Chapter 16:Chapter 16: Evolution Evolution

of of PopulationsPopulations

1.1. When Darwin developed his theory of When Darwin developed his theory of evolution, he did not understand:evolution, he did not understand:

• how heredity worked.how heredity worked.

This left him unable to explain two This left him unable to explain two things:things:

a.a. source of variationsource of variation

b. how inheritable traits pass b. how inheritable traits pass from one generation to from one generation to

the nextthe next

In the 1940’s, Mendel’s work on genetics In the 1940’s, Mendel’s work on genetics was “rediscovered” and scientists began was “rediscovered” and scientists began to combine the ideas of many branches of to combine the ideas of many branches of biology to develop a modern theory of biology to develop a modern theory of evolution. When studying evolution evolution. When studying evolution today, biologists often focus on a today, biologists often focus on a particular particular populationpopulation.. This evolution of This evolution of populations is called populations is called microevolutionmicroevolution. .

2. Vocabulary: 2. Vocabulary:

population: population: group of individuals of the group of individuals of the same species living in the same area that same species living in the same area that breed with each other. breed with each other.

2. gene pool: 2. gene pool: combined genetic info. combined genetic info. for all members of a populationfor all members of a population

2. allele: 2. allele: one form of a geneone form of a gene

2. relative frequency of an allele: 2. relative frequency of an allele: # # times an allele occurs in the gene pool times an allele occurs in the gene pool compared to other alleles (percent)compared to other alleles (percent)

Example

Relative Frequency:

70% Allele B

30% Allele b

3. 3. Sources of VariationSources of Variation: :

a. a. mutationsmutations: : any change in DNA sequenceany change in DNA sequence

♦ Can occur because of: Can occur because of:

♦mistakes in replicationmistakes in replication

♦ environmental chemicalsenvironmental chemicals

♦ May or may not affect an organism’s May or may not affect an organism’s phenotypephenotype

3. Sources of Variation3. Sources of Variationb. b. Gene ShufflingGene Shuffling: : recombination of genes recombination of genes that occurs during production of gametesthat occurs during production of gametes♦ Cause most Cause most inheritableinheritable differences between differences between

relativesrelatives♦ Occurs during Occurs during meiosismeiosis♦ As a result, As a result, sexualsexual reproduction is a major reproduction is a major

source of variation in organisms.source of variation in organisms.♦ Despite gene shuffling, the frequency of alleles Despite gene shuffling, the frequency of alleles

does not change in a population. does not change in a population. Explain why Explain why this is truethis is true..

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

4. Gene Traits: 4. Gene Traits:

A) Single gene trait:A) Single gene trait: controlled by single controlled by single gene with two allelesgene with two alleles

♦ ExamplesExamples: widow’s peak, hitchhiker’s thumb, : widow’s peak, hitchhiker’s thumb, tongue rollingtongue rolling

(4. Gene Traits:)(4. Gene Traits:)

B) Polygenic trait: B) Polygenic trait: controlled by 2 or more controlled by 2 or more genes, each with 2 or more allelesgenes, each with 2 or more alleles

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

Most human traits are Most human traits are polygenic.polygenic.

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

5. Natural selection acts on 5. Natural selection acts on phenotypesphenotypes, not , not genotypesgenotypes. .

Example: in a forest covered in brown leaves, Example: in a forest covered in brown leaves, dirt and rocks which mouse will survive better dirt and rocks which mouse will survive better brown or white? brown or white?

Brown, more hidden. Brown, more hidden.

5. If brown is dominant can the a predator tell the difference 5. If brown is dominant can the a predator tell the difference between: between:

Mouse with highest fitness will have the most Mouse with highest fitness will have the most allelesalleles passed on to passed on to the next generation.the next generation.

White mouse will have low fitnessWhite mouse will have low fitness

BB Bb

?

5. Which mouse will have the lowest fitness? 5. Which mouse will have the lowest fitness?

White, bb (recessive)White, bb (recessive)

Will the fitness of BB and Bb differ? Why?Will the fitness of BB and Bb differ? Why?

