Chapter 20Genes Within Populations

AP Biology 2012

20.1 Genetic Variation and Evolution

Evolution – changes over timeDarwin – descent with modification

Darwin’s mechanism is natural selectionorganisms with a desirable characteristic produce more offspring that live than

those that do not

Opposing theory – inheritance of acquired characteristics – giraffe example

Based on genetics – which mechanism makes the most sense?

Enzyme polymorphismpolymorphism – the presence in a population of more than one allele of a gene at a frequency greater than that of newly arising mutations

Common in insects and plants

20.2 Changes in Allele Frequency

Was once believed that genetic variation was blended from one generation to the next

- blending inheritanceThought that variation was lost due to blending

Question – Wouldn’t all members of a population eventually only exhibit dominant traits?

Hardy-Weinberg Principle – proportions of a genotype remain constant as long as:

1. No mutation takes place2. No immigration or emigration takes

place3. Random mating is occurring4. The population size is very large5. No selection occurs

Hardy-Weinberg Equation

(p + q)2 = p2 + 2pq + q2

Where did we see this before?

Cat Example:If 16% of cats are white and white is recessive Then q2 = 0.16 q = 0.4

p = 0.6 b/c p + q = 1

Genotypic Frequencies would be:BB p2 = (0.6)2 = .36 or 36%Bb 2pq = 2(0.6)(0.4) = .48 or 48%

HW can be used to find evidence for evolution Populations changeNatural Selection occursMating is not randomImmigration and emigration

20.3 Five Agents of Evolutionary Change

1. Mutation changes allelesmutation rates are very low

1/100,000 cell divisionshowever – ultimate source of change

*mutation is not a result of natural selection

2. Gene flow occurs when alleles move between populations

a. New organism with different alleles comes to the area

b. gametes spread – seeds or pollenc. mating between adjacent populations

3. Nonrandom mating shifts genotype frequencies

a. assortative mating – phenotypically similar individuals mate – increases homozygous

b. disassortative mating – phenotypically different individuals mate – increases heterozygous

4. Genetic driftchange in genetic frequencies

A. Founder Effect – few individuals “found” a new population, alleles they do not have are lostB. Bottleneck Effect – few individuals survive and then produce the new population

5. Selection favors some genotypes over othersA. Artificial selectionB. Avoiding predatorsC. Matching climatic conditionsD. Pesticide resistance

20.4 Fitness and Its Measurement

A phenotype with greater fitness usually increases in frequency

Why?

Toad Example2 phenotypes of toads – green and brown

Green 4.0 to next generationBrown 2.5 to the next generationG – 4.0/4.0 = 1 B – 2.5/4.0 = 0.625What do you think should eventually happen?

20.5 Interactions Among Evolutionary Forces

Sometimes drift doesn’t favor the allele favored by selection?

How would you explain this statement?

Gene flow may promote or constrain evolutionary change

How would you explain this?

20.6 Maintenance of Variation

Frequency-dependent selection – fitness of a phenotype depends on its frequency within the population

Negative frequency-dependent selection – rare phenotypes are selected

Positive frequency-dependent selection – common phenotypes are selected

Oscillating selection – favored phenotype changes as the environment changesExample – Ground Finch

A. large bills favored during droughtB. small bills favored during wet seasons

In some cases heterozygous may exhibit greater fitness than homozygous

Example – Sickle Cell AnemiaHeterozygous individuals are resistant to malaria

20.7 Selection Acting on Traits Affected by Multiple Genes

Disruptive Selection removes intermediatesExample

African black-bellied seedcracker finchSome individuals have large beaks –

for large seedsSome have small beaks – for small

seedsNo intermediates – no medium seeds

Directional Selection – selection that eliminates an extreme, causing those genes to become less frequentExample – Fruit flies

Some have genes that cause them to move toward light – often leading to death

Over time less flies are moving toward light

Stabilizing Selection – selection that eliminates both extremes, increase of the common phenotypeExample – Infant birth weights

Highest survival is found between 6 & 7 lbs

20.8 Experimental Studies of Natural Selection

In groups• Identify the biology of the guppy• Identify why the guppy is a good organism to

study• Describe the laboratory experiment• Describe the field experiment• What were the results

20.9 The Limits of Selection

What we cannot do:Chickens cannot lay larger or more eggs

b/c shells would become thinnerRacehorses cannot get any faster

b/c we bred them faster than mutations occur – no faster horses in 50 yrs

Some selection on one gene can be affected by another -- seen in epistasis

Epistasis – when one gene modifies the effect of another – example – fur color in cats