genetic variation interest grabber yes, no, or maybe some traits, such as a widow’s peak, fall...

Genetic Variation

Interest Grabber

Yes, No, or Maybe

Some traits, such as a widow’s peak, fall into neat categories: You either have a widow’s peak or you don’t. Other traits, such as height, aren’t so easy to categorize.

Section 16-1

Genetic Variation

Interest Grabber continued

1. Make a list of physical traits that you think are influenced by genes.Then, write next to each trait whether you have the trait or not(e.g., a widow’s peak) or whether there are many variations of the trait (e.g., hair color).

2. Are most of the traits you listed clear-cut or are they mostly traitsthat have many variations? Which traits in your list are difficult tocategorize?

3. Compare your list with that of another student. Did he or she think of any traits that you missed? Why do you think some traits are clear-cut, while others are not?

Section 16-1

Genetic Variation

Section Outline

Genes and VariationA. Darwin’s Ideas Revisited

B. Gene Pools

C. Sources of Genetic Variation

1. Mutations

2. Gene Shuffling

D. Single-Gene and Polygenic Traits

Section 16-1

Genetic Variation

Review of Natural Selection

• Natural Selection - 5 Major Elements

1) All species have genetic variation.

2) The environment presents many different challenges to an individual's survival

3) Organisms tend to produce more offspring than their environment can support : thus, individuals of a species often compete with one another to survive (struggle for survival)

Genetic Variation

4) Survival of fittest : Individuals who are better able to cope with the challenges of their environment tend to leave more offspring than those less suited to the environment

5) The characteristics (genes) of the individual best suited to an environment tend to increase in a population over time

Genetic Variation

Review

EVOLUTION –

change in a population’s genes/traits over time

NATURAL SELECTION –

mechanism of evolution in which individuals with better adaptations (traits) for their environment are more likely to survive and reproduce, contributing their traits to the next generation

ALLELES –

different expressions of same gene for a single trait

Genetic Variation

A. Modern Evolution Theory

Darwin did not know about genetics.

Today evolution is expressed in genetic terms because natural selection acts on the heritable traits of organisms which we know to be the result of their GENES.

Natural selection (environmental pressures) determines which, if any, allele is favorable

Genetic Variation

KEY CONCEPT A population shares a common gene pool.

Genetic Variation

1. Population Genes

Gene Pool– common group of genes shared by a population

• **All of the alleles in all of the individuals that make up a population

• where a population’s

genetic variation is stored (next generation draws from pool).

Genetic Variation A. Population Genes

**Gene frequency – how much each allele is present within a population’s gene pool– Also allele frequency or relative frequency

Gene frequency is a percentage

= # of alleles / total alleles in population

Bb

BBbb

Genetic Variation

a) Determine which alleles in gene pool.

b) List alleles for population.

c) Count how many of each allele = # each allele.

d) Add up all alleles = total # of alleles (2 x pop. size)

e) GF = # each allele/total # of alleles

1) Calculating Gene FrequencyFrequency of Alleles “b” “B”

Population = 25 mice

BbBB

bb

12 mice

9 mice

4 mice

Genetic Variation

**Genetic variation in a population increases the chance that some individuals will survive.

• Genetic variation leads to phenotypic variation.• **Phenotypic variation is necessary for natural selection.• Genetic variation is stored in a population’s gene pool.

– made up of all alleles in a population– allele combinations form when organisms have offspring

Genetic Variation

• **Allele frequencies measure genetic variation.

– measures how common allele is in population– can be calculated for each allele in gene pool– Frequency: #of specific allele/total alleles for trait

Genetic Variation

**Genetic variation comes from several sources.

• Mutation is a random change in the DNA of a gene.

• Recombination forms new combinations of alleles.

– can form new allele– can be passed on to offspring if in reproductive cells

– usually occurs during meiosis – parents’ alleles arranged in new ways in gametes

Genetic Variation

Genetic variation comes from several sources.

• Hybridization is the crossing of two different species.– occurs when individuals can’t find mate of own

species– topic of current scientific research

Genetic Variation

Sources of Genetic Variation

Lateral Gene Transfer – genes swapped between cells without sex

– Esp. important to single-celled organisms– Transformation - Griffith’s experiment; antibiotic

resistance– Viruses (HIV), bacteriaphages

Genetic Variation

Checkpoint

1. Modern evolution is explained using which field of biology?

2. What is a population’s gene pool?

3. Define gene frequency?

4. If a population has 2 BB mice, 6Bb mice and 2 bb mice; what is the gene frequency of B?

5. Where does genetic variation in populations come from?

17ANY QUESTIONS?

