Evolution of Populations Chapter 16

Gene Pool • The combine genetic information of

a particular population• Contains 2 or more Alleles for each

inheritable trait

Population

• A collection of individuals of the same species in a given area

Relative Frequency • Is the number of times an allele occurs

in a gene pool • Compared with the number of times

other alleles occur

• Example

• Relative Frequency:

• 70% Allele B

• 30% Allele b

Sources of Genetic Variation

• 2 main sources of Genetic Variations are Mutations and Genetic Shuffling that result from sexual reproduction

Mutations: any change in DNA sequence

♦ Can occur because of: ♦mistakes in replication♦ environmental chemicals

♦ May or may not affect an organism’s phenotype

Genetic Shuffling • Occurs during the formation of gametes• Chromosomes move independently during…

23 chromosomes• Can produce 8.4 million different combo’s of

genes• Crossing over can also occur during meiosis

further increasing the possible number of gene variation

Despite gene shuffling, the frequency of alleles does not

change in a population. Explain why this is true.

Similar to a deck of cards – no matter how many times you shuffle, same cards (alleles) are always there.

Gene Traits

• The number of phenotypes produced for a given trait depends on how many genes control that trait

Single Gene Trait • Controlled by a single gene (2alleles)• Examples: widow’s peak, hitchhiker’s

thumb, tongue rolling

Polygenic Trait• Controlled by 2 or more genes can

produce many genotypes and thus many phenotypes

♦Examples: height, hair color, skin color, eye color

Most human traits are polygenic.

Do the following graphs show the distribution of phenotypes for single-gene or polygenic traits? Explain.

• type: single gene

• why? Only two phenotypes possible

• Example: tongue roller or non-tongue roller

• type: polygenic

• why? Multiple (many) phenotypes possible

• Example: height range 4feet to 9 feet all

Natural Selection

• On single gene traits can lead to changes in allele frequency and thus to evolution

• Can effect the distribution of phenotypes in any of 3 ways: directional selection, stabilizing selection, or disruptive selection

Directional Selection• Evolution causes an increase in the

number of individuals with a certain trait at one end of the bell curve

♦ Individuals with highest fitness: those at one end of the curve

♦ Example: Galapagos finches – beak size

Stabilizing Selection• Evolution causes individuals at the center of the

curve to have a higher fitness than those at either end of the curve thus narrowing the curve

– Individuals with highest fitness: near the center of the curve (average phenotype)

– Example: human birth weight

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Birth Weight

Selection against both extremes

keep curve narrow and in same place.

Low mortality, high fitness

High mortality, low fitness

Stabilizing Selection

Disruptive Selection• individuals at both ends of the curve survive better

than the middle of the curve

• Example: birds where seeds are either large or small

Disruptive Selection

Largest and smallest seeds become more common.

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Beak Size

Population splits into two subgroups specializing in different seeds.

Beak Size

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Low mortality, high fitness

High mortality, low fitness

3 Distribution of Natural Selection

Genetic Drift • Random change in allele frequency that

occurs in small populations• Usually in smaller populations where the laws

of probability are less likely to be predicted • In small populations, individuals that carry a

particular allele may leave more descendants than others by chance. Over time, this type of chance occurrence can cause an allele to be more common in a population

a. Founder effect: allele frequencies change due to migration of a small subgroup of a population

b. Example: fruit flies on Hawaiian islands

Two phenomena that result in small populations and cause genetic drift

1.Founder Effect2.Bottleneck Effect

Founder effect

• Allele frequencies change due to migration of a small subgroup of a population

Founder Effect: : Fruit Flies on Hawaiian islands

• Sample of • Original Population

• Founding Population A

• Founding Population B

• Descendants

Bottleneck Effect

major change in allele frequencies when population decreases dramatically due to catastrophe

♦ Example: northern elephant sealsdecreased to 20 individuals in 1800’s, now

30,000no genetic variation in 24 genes

Bottleneck Effect: Northern Elephant Seal Population

♦ Hunted to near extintion♦ Population decreased to♦ 20 individuals in 1800’s,♦ those 20 repopulated so ♦ today’s population is ~30,000♦ No genetic variation in 24 genes

Evolution vs Genetic Equilibrium

• Hardy-Weinberg Principle– States that allele frequencies in a

population will remain constant unless 1 or more factors cause the frequencies to change

• This situation is called Genetic Equilibrium

• If allele freq.’s do not change… the population will not evolve

5 Conditions are required

to maintain genetic equilibrium form

generation to generation:• Random mating• Large Population• No movement into or out of the

population• No Mutation• No Natural Selection• If these conditions are not met then

equilibrium is lost and the population will evolve

The Process of Speciation

I. The formation of new biological species,

II. Caused by natural selection and chance events that cause a change in relative allele frequencies

III. As new species evolve, they become reproductively isolated from one another

Reproductive Isolation

• Is when 2 species evolve so that they cannot interbreed

• Populations now have separate gene pools

• Develop by Behavioral Isolation, Geographic Isolation, and Temporal Isolation

Behavioral Isolation

• Two populations are capable of interbreeding but do not interbreed because they have different ‘courtship rituals’ or other lifestyle habits that differ.

Example: Eastern and Western Meadowlark

• Eastern and Western Meadowlark

populations overlap in the middle of the US

Geographic Isolation

• Two populations separated by a geographic barrier; river, lake, canyon, mountain range.

Temporal Isolation

• Populations reproduce at different times

January

1 2 3 4 5 6

7 8 9 10 11 12 13

• Speciation in the Galapagos islands finches occurred by founding of a new population, geographical isolation, changes in the new pop.’s gene pool, reproductive isolation, and ecological competition