16-2 Evolution As Genetic Change

Discuss w/ Partner:Use figure 16-5 on page 397 to answer the

following questions1. How does color affect the fitness of the

lizards?2. What would you predict the lizard

population will look like by generation 50? Explain.

Answers:1. How does color affect the fitness of the

lizards? Both red and brown lizards are less fit than the black lizards.

2. What would you predict the lizard population will look like by generation 50? Explain. There will be no red lizards, less brown lizards and mostly black lizards by generation 50.

I. Natural Selection on Single-Gene Traits

1. Can result in evolution b/c of changes in allele frequencies.

Ex: organisms of one color may produce fewer offspring than organisms of other colors

II. Genetic Drift

1. Genetic Drift- random changes in allele frequencies that happens in small populations

a) may occur when a small group of individuals colonizes a new habitat

2. How does this happen?a) In each generation,

some individuals may, just by chance, leave behind a few more descendants (and genes) than other individuals.

b) The genes of the next generation will be the genes of the “lucky” individuals, not necessarily the healthier or “better” individuals

DescendantsPopulation A Population B

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

II. Evolution Vs. Genetic Equilibrium

1. To figure out how evolution happens scientists learned what happens when there are no changes

2. Hardy-Weinburg- the frequencies of alleles in a population don’t change unless one or more factors make the frequencies change.

3. Genetic Equilibrium- happens when allele frequencies in a population do not change.

4. The population will not evolve if this happens

4. Five conditions are needed to maintain genetic equilibrium from generation to generation

a) Must be random matingAll members of a

population have an equal chance to reproduce

There is no sexual selection

Sexual selection-acts on an organism's ability to obtain (often by any means necessary!) or successfully copulate with a mate.

b) Population has to be Large

Genetic drift has less effect on larges populations

c) No movement in or out of the population

Individuals can’t enter a population bringing their new alleles with them

Individuals can’t leave a population taking their alleles with them

d) No mutationIf mutations occur

the genes change from one form to another

New alleles may be introduced to the population

All frequencies will change.

e) No Natural Selection

Every genotype in the population has to have the same possibility of survival and reproduction

No phenotype can have a selective advantage over another

Discuss w/ PartnerIn addition to allele frequencies remaining

constant when a population is in genetic equilibrium, genotype proportions also remain constant and can be calculated from the allele frequencies. If p is the frequency of an allele A for a trait and q is the frequency of allele a for the same trait, then genotype proportions are give by (p+q)2 = p2 (AA) + 2pq (Aa) + q2 (aa)

If a population is an genetic equilibrium and the value of p is 0.3 , what proportion of the population has each genotype?

Answer:If a population is an genetic

equilibrium and the value of p is 0.3 , what proportion of the population has each genotype? The proportion of AA individuals is p2, or 0.09; the proportion of Aa individuals is 2pq, or 0.42; and the proportion of aa individuals is p2 or 0.49)

The genotype proportions must add up to 1.00

Discuss w/ PartnerSelection for heterozygous can also lead to equilibrium

in allele frequencies. Ex: In some African populations where malaria is

prevalent, heterozygous for sickle cell hemoglobin have the highest fitness, b/c they are somewhat resistant to malaria and largely unaffected by sickle cell anemia. Homozygous for sickle cell hemoglobin have the lowest fitness, because they have sickle cell anemia. Normal homozygous have somewhat reduced fitness, b./c they have not resistance for malaria. As a result, the allele for sickle cell hemoglobin persists in these populations.

What do you think would happen to the sickle cell allele in these populations if malaria were eradicated (gotten rid of)?

Answer:What do you think would happen to

the sickle cell allele in these populations if malaria were eradicated (gotten rid of)? The allele would be selected against and become less common.