Chapter 20 Genes Within Populations AP Biology 2012

20.1 Genetic Variation and Evolution Evolution – changes over time Darwin – descent with modification Darwin’s mechanism is natural selection organisms 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 polymorphism polymorphism – 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 inheritance Thought 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 place 2. No immigration or emigration takes place 3. Random mating is occurring 4. The population size is very large 5. 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 change Natural Selection occurs Mating is not random Immigration and emigration

20.3 Five Agents of Evolutionary Change Mutation changes alleles mutation rates are very low 1/100,000 cell divisions however – ultimate source of change *mutation is not a result of natural selection

2. Gene flow occurs when alleles move between populations a 2. Gene flow occurs when alleles move between populations a. New organism with different alleles comes to the area b. gametes spread – seeds or pollen c. mating between adjacent populations

3. Nonrandom mating shifts genotype frequencies a 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 drift change in genetic frequencies A 4. Genetic drift change in genetic frequencies A. Founder Effect – few individuals “found” a new population, alleles they do not have are lost B. Bottleneck Effect – few individuals survive and then produce the new population

5. Selection favors some genotypes over others A 5. Selection favors some genotypes over others A. Artificial selection B. Avoiding predators C. Matching climatic conditions D. Pesticide resistance

20.4 Fitness and Its Measurement A phenotype with greater fitness usually increases in frequency Why?

Toad Example 2 phenotypes of toads – green and brown Green 4 Toad Example 2 phenotypes of toads – green and brown Green 4.0 to next generation Brown 2.5 to the next generation G – 4.0/4.0 = 1 B – 2.5/4.0 = 0.625 What 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 changes Example – Ground Finch A. large bills favored during drought B. small bills favored during wet seasons

In some cases heterozygous may exhibit greater fitness than homozygous Example – Sickle Cell Anemia Heterozygous individuals are resistant to malaria

20.7 Selection Acting on Traits Affected by Multiple Genes Disruptive Selection removes intermediates Example African black-bellied seedcracker finch Some individuals have large beaks – for large seeds Some have small beaks – for small seeds No intermediates – no medium seeds

Directional Selection – selection that eliminates an extreme, causing those genes to become less frequent Example – 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 phenotype Example – 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 thinner Racehorses 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