Population Genetics Ch 17 and Ch 18.2. Genes and Variation Combine Darwin’s theory of Natural...

Population Genetics

Ch 17 and Ch 18.2

Genes and Variation• Combine Darwin’s theory

of Natural Selection with Mendel’s studies on genetics.

• Which phenotype will be more often expressed in a population?

• For example why might the black coat show up more in a population than the brown?

Which moth is more likely to survive due to the theory of Natural Selection?

How would happen to the black moth’s population here?

Natural selection never acts on genes. Why?

The organisms that have phenotypes that are better suited to their environment, will produce more offspring. Organisms more adapted to their environment, can pass these traits onto their offspring.

Populations and Gene Pools• Population – a group of

individuals of the same species that mate and produce offspring.

• Gene pool- consists of all the genes (different alleles), found in that population.

• Allele frequency – the number of times an allele occurs in a population

Evolution, in genetic terms involves a change in the frequency of alleles in a population over time.

Populations evolve, not individuals.

Some alleles affect the species ability to survive.

Determing Allele Frequency

You have a population of 25 mice. How many alleles for coat color are found in this population?

Allele Frequencies of Different Blood types

Location of O Blood type globally

Location of A blood type globally

Location of B blood type globally

Sources of Genetic Variation

• Mutations

• Genetic Recombination in Sexual Reproduction– Crossing over

• Lateral Gene Transfer– Bacteria swapping

plasmid DNA

What determines the number of phenotypes for a given trait?

• Single gene trait– Absence or presence of

a band on shells

• Polygenic traits– Over 180 genes help

code for height

How many alleles are found in each example?

How does Natural Selection Work?

Pesticide Resistance

What changes Allele Frequenciesbesides Natural Selection?

Lesson OverviewLesson Overview The Process of SpeciationThe Process of Speciation

Genetic Drift• Random change in an allele frequency in small

populations. (Change is not caused by Natural Selection)

– Genetic Bottlenecks – change in allele frequency after a dramatic reduction in size of the population

• Disaster such as disease

Potato Famine in IrelandIrish poor grew one variety of potatoes called lumpers, which were harvested in the fall. Airborne fungus wiped out the crop in September of 1845 and wiped out the potato crops for the next 6 years.

Genetic Drift• The Founder Effect – allele frequencies change

as a result of the migration of a small group of a population.

Finches on G. Islands

Random loss of alleles; leads to reduced genetic variationSmaller population that changed from the original population found on South America.

Allele frequency in a Population

• A population that does not evolve should have allele frequencies that do not change in a population – Genetic Equilibrium

• Hardy-Weinberg Principle – allele frequencies in a population should remain constant unless one or more factors cause those frequencies to change. – Used to make predictions about allele frequencies

in populations. (Similar to what Mendel did with offspring)

What factors would cause a change in allele frequency?

• Five conditions that affect genetic equilibrium– Nonrandom mating

• Sexual Selection – genes that are used for selection of a mate, are not longer in equilibrium

– Small Population Size• Genetic drift has a greater chance of affecting small

populations. Why?

– Immigration and Emigration• Addition and removal of alleles from a population

Factors that affect genetic equilibrium? (cont.)

– Mutations• New alleles are introduced into gene pool

– Natural Selection• A allele makes the organisms better able to survive,

genetic equilibrium will be disrupted. More organism will show up with the better adapted trait over time.

How do you determine allele frequency using Hardy-Weinberg?

• Hardy-Weinberg Equation– p2 +2pq + q2 = 1

• q represents the frequency for the recessive allele (a)

• P represents the frequency for the homozygous dominant allele (A)

• pq represents the frequency for heterozygous (Aa)

Solving allele frequency with Hardy-Weinberg Equation

Remember that p + q = 1

Always solve for q first and then p. Why?

Example. In a population of mice, the recessive brown coat has an allele frequency of .36.

We need to solve for q. .36 represents q2. Why?

To Solve for q take the √ of .36 q = How would you solve for p? (Remember p+q =1)

How would you determine the allele frequency for heterozygous?

