Bio Unit 8 Part 2.pptx

Unit 8 Notes

 Part 2

Evolution of Populations

A. Genetic Variation

1.

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

2.

Genetic variation is stored as alleles in a gene pool – all the alleles of a population

3.

Genetic variation is measured with allele frequencies – how common an allele is in a population

4.

Genetic variation happens by:

a. Mutations – a change in DNA sequence

Evolution of Populations

B. Natural Selection on Populations 

1.

Normal distribution – when the frequency of a trait is highest

at the mean value and decreases at each end of the

Evolution of Populations

a.

Phenotypes near the middle tend to be the most common

b.

But if environmental conditions change, they may favor the extreme ends (natural selection)!

Evolution of Populations

2.   Natural selection can then change the distribution of a

trait in 3 ways:

a.

Directional selection – causes a shift in a population's phenotypic distribution

1. An extreme phenotype that was uncommon now becomes common

Evolution of Populations

Evolution of Populations

Evolution of Populations

C. 3 Other Mechanisms of Evolution 

1.

Gene flow

a.

When an organism joins a new population and

reproduces, its alleles become part of that population's gene pool

b.

These alleles are then removed from the gene pool of its former population

c.

The movement of alleles from one population to another

d.

Increases genetic variation of the receiving population

Evolution of Populations

2.   Genetic Drift 

a.

The change in allele frequencies due to change

b.

Occurs in smaller populations

c.

Causes a loss of genetic diversity in a population

d.

2 causes:

       1. bottleneck effect

        a. genetic drift that occurs after an event greatly reducing       the population size

      b. example – overhunting

Evolution of Populations

2. founder effect

         a. genetic drift that occurs after a small number of

individuals colonize a new area

      b. example – Amish

e. problems of genetic drift:

1. population loses genetic variation

Evolution of Populations

3.   Sexual selection

a.

Certain traits increase mating success

b.

Some traits that increase mating success are not

always adaptive for their survival though!

c.

Males produce a lot of sperms, making their value

relatively small

Evolution of Populations

D. Hardy-Weinberg Equilibrium

1.

Godfrey Hardy and Wilhelm Weinberg

a. In 1908, they showed that genotype frequencies in a population stay the same over time as long as certain conditions are met

b. 5 conditions must be met for the population to stay in equilibrium:

1. Very large population – no genetic drift can occur

2. No emigration or immigration – no gene flow can occur

3. No mutations – no new alleles can be added to the gene pool 4. Random mating – no sexual selection can occur

Evolution of Populations

        c. Real populations rarely meet these requirements!

        d. The Hardy-Weinberg equation is used to predict genotype 

       frequencies in a population and to see how populations are             evolving

        e. If the genetic data does not match the equation, the         population is not in equilibrium , it is evolving

p2 +2pq + q2 = 1

Evolution of Populations

    Example:

       B (brown eyes) is dominant over b (blue eyes)

       A population has 2 people. Their genotypes are Bb and bb        The frequency of the B allele - ¼ or .25 or 25% (p)

       The frequency of the b allele - ¾ or .75 or 75% (q)         p + q = 1

Evolution of Populations

    Example:

       If there are 49% of a population with the recessive condition, 

       then what are the allele frequencies?

1. q2 _{= .49} 

2. q = .7

3. since p + q = 1, then p + .7 = 1

4. p = .3

5. p2 = 9% of population are AA

6. 2pq = 42% of population are Aa

Evolution of Populations

In summary: 5 Factors Leading to Evolution

1.

Genetic drift

2.

Gene flow

3.

Mutation

4.

Sexual selection

Evolution of Populations

E. Isolation of Populations

1. if gene flow stops between populations, the populations are said to be isolated

2. reproductive isolation

a. occurs when members of different populations can no longer mate successfully

b. final step in becoming separate species

Evolution of Populations

3. how isolation occurs:

a. behavioral barriers – isolation caused by differences in

courtship or mating behaviors

b. geographic barriers – physical barriers that divide a

population

1. rivers, mountains, dried lakebeds

2. over time, the isolated populations become

genetically different

Evolution of Populations

F. Natural Selection

1. natural selection – mechanism by which individuals that have inherited beneficial adaptations produce more offspring on average then do other individuals

a. nature is the “breeder” selecting for certain advantages and passing those traits down through heredity

Evolution of Populations

d. individuals with traits that are better adapted for their environment have a better chance of surviving and

reproducing

e. it pushes a population’s traits in an advantageous direction f. the environment controls the direction of natural selection

G. Extinction

1. the elimination of a species from the Earth