Mr. Bragg 2013-2014

Chapter 17: Evolution of Populations

17.1 Essential question: How do

genes make evolution possible?

• Explain how evolution is defined in genetic terms•Identify the main sources of genetic variation within a population•State what determines the number of phenotypes for a trait

I. 17.1 Genes and Variation

A. Genotype and Phenotype in evolution1. How does it work?

a. Some individuals within a population have phenotypes that make them more fit

b. Organisms with higher fitness pass on more copies of their genes to the next generation

B. Populations and Gene Pools

1. Gene Pool – consists of all of the genes, including all of the different alleles for each gene, that are present in a population

2. Allele frequency – the number of times an allele occurs in a gene pool, compared to the total number of alleles in that pool for that same gene.

a. Example: % of each color of beetle in a beetle population

C. Sources of Genetic Variation

1. Mutationsa. Mutations are genetic changesb. Most mutations are neutral in their affectc. Must be passed on to the next generation to

play a role

2. Sexual reproductiona. Genes from two parents fuse in new waysb. Crossing over during Meiosis allows for new

combinations as wellc. Doesn’t affect allele frequencies, more like

shuffling of a deck of cards

3. Gene Flow

a. Gene flow—also called migration—is any movement of genes from one population to another. b. Many ways it can happen;• pollen being blown to a new destination• people moving to new cities or countries

c. If genes are carried to a population where those genes previously did not exist, gene flow can be a very important source of genetic variation.

4. Lateral gene transfera. defined: the passing of genes to an organism

that is not an offspringb. Useful for the evolution of single celled

organisms

D. Single Gene and Polygenic Traits

1. Single gene traits - a trait that is controlled by just one gene

a. may have just two or three phenotypes

b. Ex: In some snails, a single gene controls stripes on the shell

– there are only two possible phenotypes- stripes or no stripes

2. Polygenic trait - a trait that is controlled by two or more genes

a. Can have many genotypes and phenotypes

b. Ex: human height

Comparison

c. When graphed, polygenic traits have a bell shaped curve

17.1 Essential question: How do

genes make evolution possible?

• Explain how evolution is defined in genetic terms•Identify the main sources of genetic variation within a population•State what determines the number of phenotypes for a trait

Follow-up questions:

1. How is evolution defined in genetic terms?

2. What are the sources for genetic variation?

3. What determines the number of phenotypes for a given trait?

17.2 Essential question: What causes a

population’s gene pool to change?

• Explain how natural selection affects single gene and polygenic traits•Describe genetic drift•Explain how different factors affect genetic equilibrium

II. 17.2 Evolution as Genetic Change in Populations

A. How Natural Selection Works1. Natural selection on single gene traits can

lead to changes in allele frequenciesa. When one trait is favored over another, the

allele favored becomes more common over time

b. This can lead to changes in phenotypic frequencies

b. Ex: Hypothetical Brown Lizard population» a single gene that controls color mutates and creates two

new alleles - red and black» Red lizards are conspicuous and subsequently eaten more

often» Black lizards heat up faster, which allows them to be

active quicker when compared to typical brown lizards» Being more active means escaping predation and thus

surviving to become the more common color over time

2. Natural Selection on Polygenic Traits

a. Natural selection can act on polygenic traits in 3 ways

–Directional–Stabilizing –Disruptive

b. Directional selection- when individuals on one end of the bell curve have a higher fitness level–Ex: A cacti population

with a large number of spines due to herbivory by peccaries

c. Stabilizing - when individuals near the center have higher fitness–Ex: Human babies: very

small babies have more health problems. Large babies have trouble being born. Those that are of average size have higher fitness

d. Disruptive - individuals at the upper and lower ends of the bell curve have higher fitness than the ones in the middle–Ex: An island with large, medium, and small

seeds. The medium sized seeds are rare. Birds on opposite extremes are more likely to survive. Population might split into

two.

Activity!

• Groups of 3-4• One sheet of paper• Pick up packet• Read scenarios• Determine the type of selection• Support your answer – why?

B. Genetic Drift1. Defined - a random change in the allele

frequency caused by a series of chance occurrences that cause an allele to become more or less common

2. Bottleneck effect - a change in allele frequency following a dramatic reduction in the size of a population

a. Ex: The American Bison (Bison bison) had a population estimated at around 60,000,000 individuals prior to European colonization.