Genetic Variation

Interest Grabber• . . . All the Help I Can Get

Natural selection operates on traits in different ways. You might be ableto predict which traits natural selection would favor if you think about the demands of an organism’s environment.

1. Choose an animal that you know something about, such as a deer, and write its name at the top of a sheet of paper. Then, divide your paper into two columns, and write the heading Trait in one column and Advantage in the other.

2. Under Trait, write in several of the animal’s traits.

3. Under Advantage, write in how you think the trait would be helpful to the animal.

Genetic Variation

Section Outline

Evolution as Genetic ChangeA. Natural Selection on Single-Gene Traits

B. Natural Selection on Polygenic Traits

1. Directional Selection

2. Stabilizing Selection

3. Disruptive Selection

C. Genetic Drift

D. Evolution Versus Genetic Equilibrium

1. Random Mating

2. Large Population

3. No Movement Into or Out of the Population

4. No Mutations

5. No Natural Selection

Section 16-2

Genetic Variation

KEY CONCEPT Populations, not individuals, evolve.

Genetic Variation

Founder Effect

Change in allele frequency as a result of migration by a small group

Hardy Weinberg Principle

Allele frequencies in a population will remain constant unless factors cause change

Genetic Equilibrium

Situation in which allele frequencies remain constant

**Phenotypic Distribution

Distribution curve of phenotypes, genetic drift

Genetic Variation

Directional Selection

When Natural selection forces the distribution of traits to move to one end.

Stabilizing Selection

When Natural selection forces the distribution of traits toward the center

Disruptive Selection

When Natural selection forces a split at either end.

Genetic Drift

Random changes in allele frequencies that occurs in small populations

Genetic Variation

KEY CONCEPT Natural selection is not the only mechanism through which populations evolve.

Genetic Variation

Natural Selection: environmental pressures change the gene pool

1) Selection causes an increased frequency of the most favorable allele.

Changing the Gene Pool

Genetic Variation

Gene flow is the movement of alleles between populations.

• Gene flow occurs when individuals join new populations and reproduce.

• Gene flow keeps neighboring populations similar.

• Low gene flow increases the chance that two populations will evolve into different species.

bald eagle migration

Genetic Variation

Genetic drift is a change in allele frequencies due to chance.

• Genetic drift causes a loss of genetic diversity.• It is most common in small populations.• A population bottleneck can lead to genetic drift.

– It occurs when an eventdrastically reducespopulation size.

– The bottleneck effect isgenetic drift that occursafter a bottleneck event.

Genetic Variation

Genetic Drift

• Creates small populations = small gene pool so lower genetic diversity and fewer traits available for adaptation

• often leads to extinction

Genetic Variation

Sample of Original Population

Founding Population A

Founding Population B

Descendants

Genetic DriftSection 16-2

Genetic Variation

• The founding of a small population can lead to genetic drift.

– It occurs when a few individuals start a new population.– The founder effect is genetic drift that occurs after start

of new population.

Genetic Variation

• Genetic drift has negative effects on a population.

– less likely to have some individuals that can adapt – harmful alleles can become more common due to

chance

Genetic Variation

Sexual selection occurs when certain traits increase mating success.

• Sexual selection occurs due to higher cost of reproduction for females.– males produce many

sperm continuously– females are more

limited in potential offspring each cycle

Genetic Variation

• There are two types of sexual selection.

– intrasexual selection: competition among males– intersexual selection: males display certain traits to

females

Genetic Variation

Balancing Selection = selection that acts in opposite directions

1) tends to eliminate the sickle cell gene because it is lethal to homozygous individual

2) tends to preserve the sickle cell gene because

heterozygous individual is resistant to malaria

Thus even defective genes will remain in a population as long as the benefit outweighs the cost.

Genetic Variation

Directional Selection = causes a gene to become either more or less common due to the environment.

Example: sickle cell gene becoming less common

among African Americans since malaria no longer occurs in North America.

When natural selection is unopposed, frequency of a particular gene tends to go in one direction only (elimination)

EX: peppered moth

Genetic Variation

Natural selection acts on distributions of traits.

• **A normal distribution graphs as a bell-shaped curve.

• **Traits not undergoing natural selection have a normal distribution.

