in the Galapagos finches occurred by founding of a new population, geographic isolation, changes in the new population’s gene pool, reproductive isolation, and ecological competition.

16.3 Studying Evolution Since Darwin

• Read: Unanswered Questions (p. 410)• “New data from genetics, physics,

and biochemistry could have proved him wrong on many counts. They didn’t.”• “…while the Grants observed

changes in the size of the finches’ beaks, they did not observe the formation of a new species.”• Why is understanding evolution

important?