Bison (cont.)• by the late 1800’s, the overall

population was down to less than 1,000

• Through conservation efforts since that time the population has risen to an estimated 360,000 bison

• The Bison that are around today do not have the same allele frequency as the original population

b. Bottlenecks lead to greatly reduced genetic diversity

3. The Founder Effect - a change in the allele frequencies as a result of the migration of a small group of a population

a. The new group may have allele frequencies that differ from the main population

b. Ex: High rates of polydactyly in the Amish

C. Evolution vs. Genetic Equilibrium

1. Genetic equilibrium - a situation in which the allele frequencies in a population remain the same

2. Sexual reproduction does not affect genetic equilibrium

3. What then disrupts genetic equilibrium and causes populations to evolve?

a. Hardy-Weinberg Principle - allele frequencies remain constant unless one or more factors cause the frequencies to change

b. There are 5 conditions that can cause a population to break equilibrium

• Non-random mating- in many species organisms choose mates for particular traits

• Small population - genetic drift causes evolution to happen much more easily with smaller populations

Hardy-Weinberg (cont.)

• Gene flow: movement into or out of population - new individuals can add new genes to a gene pool (provides variety)

• Mutations - can create new alleles • Natural selection - if different genotypes have

differing levels of fitness, then those that are the most fit will survive

17.2 Essential question: What causes a

population’s gene pool to change?

• Explain how natural selection affects single gene and polygenic traits•Describe genetic drift•Explain how different factors affect genetic equilibrium

Do NowCheetahs nearly escaped extinction about 10,000 years ago. With already low numbers, humans have reduced the population further due to poaching and habitat loss. Evidence suggest that their prey, the gazelle- are getting faster with each generation. 1. What has happened to the cheetah

population?2. Is it possible for the cheetah, as a species, to

become faster? Explain.

Answers:1. Bottleneck effect2. Selection can only operate on the available

genetic variation. A cheetah might run faster if it had “faster genes”—but if “faster” genes are not in the population from mutation or gene flow, evolution in this direction will not happen.

Trade-offs???

“From Wolf to Dog” Useful Definitions

1. Gene Pool – consists of all of the genes, including all of the different alleles for each gene, that are present in a population 2. Gene Flow - is any movement of genes from one population to another (Ex: migration)3. Stabilizing selection - when individuals near the center of a bell curve have higher fitness4. Directional selection- when individuals on one end of the bell curve have a higher fitness level

17.3 Essential question: How do new species form?

• Identify the types of isolation that can lead to the formation of new species.

• Describe the current hypothesis about Galápagos finch speciation.

III. 17.3 The Process of Speciation

A. Isolating Mechanisms1. How do new species form?

a. Speciation – the formation of a new species

b. Species – a population whose members can breed and produce fertile offspring

2. Breeding connects the gene pool of a species

a. If a species is split into 2 parts, genetic changes in one part don’t often transfer to the other

b. Reproductive isolation – the separation of a species or a population so that members can no longer interbreed

Cayos Cochinos Boa constrictor

3. Geographic isolation – when populations are separated by a barrier, such as a river, mountain, or an ocean

a. Ex: Death Valley Pupfish, Abert’s and Kaibab squirrels, Darwin’s finches

b. Death Valley pupfish – inhabit an area in California that used to be a large lake that dried up at the end of the last ice age

• The lake bed is still fed by an underground basin, so the deepest pockets were able to still maintain water• The pupfish were stuck in

these isolated ponds• Environmental conditions

in each pond were different• Those that could survive

reproduced and formed the 13 modern day pupfish species

Pupfish in Death Valley, CA

13 species known!

4. Behavioral isolation-a form of reproductive isolation in which two populations develop differences in courtship rituals or other behaviors that prevent them from breeding

a. Occurs mostly in animalsb. Ex: Male moths only respond to

chemical pheromones given off by females of their species

c. Courtship displays in birds are species specific

5. Temporal isolation- a form of reproductive isolation in which two or more species reproduce at different times

a. Ex#1: Wood frog and Leopard frog breeding seasons• Wood frogs breed in Late March to

early April and Leopard frogs breed in late April.

Wood Frog vs. Leopard Frog

Similarities: •eat same food• similar distribution• similar life cycles•about the same size

Differences:• different breeding months•Different color patterns•Wood frogs are obligate vernal pond breeders

b. Ex #2: The pine trees Pinus radiata and P. muricata, which grow together in California.

• P. radiata sheds its pollen in early February and P. muricata in April. Hybrids do occur but form only a small fraction of the population and are less vigorous and productive than either parent species

6. Mechanical isolation - This happens when there is an anatomical incompatibility between individuals from two populations

a. More common in plantsb. Example: the floral structure of some

plant species prevents some pollinators that visit the plant from actually transferring pollen.

17.3 Essential question: How do new species form?

• Identify the types of isolation that can lead to the formation of new species.

• Describe the current hypothesis about Galápagos finch speciation.

• Follow-up Questions:1. What is speciation?

2. List 4 ways that reproductive isolation can occur.

3. Give an example for each of the 4 types of reproductive isolation