15-3 Shaping Evolutionary Theory:

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Georgia Performance Standards:

SB5b: Explain the history of life in terms of biodiversity, ancestry, and the rates of evolution.

SB5d: Relate natural selection to changes in organisms.

Essential Questions:

1. Why is important to understand evolutionary theory? 2. What is the role of natural selection in speciation?

Gene Pools:

• All members of a population can interbreed, they share a common group of genes, called a gene pool. – A gene pool is the combined genetic information

of all the members of a particular population.• Typically contains two or more alleles—or forms of a

certain gene—for each inheritable trait.

– The relative frequency of an allele is the number of times that allele occurs in a gene pool compared with the number of times other alleles occur.

Sample Population

48% heterozygous

36% homozygous

16% homozygous

Frequency of Alleles

allele for brown fur allele for black fur

Relative Frequencies of AllelesSection 16-1

Evolution as Genetic Change

• Natural selection on single-gene traits can lead to changes in allele frequencies and, thus, to evolution.

– Ex: Color Mutations

Single-Gene and Polygenic Traits

• Inheritable variation can be expressed in a variety of ways.

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

Single-gene trait

• Trait controlled by a single gene

• Variation in this gene leads to only two distinct phenotypes

• The number of phenotypes a given trait has is determined by how many genes control the trait.

In humans, having a widow’s peak or not having a widow’s peak is controlled by a single gene with two alleles. As a result, only two phenotypes are possible.

Polygenic Traits:

• Most traits are controlled by two or more genes and are, therefore, called polygenic traits.

• Each gene of a polygenic trait often has two or more alleles.

• As a result, one polygenic trait can have many possible genotypes and even more possible phenotypes.

EX: height (A bell-shaped curve is also called a normal distribution)

• Natural selection

– does not act directly on genes, but on phenotypes.

– affects which individuals having different phenotypes survive and reproduce and which do not.

– determines which alleles are passed from one generation to the next.

– can change the relative frequencies of alleles in a population over time.

Exactly what factors change the relative

frequencies of alleles in a population? • In genetic terms, any factor that causes alleles

to be added to or removed from a population will change the relative frequencies of alleles.

• Evolution is any change in the relative frequencies of alleles in a population’s gene pool.

• Evolution acts on populations, not on individuals.

Natural Selection on Single-Gene Traits

• Natural selection on single-gene traits can lead to changes in allele frequencies and, thus, to evolution.

– EX: Color Mutations (organisms of one color may produce fewer offspring than organisms of another color.

Equilibrium v/s Evolution

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

• The situation in which allele frequencies remain constant is called genetic equilibrium (juh-net-ik ee-kwih-lib-ree-um).

• If the allele frequencies do not change, the population will not evolve.

Evolution

This equation allows us to determine the equilibrium frequency of each genotype in the population.

Homozygous dominant (p2)

Heterozygous (2pq)

Homozygous recessive (q2)

15.3 Shaping Evolutionary Theory

Chapter 15

• Hardy-Weinberg equilibrium states that the shuffling of genes during sexual reproduction does not alter the proportions of different alleles in a gene pool– To test this, let’s look at an

imaginary, nonevolving population of blue-footed boobies

13.8 The gene pool of a nonevolving population remains constant over the generations

Figure 13.8A

Webbing No webbing

• We can follow alleles in a population to observe if Hardy-Weinberg equilibrium exists

Figure 13.8B

Phenotypes

Genotypes

Number of animals(total = 500)

Genotype frequencies 320/500 = 0.64 160/500 = 0.32 20/500 = 0.04

640 W 160 W + 160 w 40 w

800/1,000 = 0.8 W 200/1,000 = 0.2 w

Number of allelesin gene pool(total = 1,000)

Allele frequencies

Figure 13.8C

Recombinationof alleles fromparent generation

Next generation:

Genotype frequencies

Allele frequencies

SPERM EGGS

0.64 WW 0.32 Ww 0.04 ww

0.8 W 0.2 w

WWp2 = 0.64

WWqp = 0.16

Wwpq = 0.16

wwq2 = 0.04

W sperm

p = 0.8

w sperm

q = 0.2

Understanding Evolution: Problem-based Understanding Evolution: Problem-based discussiondiscussion

1) What are the genotype frequencies at the flower-color locus in this generation?

2) What are the allele frequencies at the flower-color locus in this generation?

3) If the individuals in this generation mate randomly, what would you expect the genotype frequencies to be in the next generation?