No, Both BB and Bb have the same fitness advantage of being No, Both BB and Bb have the same fitness advantage of being brownbrown

BB Bb

?

6. Three ways in which 6. Three ways in which natural selection affects natural selection affects

polygenic traits.polygenic traits.

a. a. Directional SelectionDirectional Selection: : individuals at one individuals at one end of the curve have higher fitness so end of the curve have higher fitness so evolution causes increase in individuals evolution causes increase in individuals with that traitwith that trait

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

♦ Example: Example: Galapagos finches – beak sizeGalapagos finches – beak size

Food becomes scarce.

KeyLow mortality, high fitness

High mortality, low fitness

Directional Selection (page 398)Directional Selection (page 398)

Food becomes scarce.

Key

Low mortality, high fitness

High mortality, low fitness

Directional Selection

b. Stabilizing Selection: b. Stabilizing Selection: individuals at the individuals at the center of the curve have highest fitness; center of the curve have highest fitness; evolution keeps center in the same position evolution keeps center in the same position but narrows the curvebut narrows the curve

Key

Per

cen

tag

e o

f P

op

ula

tio

n

Birth Weight

Selection against both

extremes keep curve narrow and

in same place.

Low mortality, high fitnessHigh mortality, low fitness

Stabilizing Selection

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

Example: Example: human birth human birth weightweight

Key

Per

cen

tag

e o

f P

op

ula

tio

n

Birth Weight

Selection against both

extremes keep curve narrow and in same

place.

Low mortality, high fitness

High mortality, low fitness

Stabilizing Selection

Stabilizing Selection

c. c. Disruptive SelectionDisruptive Selection: : individuals at individuals at both ends of the curve survive better both ends of the curve survive better than the middle of the curve. than the middle of the curve.

♦ Individuals with highest fitness: Individuals with highest fitness: both ends both ends of curveof curve

♦ Example: Example: birds where seeds are either birds where seeds are either large or smalllarge or small

Disruptive Selection

Largest and smallest seeds become more common.

Nu

mb

er o

f B

ird

sin

Po

pu

lati

on

Beak Size

Population splits into two subgroups specializing in different seeds.

Beak Size

Nu

mb

er o

f B

ird

sin

Po

pu

lati

onKey

Low mortality, high fitness

High mortality, low fitness

Disruptive Selection (pg 399)Disruptive Selection (pg 399)

Disruptive Selection

Largest and smallest seeds become more common.

Nu

mb

er o

f B

ird

sin

Po

pu

lati

on

Beak Size

Population splits into two subgroups specializing in different seeds.

Beak Size

Nu

mb

er o

f B

ird

sin

Po

pu

lati

onKey

Low mortality, high fitness

High mortality, low fitness

Quiz Monday!!Quiz Monday!!

•Evolution review ½ sheet. Evolution review ½ sheet. (yes some questions are (yes some questions are missing)missing)

•Thursday and Friday’s Thursday and Friday’s concepts will be on the quiz:concepts will be on the quiz:–Directional, Stabilizing and Directional, Stabilizing and Disruptive selection.Disruptive selection.

–Geographic, Behavioral, Geographic, Behavioral, Temporal IsolationTemporal Isolation

–Small populations caused by Small populations caused by bottleneck and founder effectbottleneck and founder effect

The Process of The Process of SpeciationSpeciation

•The formation of new The formation of new biological species, usually biological species, usually by the division of a single by the division of a single species into two or more species into two or more genetically distinct one. genetically distinct one.

Three Isolating Three Isolating MechanismsMechanisms: :

Isolate species forming Isolate species forming subspecies and perhaps subspecies and perhaps

causing speciation. causing speciation.

1.1.Geographic Geographic IsolationIsolation

2.2.Behavioral IsolationBehavioral Isolation

3.3.Temporal IsolationTemporal Isolation

1. Geographic 1. Geographic IsolationIsolation•Two populations Two populations

separated by a separated by a geographic geographic barrierbarrier; ; river, lake, canyon, river, lake, canyon, mountain range.mountain range.

