Chapter 16

Evolution of Populations

Ch. 16 Evolution of Populations

• 16–1Genes and VariationA. How Common is Genetic Variation?

– Most genes have two forms.– Many genes have multiple alleles.– Many traits are polygenic.

B. Variation and Gene Pools• Gene pools have frequency of certain traits.• This frequency changes over time.

– This increases or decreases the variation in the population

Ch. 16 Evolution of Populations

C. Sources of Genetic Variation1. Mutations

• http://www.koshlandsciencemuseum.org/exhibitdna/crops04activity.jsp

2. Gene Shuffling• Independent assortment of

chromosomes during meiosis.

D. Single-Gene and Polygenic Traits– 2 phenotypes vs. many phenotypes

Click here to go to next outline

Ch. 16 Evolution of Populations

Fre

qu

en

cy

O

f p

he

no

typ

e

Phenotype (height)

Polygenic

Traits will make a bell curve when graphed

Click here to return to outline-

Ch. 16 Evolution of Populations

• A single gene trait makes a bar graph with two phenotypes

Click here to return to outline

Ch. 16 Evolution of Populations

• 16–2 Evolution as Genetic ChangeA. Natural Selection on Single-Gene

Traits– Natural selection on single-gene

traits can lead to changes in allele frequencies and thus to evolution.

• Run this simulation of the perrered moth evolution.

http://www.biologycorner.com/worksheets/pepperedmoth.html

Ch. 16 Evolution of Populations

B. Natural Selection on Polygenic Traits

Natural selection can affect the distributions of phenotypes in any of three ways 1. Directional Selection

2. Stabilizing Selection

3. Disruptive Selection

Ch. 16 Evolution of Populations

– C. Genetic Drift

Sample of Original Population

Founding Population A

Founding Population B

Descendants

Click here to continue with outline

Ch. 16 Evolution of Populations

• Click to return to outline

Low mortality, high fitness

High mortality, low fitness

Directional Selection

Ch. 16 Evolution of Populations

• Click here to return to outline

Selection against both

extremes keep curve narrow and in same

place.

Low mortality, high fitness

High mortality, low fitness

Stabilizing Selection

Selection against both

extremes keep curve narrow and in same

place.

Low mortality, high fitness

High mortality, low fitness

Stabilizing Selection

Ch. 16 Evolution of Populations

• Click here to continue with outline

Disruptive Selection

Largest and smallest seeds become more common.

Beak Size

Population splits into two subgroups specializing in different seeds.

Beak Size

Low mortality, high fitness

High mortality, low fitness

Disruptive Selection

Largest and smallest seeds become more common.

Beak Size

Population splits into two subgroups specializing in different seeds.

Beak Size

Low mortality, high fitness

High mortality, low fitness

Ch. 16 Evolution of Populations

D. Evolution Versus Genetic Equilibrium

The Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change.

In other words, as long as everything stays the same, evolution will not happen.

Ch. 16 Evolution of Populations

Genetic Equilibrium Five conditions are required to maintain

genetic equilibrium from generation to generation

1. Random Mating2. Large Population3. No Movement Into or Out of the

Population4. No Mutations5. No Natural Selection

Ch. 16 Evolution of Populations

16–3 The Process of Speciation-how do species evolve?A. Isolating Mechanisms-ALL types of

isolation result in Reproductive Isolation.1.Behavioral Isolation-have differences in courtship

rituals

2.Geographic Isolation-are separated by geographic barriers such as rivers, mountains, or bodies of water.

3.Temporal Isolation-reproduce at different times.

Ch. 16 Evolution of Populations

B. Testing Natural Selection in Nature

Examples of Evolution observed in Nature• Peppered Moth• Antibiotic resistance in bacteria• Human adults who can digest milk• Gypsy Moth• Insects able to survive insecticides• Plants able to survive herbicides• Darwin’s finches-see next page• Sickle cell trait in humans

Ch. 16 Evolution of Populations

C. Speciation in Darwin’s Finches1. Founders Arrive-birds arrive from S. America

perhaps blown there by a storm2. Separation of Populations-The small population is

separated on different islands.3. Changes in the Gene Pool-Natural variation due to

mutation occurs4. Reproductive Isolation-Birds are isolated by

behavior and geography5. Ecological Competition-Each island has specific

foods in limited amounts6.Continued Evolution-all of these drive changes in

the population (evolution) which is documented even today

Ch. 16 Evolution of PopulationsD. Studying Evolution Since Darwin• Advances in genetics, molecular biology,

ecology, physics and chemistry increase the evidence that species change over time and continue to change.

E. Limitations on Research• Evidence of change exists. But no one has yet

witnessed the formation of a brand new species.• Many new discoveries have led to new

hypotheses that refine and expand Darwin's original ideas. Even Darwin has evolved.

• There are still many unanswered questions.