Ribozyme structure comes from Scott, W.G., Finch, J.T., Klug, A. (1995) The crystal structure of an all-RNA

hammerhead ribozyme: a proposed mechanism for RNA catalytic cleavage. Cell 81: 991-1002

Population genetics calculations and Hardy Weinberg

4) If the individuals in this generation mate randomly, what would you expect the allele frequencies to be in Generation 2?

5) If the resulting individuals in Generation 2 mate randomly, what would you expect the allele frequencies to be in Generation 3?

6) Is this population evolving? Why or why not?

Population genetics calculations and Hardy Weinberg

Evolution

Chapter 15

Process of Speciation:

• Factors such as natural selection and chance events can change the relative frequencies of alleles in a population.

• But how do these changes lead to the formation of new species, or speciation?

Process of Speciation:

• A species as a group of organisms that breed with one another and produce fertile offspring.

– They share a common gene pool. – A genetic change that occurs in one individual can

spread through the population as that individual and its offspring reproduce.

– If a genetic change increases fitness, that allele will eventually be found in many individuals of that population.

Genes and Variation:

• Genetics, molecular biology, and evolutionary theory work together to explain how inheritable variation appears and how natural selection operates on that variation

Checkpoint Questions:

1. Describe how natural selection can affect traits controlled by single genes.

2. Describe three patterns of natural selection on polygenic traits. Which one leads to two distinct phenotypes?

3. How does genetic drift lead to a change in a population’s gene pool?

4. What is the Hardy-Weinberg principle?

5. How are directional selection and disruptive selection similar? How are they different?

Evolution

Genetic Drift

A change in the allelic frequencies in a population that is due to chance

In smaller populations, the effects of genetic drift become more pronounced, and the chance of losing an allele becomes greater.

Chapter 15

Examples: Founder Effect & Bottleneck

Sample of Original Population

Founding Population A

Founding Population B

Descendants

Genetic Drift

1) Explain what is shown on the x- and y-axes.

2) Choose two lines on graph A, one that goes to the top of the graph and one that goes to the bottom. For each line, explain what the line represents and how it changes over time. Also, explain what it means when a line goes to the top of the graph versus what it means when a line goes to the bottom of the graph.

Population genetics simulation of drift

3) Explain the difference between the simulations that generated graphs A and B.

4) Why do y-values fluctuate in each graph?

5) In graph A, how many of the trials resulted in a frequency of A = 1.0 and how many resulted in a frequency of a = 1.0? Why might this occur?

6) The populations represented by graph A and graph B demonstrate very different behavior. What concept regarding genetic drift does this illustrate?

7) If one allele were under selection in graph B, how would this graph differ?

8) For each population represented on graph A, would you expect the population to be in Hardy-Weinberg equilibrium? For graph B?

9) How would each of these graphs differ if starting allele frequencies in the simulations were not 50/50—for example, if the populations began with 30% A alleles and 70% a alleles? Would drift occur?

Evolution

Founder Effect

Occurs when a small sample of a population settles in a location separated from the rest of the population

Alleles that were uncommon in the original population might be common in the new population.

Chapter 15

Evolution

BottleneckOccurs when a population declines to a

very low number and then rebounds

Chapter 15

Evolution

Gene Flow

Increases genetic variation within a population and reduces differences between populations

Nonrandom MatingPromotes inbreeding and could lead

to a change in allelic proportions favoring individuals that are homozygous for particular traits

Chapter 15

Sources of Genetic Variation

• The two main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction.

– Sexual reproduction can thus produce many different phenotypes, but this does not change the relative frequency of alleles in a population. (Card deck analogy)

1. What two processes can lead to inherited variation in populations?

2. How does the range of phenotypes differ between single-gene traits and polygenic traits?

3. What is a gene pool? How are allele frequencies related to gene pools?

4. How could you distinguish between a species in which there is a lot of variation and two separate species?

Natural Selection on Polygenic Traits

• Fitness varies in polygenic traits.

• Where fitness varies, natural selection can act.

• Natural selection can affect the distributions of phenotypes in any of three ways:

– directional selection – stabilizing selection– disruptive selection.

Evolution

Natural Selection

Acts to select the individuals that are best adapted for survival and reproduction

Chapter 15

Evolution

Stabilizing selection operates to eliminate extreme expressions of a trait when the average expression leads to higher fitness.

Chapter 15

Evolution

Directional selection makes an organism more fit.

Chapter 15

Evolution

Disruptive selection is a process that splits a population into two groups.

Chapter 15

Evolution

Sexual selection operates in populations where males and females differ significantly in appearance.