Example: 10,000 years Example: 10,000 years ago the ago the Colorado RiverColorado River separated two squirrel separated two squirrel populations. populations.

•Kaibab Squirrel Kaibab Squirrel Abert SquirrelAbert Squirrel

This resulted in a This resulted in a subspecies, but subspecies, but did notdid not result in speciation result in speciation because the two can still because the two can still matemate if brought if brought togethertogether

•Kaibab Squirrel Kaibab Squirrel Abert SquirrelAbert Squirrel

2. 2. Behavioral Behavioral IsolationIsolation

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

Example: Eastern and Example: Eastern and Western Meadowlark Western Meadowlark

•Eastern and Western Eastern and Western Meadowlark populations Meadowlark populations

overlapoverlap in the in the middle of the USmiddle of the US

Example: Eastern and Western Example: Eastern and Western Meadowlark Meadowlark

•Male birds sing a Male birds sing a

matting songmatting song that that females like, East and females like, East and West have different songs. West have different songs. Females only respond to Females only respond to their subspecies song. their subspecies song.

3. Temporal 3. Temporal IsolationIsolation

Populations reproduce Populations reproduce at different timesat different times

January

1 2 3 4 5 6

7 8 9 10 11 12 13

Example: Northern Leopard Example: Northern Leopard Frog Frog

& North American Bullfrog & North American Bullfrog •Mates in: Mates in:

Mates in: Mates in:

AprilApril JulyJuly

ConclusionConclusion::•Geographic, Behavioral Geographic, Behavioral and Temporal Isolation and Temporal Isolation are all believed to lead are all believed to lead

to to speciation.speciation.

However:However:–No examples ever No examples ever

observed in observed in animalsanimals–A couple examples that A couple examples that may demonstrate may demonstrate speciation exist in speciation exist in plants plants and some insectsand some insects. .

Genetic DriftGenetic Drift• random change in allele random change in allele

frequency that occurs in small frequency that occurs in small populationspopulations

The results of genetic crosses can The results of genetic crosses can usually be predicted using the laws of usually be predicted using the laws of

probabilityprobability. In . In smallsmall populations, however, these populations, however, these predictions are not always accurate.predictions are not always accurate.a. a. Founder effect: Founder effect: allele frequencies change due to allele frequencies change due to migration of a small subgroup of a populationmigration of a small subgroup of a population

♦ Example: Example: fruit flies on Hawaiian islandsfruit flies on Hawaiian islands

Two phenomena that Two phenomena that result result in small populations in small populations and cause genetic and cause genetic

driftdrift1.1.Founder EffectFounder Effect

2.2.Bottleneck Bottleneck EffectEffect

Founder effectFounder effect

allele frequencies change due allele frequencies change due to migration of a small to migration of a small subgroup of a populationsubgroup of a population

Sample of Original Population

Founding Population A

Founding Population B

Descendants

Founder Effect: : Fruit Fruit Flies on Hawaiian Flies on Hawaiian

islandsislands

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

♦ Example: Example: northern elephant sealsnorthern elephant seals

decreased to 20 individuals in 1800’s, now 30,000decreased to 20 individuals in 1800’s, now 30,000

no genetic variation in 24 genesno genetic variation in 24 genes

2. Bottleneck effect2. Bottleneck effect

Bottleneck Effect: Northern Bottleneck Effect: Northern Elephant Seal PopulationElephant Seal Population

♦ Hunted to near extintionHunted to near extintion

♦ Population decreased to Population decreased to 20 individuals in 1800’s, 20 individuals in 1800’s, those 20 repopulated so those 20 repopulated so today’s population is today’s population is ~30,000~30,000

♦ No genetic variation in 24 No genetic variation in 24 genesgenes

Bottleneck EffectBottleneck Effect

Original population

Bottleneck EffectBottleneck Effect

Original population

Catastrophe

Bottleneck EffectBottleneck Effect

Original population

Catastrophe

Surviving population

Another picture to Another picture to illustrate bottleneck effect illustrate bottleneck effect