– highest frequency near

mean value– frequencies decrease toward each extreme value

Genetic Variation

Natural selection can change the distribution of a trait in one of three ways.

• Microevolution is evolution within a population.– observable change in the allele frequencies – can result from natural selection

Genetic Variation

• Natural selection can take one of three paths.

– **Directional selection favors phenotypes at one extreme.

Genetic Variation

– **Stabilizing selection favors the intermediate phenotype.


Genetic Variation


– **Disruptive selection favors both extreme phenotypes.

Genetic Variation

KEY CONCEPT Hardy-Weinberg equilibrium provides a framework for understanding how populations evolve.

Genetic Variation

Hardy-Weinberg equilibrium describes populations that are not evolving. • Biologists use models to study populations.• Hardy-Weinberg equilibrium is a type of model.

Genetic Variation

Hardy-Weinberg equilibrium describes populations that are not evolving. • Genotype frequencies stay the same if five conditions are

met.– very large population: no genetic drift – no emigration or immigration: no gene flow – no mutations: no new alleles added to gene pool– random mating:

no sexual selection– no natural selection:

all traits aid equallyin survival

Genetic Variation

Hardy-Weinberg equilibrium describes populations that are not evolving. • Real populations rarely meet all five conditions.

– Real population data iscompared to a model.

– Models are used tostudying how populationsevolve.

Genetic Variation

The Hardy-Weinberg equation is used to predict genotype frequencies in a population.

• Predicted genotype frequencies are compared with actual frequencies.– used for traits in simple dominant-recessive systems– must know frequency of recessive homozygotes– p2 + 2pq + q2 = 1

Genetic Variation

There are five factors that can lead to evolution.

Genetic Variation

• Genetic drift changes allele frequencies due to chance alone.

Genetic Variation

• Gene flow moves alleles from one population to another.

Genetic Variation

• Mutations produce the genetic variation needed for evolution.

Genetic Variation

• Sexual selection selects for traits that improve mating success.

Genetic Variation

• Natural selection selects for traits advantageous for survival.

Genetic Variation

• In nature, populations evolve.

– expected in all populations most of the time

– respond to changing environments

Genetic Variation

Section OutlineThe Process of Speciation

A. Isolating Mechanisms

1. Behavioral Isolation

2. Geographic Isolation

3. Temporal Isolation

B. Testing Natural Selection in Nature

1. Variation

2. Natural Selection

3. Rapid Evolution

C. Speciation of Darwin’s Finches

1. Founders Arrive

2. Separation of Populations

3. Changes in the Gene Pool

4. Reproductive Isolation

5. Ecological Competition

6. Continued Evolution

Section 16-3

Genetic Variation

Speciation

Formation of a new species

Reproductive Isolation

Separation of a species that does not allow interbreeding

Behavioral Isolation

Reproductive isolation due to differing mating rituals

Geographic Isolation

Reproductive isolation due to physical barriers.

Temporal Isolation

Reproductive isolation caused by differing reproductive timing

Genetic Variation

KEY CONCEPT New species can arise when populations are isolated.

Genetic Variation

A. Speciation

SPECIATION - Formation of new species.• A new species has evolved when enough shifts in the

gene pool results in it becoming reproductively isolated from other populations.

1. Reproductive Isolation – defines a species as a population that no longer interbreed with other populations to produce fertile offspring

Genetic Variation

2. Isolation Mechanisms

Behavioral Isolation – do not mate because of incorrect behavior cues – displays, courting rituals, songs, etc.

4 leopard frog species overlap habitat but have different mating calls

Genetic Variation


Geographic Isolation –separated by physical barriers – water, mountains, habitat differences, etc.

Squirrels living on opposite sides of the Grand CanyonSquirrels living on opposite sides of the Grand Canyon

Genetic Variation


Temporal Isolation – no longer reproduce at the same time/same season

Different species of frogs may live in same pond, but reproduce at different times. One in March and one in June.

Genetic Variation

The isolation of populations can lead to speciation.

Populations become isolated when there is no gene flow. – Isolated populations adapt to their own environments.– Genetic differences can add up over generations.

Genetic Variation

Species Formation Begins with Microevolution

Divergence is the accumulation of differences between groups (population, species etc.)

Separate populations of same species may become very different.

Ecological Races occur when separate populations continue to accumulate genetic changes due to the environment.

Genetic Variation

(Ex: Seaside sparrows)

In each environment, local population of species become better adapted to environment.

Genetic change may continue to diverge until the populations can no longer interbreed successfully. A new species has formed.