Within a population of butterflies, the color brown (B) is dominant over the color white (b). And, 40% of all butterflies are white. Calculate the following:

The frequency of the recessive allele.

The frequency of homozygous dominant individuals.

The percentage of butterflies in the population that are heterozygous.

In the U.S., 16% of the population is Rh-, due to a homozygous recessive pair of alleles. From this data determine:

a. The frequency of the recessive allele

b. the frequency of the dominant allele

c. The percentage of homozygous dominants and percentage of heterozygotes in the population.

Allele W for white wool is dominant over allele w for black wool. In a sample of 900 sheep, 891 are white and 9 are black. Estimate the allele frequencies in this sample.

For w:

For W:

For Ww:

In a given population, only the "A" and "B" alleles are present in the ABO system; there are no individuals with type "O" blood or with O alleles in this particular population. If 200 people have type A blood, 75 have type AB blood, and 25 have type B blood, what are the allele frequencies of this population (i.e., what are p and q)?


Speciation is the formation of a new species. A species is a population whose members can interbreed and produce fertile offspring.

How does one species become two?

When populations become reproductively isolated, they can evolve into two separate species. Reproductive isolation can develop in a variety of ways, including behavioral isolation, or geographic isolation.


Isolating Mechanisms

Reproductive isolation occurs when a population splits into two groups and the two populations no longer interbreed. When populations become reproductively isolated, they can evolve into two separate species.


Behavioral IsolationBehavioral isolation occurs when two populations that are capable of interbreeding develop differences in courtship rituals or other behaviors.

http://www.allaboutbirds.org/guide/Eastern_meadowlark/id

http://www.allaboutbirds.org/guide/western_meadowlark/id


Geographic isolation occurs when two populations are separated by geographic barriers such as rivers, mountains, or bodies of water. An example: Kaibab squirrel is a subspecies of the Abert’s squirrel that formed when a small population became isolated on the north rim of the Grand Canyon. Why would this cause speciation?


Testing Natural Selection in Nature

What did the Grants’ scientific investigation show about Galápagos finches? (pages 496-497)

Founder effect – allele frequencies change as a result of the migration of a small subgroup of a population.

Variation within a species increases the likelihood that the species can adapt and survive environmental change.


Testing Natural Selection in Nature

Darwin hypothesized that the Galápagos finches had descended from a common ancestor. He proposed that natural selection shaped the beaks of different bird populations as they became adapted to eat different foods.


A Testable Hypothesis

Peter and Rosemary Grant from Princeton University realized that Darwin’s hypothesis rested on two testable assumptions: For beak size and shape to evolve, there must be enough heritable variation in those traits to the material for natural selection.• research found that there was a lot of diversity Differences in beak size and shape must produce differences in fitness.


Natural SelectionThe Grants’ data showed individual finches with different-sized beaks had different chances of surviving drought. When food was scarce, individuals with the largest beaks were more likely to survive. The Grants observed that average beak size in that finch population increased dramatically over time.


Founders Arrive

Many years ago, a few finches from South America—species M—arrived on one of the Galápagos islands. Because of the founder effect, the allele frequencies of this founding finch population could have differed from those in the South American population.

What is a current hypothesis about Galápagos finch speciation?


Geographic Isolation

Over time, GI and NS enabled the island finch population to evolve into a new species – Species A. These species (A) crossed to another island.

As a result we have two populations who are geographically isolated.


Changes in Gene Pools

Over time, populations on each island adapted to local environments. Natural selection could have caused two distinct populations to evolve (A and B), each characterized by a new phenotype.


Behavioral Isolation

Over time, populations on each island adapted to local environments. A few birds (B) come back to main island, but since they have evolved differently, they don’t mate with each other.

We now have two distinct species.


Competition and Continued Evolution

Birds that are most different from each other have the highest fitness. More specialized birds have less competition for food. Over time, species evolve in a way that increases the differences between them, and new species may evolve (C, D, and E).

This process could have produced the `13 different finch species found today.

Population Genetics Ch 17 and Ch 18.2. Genes and Variation Combine Darwin’s theory of Natural...

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