Qualities of sexual attractiveness appear to be the opposite of qualities that might enhance survival.

Chapter 15

Natural Selection

Isolating Mechanisms

• What happens to a gene pool as one species evolves into one or more species?

• As new species evolve, populations become reproductively isolated from each other.

• When the members of two populations cannot interbreed and produce fertile offspring, reproductive isolation has occurred. – At that point, the populations have separate gene

pools.

Reproductive Isolation• Develops in a variety of ways:

– Postzygotic isolation (behavioral isolation) = occurs when two populations are capable of interbreeding but have differences in courtship rituals or other types of behavior.

– Allopatric Speciation (geographic isolation) = two populations are separated by geographic barriers such as rivers, mountains, or bodies of water.

• do not guarantee the formation of new species

– Prezygotic isolation (temporal isolation) = two or more species reproduce at different times.

Section 16-3

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Reproductive Isolation

Isolating mechanisms

Behavioral isolation Temporal isolationGeographic isolation

Behavioral differences Different mating timesPhysical separation

Independentlyevolving populations

Formation ofnew species

• Courtship ritual in blue-footed boobies is an example of one kind of prezygotic barrier, behavioral isolation

• Many plant species have flower structures that are adapted to specific pollinators

– This is an example of mechanical isolation, another prezygotic barrier Figure 14.2A, B

• Hybrid sterility is one type of postzygotic barrier

– A horse and a donkey may produce a hybrid offspring, a mule

– Mules are sterile

Figure 14.2C

Evolution

Prezygotic isolation prevents reproduction by making fertilization unlikely.

Prevents genotypes from entering a population’s gene pool through geographic, ecological, behavioral, or other differences

Eastern meadowlark and Western meadowlark

Chapter 15

Postzygotic isolation occurs when fertilization has occurred but a hybrid offspring cannot develop or reproduce.

Evolution

Prevents offspring survival or reproduction Liger

Chapter 15

Evolution

Allopatric Speciation

A physical barrier divides one population into two or more populations.

Abert squirrel Kaibab squirrel

Chapter 15

Evolution

Sympatric Speciation

A species evolves into a new species without a physical barrier.

The ancestor species and the new species live side by side during the speciation process.

Chapter 15

Checkpoint Questions:1. How is reproductive isolation related to the

formation of new species? 2. What type of isolating mechanism was

important in the formation of different Galápagos finch species?

3. Explain how behavior can play a role in the evolution of species.

4. Leopard frogs and tree frogs share the same habitat. Leopard frogs mate in April; tree frogs mate in June. How are these species isolated from each other?

Adaptive Radiation

• Studies of fossils or of living organisms can show that a single species or a small group of species has evolved into several different forms that live in different ways.

• This process is known as adaptive radiation. – Implies common descent

Convergent Evolution

• Unrelated organisms that come to resemble one another, is called convergent evolution.

• Natural selection may mold different body structures, such as arms and legs, into modified forms, such as wings or flippers. – EX: Streamlined body of penguin, shark,

dolphin

Coevolution

• The process by which two species evolve in response to changes in each other over time is called coevolution.

• An evolutionary change in one organism may also be followed by a corresponding change in another organism.

• EX: Many flowering plants, for example, can reproduce only if the shape, color, and odor of their flowers attract a specific type of pollinator.

Developmental Genes and Body Plans

• First, molecular studies show that homologous hox genes establish body plans in animals as different as insects and humans

• Second, major evolutionary changes—such as the different numbers of wings, legs, and body segments in insects—may be based on hox genes.

• Finally, geneticists are learning that even small changes in the timing of genetic control during embryonic development can make the difference between long legs and short ones

Changes in developmental genes are one major pattern of macroevolution.

• Fossil evidence shows that some ancient insects (top left) had no wings, but others (top right) had winglike structures on many body segments.

• In modern insects (bottom), genes may turn off wing development in all except one or two body segments.

Rates of Evolution:

• Evolution has often proceeded at different rates for different organisms at different times during the long history of life on Earth. (Rate of Evolution)

• Gradualism - slow, steady change in a particular line of descent.

• Punctuated equilibrium - long, stable periods interrupted by brief periods of more rapid change

Is this punctuated equilibrium or gradualism?

Patterns of Evolution

• What is macroevolution? Describe two patterns of macroevolution.

• What role have mass extinctions played in the history of life?

• Use an example to explain the concept of coevolution.

• How might hox genes contribute to variation?

• Compare and contrast the theories of gradualism and punctuated equilibrium.

15-3 Shaping Evolutionary Theory:

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