Genetic Variation

Molecular and genetic evidence support fossil and anatomical evidence.

• Two closely-related organisms will have similar DNA sequences.

Genetic Variation

• Pseudogenes are sequences providing evidence of evolution.– no longer function– carried along with functional DNA– can be clues to a common ancestor

Genetic Variation

• Hox genes indicate a very distant common ancestor.

– control the development of specific structures– found in many organisms

• Protein comparisons, or molecular fingerprinting reveals similarities among cell types of different organisms.

Genetic Variation

• Scientist from any fields contribute to the understanding of evolution.

• The basic principles of evolution are used in many scientific fields.

Evolution unites all fields of biology.

Genetic Variation

Concept MapSection 16-3

results from

which include

produced by produced byproduced by

which result in

which result in

Reproductive Isolation

Isolating mechanisms

Behavioral isolation Temporal isolationGeographic isolation

Behavioral differences Different mating timesPhysical separation

Independentlyevolving populations

Formation ofnew species

Genetic Variation

Checkpoint

1. What is genetic drift?

2. What is gene flow?

3. Define gene frequency?

4. What are the three types of selection on polygenic traits?

5. Which type of selection favors two different phenotypes?

6. What is genetic equilibrium?

69ANY QUESTIONS?

Genetic Variation

II. Macroevolution

broader pattern of evolutionary change over time (like new species).

• what we see when we look at the over-arching history of life: change, stability, extinction

• Asks the big questions:– Can we trace the history of life on earth?– How are living things related?– What happened to the dinosaurs?

13.1 Ecologists Study Relationships

I. MicroevolutionChanges in a population’s gene pool

Genetic Variation

Microevolution & Macroevolution

Microevolution – change in alleles and traits in a population over time

can lead to..Macroevolution – broader pattern

of evolutionary change over time (like new species).

earth.

Genetic Variation

2. Microevolution = evolution of genes within a population

• Change in gene frequency of alleles within a population over time.

• Because shifts in gene frequency equals shifts in traits– If advantageous to be brown, then selection for more

brown mice. More brown mice = more b alleles in the population.

Genetic Variation

Macroevolution

Genetic Variation

B. Theories on evolution rates

1. Gradualism (Darwin) – Species change slowly over time branching into new species (fossil transitions)

Genetic Variation

B. Theories on evolution rates

2. Punctuated Equilibrium (Gould & Lewontin) –

Species can undergo rapid “bursts” of evolution (isolation, new niche, etc.)

BOTH CAN OCCUR!

Genetic Variation

Can we see evolution occur?

• Can you think of an example of an organism that evolves “quickly”? One that has evolved during your life time?

Genetic Variation

Bacterial EvolutionWhat allows bacteria to evolve so quickly?

Genetic Variation

Insect Evolution

Genetic Variation

C. Patterns of Evolution

1. Divergent evolution –populations branching into two or more species as they become more dissimilar

Homologous structures & recent common ancestor

Ex: Adaptive radiation – population divides into a series of related species occupying different niches (roles)

Genetic Variation


2. Convergent evolution - unrelated species with similar adaptations due to similar environmental pressures

• Analogous structures

Genetic Variation

3. Coevolution – mutual selection by two or more species in close interaction

Ex: predator/prey; parasites/host

pollinators/plants


Genetic Variation D. Extinction

1. Background extinction – routine loss of species to natural selection

conditions change, competitors or predators superior, etc.

Total loss of all members of a species

Species unable to adapt to environment changes

Ex: dodo bird, passenger pigeon, dinosaurs

Genetic Variation D. Extinction

2. Mass extinction – event in which many species lost in relatively short period

– major changes in ecosystems

– Major loss of biodiversity on planet = less raw material for evolution

– Major loss of useful genes!

Genetic Variation

Evolution not always = better.

Evolutionary sequence is NOT always progress.

Successful evolutionary Successful evolutionary pathways are littered with pathways are littered with dead-end branches, dead-end branches, chance events, and chance events, and unusual survivors.unusual survivors.

Genetic Variation

Checkpoint

1. What are the differences between microevolution and macroevolution?

2. How are speciation and reproductive isolation related?

3. What are three isolation mechanisms?

4. Compare the two rates of evolution.

5. What are the three patterns of evolution?

6. What are the two types of extinction?

86ANY QUESTIONS?

genetic variation interest grabber yes, no, or maybe some traits, such as a widow’s peak, fall...

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