Population Genetics: Populations change in genetic characteristics over time Ways to measure change: Allele frequency change (B and b) Genotype frequency change (BB, Bb, & bb)

Hardy-Weinberg Equation p + q = 1 p = the frequency of dominant allele (B) q = the frequency of recessive allele (b) Total frequency = 1 Assumes a 2 allele simple traits © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e

How to Calculate Frequencies Imagine there are 1,000 plants in a population Only two flower colors are possible: red (R = dominant) and white (r = recessive). Let there be 160 RR individuals (red homozygotes) 480 Rr individuals (red heterozygotes) 360 rr individuals (white homozygotes)

How to Calculate Frequencies Total population = 1,000 The genotype frequency is: RR = 160/1000 = .16 or 16% Rr = 480/1000 = .48 or 48% rr = 360/1000 = .36 or 36%

How to Calculate Frequencies Each plant has two alleles for flower color Since there are 1,000 individuals, there are 2,000 alleles 160 RR + 480 Rr + 360 rr = 1000 plants 2x160 RR + 2x480 Rr + 2x360 rr = 2000 alleles Allele frequency: R = (2 x 160 + 480 + 0) = 800/2000 = .40 (40%) r = (0 + 480 + 2 x 360) = 1200/2000 = .60 (60%)

How to Calculate Frequencies In 1908, Hardy & Weinberg independently discovered another way to calculate gene frequencies in an equilibrium population Equilibrium populations do not change, and assume: No mutation No gene flow (no movement of alleles in or out) No selection Mating is random The population is large

Given a Hardy-Weinberg equilibrium: Allele frequencies and genotype frequencies do not change in a population p + q = 1 p2 + 2pq + q2 = 1 160 RR + 480 Rr + 360 rr = 1000 plants p2= freq of genotype RR (p is freq of allele R) 2pq = freq of Rr q2 = freq of rr =.36 q = √¯ q2 = .6 (p = 1 – q)

Genetic Variation: The Raw Material of Change Genetic variation - differences in DNA sequences in a population Population changes occur because of these differences Two major sources of variation: Mutations Recombination

Four Mechanisms of Change Changes in allele frequencies in a population occur in four ways: Mutations Gene flow Genetic drift Natural selection Animation on mechanisms of change

Mutations Mutations are random events Ultimately, all new genetic variation comes from mutations Mutations can be harmful, beneficial, or neutral

Gene Flow Gene flow -movement of alleles between two different populations May result in new alleles being introduced or rare alleles being lost by migration

Genetic Drift The result of chance events Leads to random changes in allele frequencies The smaller the population, the greater the genetic drift By chance, the white allele at right decreased

Genetic Drift May reduce genetic variation May fix alleles that are neutral, harmful, or beneficial in a population Occurs in a genetic bottleneck

Genetic Bottlenecks Occur when there is a drop in population size Environmental disaster Founder effect New population is less genetically diverse than original population

Natural Selection When individuals with favorable traits have a higher reproductive rate Artificial selection is similar, but people pick the favorable trait not nature. Three types of Natural Selection: Directional selection Stabilizing selection Disruptive selection

Directional Selection Individuals of one extreme phenotype are favored Peppered moths

Stabilizing Selection Individuals with an intermediate phenotype are favored The extreme phenotypes are selected against Human birth weights

Disruptive Selection Where both extreme phenotypes are favored The intermediate phenotypes are selected against African seed cracker birds

Three Types of Natural Selection

Sexual Selection A form of natural selection Females place selective pressure on males Peacock’s feathers Lion’s mane

How Populations Change Mutations and recombination occur randomly These events create genetic variation Gene flow, genetic drift, and natural selection change allele frequencies Gene flow tends to equalize allele frequencies between 2 pops Genetic drift an natural selection tend to diverge allele frequencies between 2 pops

Animations on the causes of differences in the alpine skypilot population Differences in the population Variation in the population The variation is inherited There is variable survival There is variation in offspring production Conclusion Answer B is correct. If a squirrel has more than 3 pups in a littler, she has too many mouths to feed and their survival is in jeopardy. Having less than three pups in a litter may be an advantage to the single pup, but the mother will not be passing on as many of her genes compared to other mothers in the population.

Concept Quiz The average litter size for a squirrel is three. Above and below three is not favored. What type of selection is occurring? Disruptive selection Stabilizing selection Directional selection Answer B is correct. If a squirrel has more than 3 pups in a littler, she has too many mouths to feed and their survival is in jeopardy. Having less than three pups in a litter may be an advantage to the single pup, but the mother will not be passing on as many of her genes compared to other mothers in the population.

Concept Quiz A population of slugs has 230 individuals that are AA, 570 that are Aa, and 200 that are aa. What is the frequency of the A allele? .50 or 50% .65 or 65% .52 or 52% Answer C is correct. There are 1,000 total individuals in the population. Each individual has 2 alleles; there are 2,000 alleles in the population. 2 x 230 (AA) + 570 (Aa) + 0 (aa) = 1,030/2,000 = .52

Concept Quiz A few birds manage to start their own population on a new island. What best describes this situation? Genetic drift Gene flow Natural selection Answer A is correct. When a few individuals start their own population, this is known as the founder effect. The founder effect is a type of genetic drift. The birds that made it to the island are not necessarily better flyers; this is a random event. Therefore, natural selection is incorrect. Gene flow occurs when there is exchange of genes between two populations.

Process Animation 17.1 The Hardy-Weinberg Equation Macintosh Windows