Ch. 21 Warm-Up Define the following terms: population, population genetics, gene pool, fitness What does Hardy-Weinberg indicate? What is 1 thing you are grateful for today?

Warm-Up In a population of 200 mice, 98 are homozygous dominant for brown coat color (BB), 84 are heterozygous (Bb), and 18 are homozygous (bb). The allele frequencies of this population are: B allele: ___ b allele: ___ The genotype frequencies are: BB: ___ Bb: ___ bb: ___ Use the above info to determine the genotype frequencies of the next generation: B (p): ___ b (q): ___ BB (p2): ___ Bb (2pq): ___ bb (q2): ___

Warm-Up The scarlet tiger moth has the following genotypes. Calculate the allele and genotype frequencies (%) for a population of 1612 moths. AA = 1469 Aa = 138 aa = 5 A = ___ a = ___ AA = Aa = aa =

Warm-Up Use the following information to help you answer the question below: Population = 1000 people AA = 160 Aa = 480 aa = 360 What are the genotype ratios? Allele frequencies? Use directional, stabilizing or disruptive selection to explain the following: Butterflies in 2 different colors that resemble other species Birds produce 4-5 eggs per clutch Average human baby weighs 7 lbs. Darwin’s finches and beak size during drought

Warm-Up Determine if the following examples are directional, disruptive, or stabilizing selection: Butterflies in 2 different colors each represent a species distasteful to birds Brightly colored birds mate more frequently than drab birds of same species Fossil evidence of horse size increasing over time Average birth weight is 7 lbs.

The Evolution of Populations Chapter 21 The Evolution of Populations

What you must know: Mutations are the only source of new genes. The three ways in which sexual reproduction produces genetic variation. The conditions for Hardy-Weinberg equilibrium. How to use the Hardy-Weinberg equation to calculate allele frequencies to test whether a population is evolving. What effects genetic drift, migration, or selection may have on a population, and analyze data to justify your predictions.

Chapter 21 The Evolution of Populations Part A (Sec. 21.1 & 21.2): Microevolution and the Hardy-Weinberg Equation

What you must know: Mutations are the only source of new genes. The three ways in which sexual reproduction produces genetic variation. The conditions for Hardy-Weinberg equilibrium. How to use the Hardy-Weinberg equation to calculate allele frequencies to test whether a population is evolving.

The evolutionary impact of natural selection is only apparent in the changes in a population of organisms over time.

Microevolution Evolution on smallest scale Change in allele frequencies in a population over generations.

Mechanisms that cause allele frequency change:

Genetic Variation Darwin’s could not explain how inherited variations are maintained in populations - not “trait blending” A few years after Darwin’s “Origin of Species”, Gregor Mendel proposed his hypothesis of inheritance: Parents pass on discrete heritable units (genes) that retain their identities in offspring - Genetic variation: differences among individuals in the composition of their genes or other DNA sequence.

Without genetic variation, evolution cannot occur.

Variation between populations Geographic variation: differences in the genetic composition of separate populations.

Sources of Genetic Variation Mutations = only source of new genes and new alleles Mutations in gametes passed to offspring Point mutations Chromosomal mutations  gene duplication Fast reproduction Normally every1/100,000 genes per generation in animals and plants in prokaryotes: mutations can quickly generate genetic variation( short life- span), the same in viruses. Example : HIV has a 2 days life span, with RNA which ahs a higher rate of mutation than DNA( lack of repair mechanisms in the host for RNA)- Most effective medicine for it is drug” cocktail”( combination of several drugs). Sexual reproduction: shuffle existing alleles: Crossing over, independent assortment, random fertilization

Example

Population: group of individuals of same species that live in the same area and interbreed, producing fertile offspring Population genetics: study of how populations change genetically over time

Gene Pool All alleles at all loci in all the members of a population Fixed allele: all members of a population are homozygous for same allele More fixed alleles  less genetic diversity

Hardy-Weinberg Equilibrium Describes a population that is NOT evolving Frequencies of alleles & genotypes in a population’s gene pool remain constant over generations, provided that only Mendelian segregation and recombination of alleles are at work.

Conditions for Hardy-Weinberg Equilibrium No mutations. Random mating. (no sexual selection) No natural selection. Extremely large population size. (no genetic drift) No gene flow. (no emigration, immigration) If ANY of these conditions are NOT met  Microevolution occurs!

Applying the Hardy-Weinberg Equation

p + q = 1 Hardy-Weinberg Equation Note: 1 – p = q 1 – q = p Allele Frequencies: Gene with 2 alleles : p, q p = frequency of A, dominant allele q = frequency of a, recessive allele Note: 1 – p = q 1 – q = p p + q = 1

p2 + 2pq + q2 = 1 Hardy-Weinberg Equation Genotype Frequencies: 3 genotypes (AA, Aa, aa) p2 + 2pq + q2 = 1 p2 = AA (homozygous dominant) 2pq = Aa (heterozygous) q2 = aa (homozygous recessive)

Example 1: PTC Tasters Tasters (TT or Tt), Nontasters (tt) Tasters = ____ Nontasters = ____ Total = ____ q2 = q = p + q = 1 p = 1 – q = p2 + 2pq + q2 = 1

Example 2: Plant Population Suppose in a plant population, red flowers (R) is dominant to white flowers (r). In a population of 500 individuals, 25% show the recessive phenotype. How many individuals would you expect to be homozygous dominant and heterozygous for this trait?

Chapter 21 The Evolution of Populations Part B (Sec. 21.3 & 21.4): Mechanisms of Evolution and Types of Selection

What you must know: What effects genetic drift, migration, or selection may have on a population, and analyze data to justify your predictions.

Hardy-Weinberg Equilibrium In nature, it is NOT likely all the conditions for H-W Equilibrium will be met  Populations are evolving Allele/genotype frequency changes due to mutations and nonrandom mating are minor Three MAJOR mechanisms of evolution: Natural Selection Genetic Drift Gene Flow

Mechanisms of Microevolution Natural Selection: differential reproductive success Rock Pocket Mice

Mechanisms of Microevolution Genetic Drift: unpredictable fluctuation of alleles from one generation to next Significant genetic drift in small populations Allele frequencies change at random. It can cause the frequency of such an allele to decrease and also can cause it to slightly increase. Can lose genetic variation in populations. Can cause harmful alleles to become fixed Types: Founder Effect Bottleneck Effect

Genetic drift

Certain alleles may be over/under represented Bottleneck Effect: severe drop in population size because of sudden changes in environment( flood, fire). Certain alleles may be over/under represented Human actions sometimes create severe bottlenecks for other species Elephant seals have reduced genetic variation due to hunting Florida panthers in danger of extinction

Polydactyly in Amish population Founder Effect: few individuals become isolated from larger population, they may establish a new population whose gene pool differs from the source population.  certain alleles over/under represented. Blown by storm in new island High frequency of certain inherited disorder in human Polydactyly in Amish population

Mechanisms of Microevolution Gene Flow: population gains/loses alleles due to immigration or emigration. Increase the frequencies of alleles that improve the match between organisms and their environment, but neither does so consistently. May introduce alleles that are advantageous or ones that are disadvantageous. Worldwide spread of insecticide-resistant alleles in Culex pipiens mosquitoes (West Nile vector)

A Closer Look at Natural Selection

Relative Fitness: contribution an individual makes to the gene pool of the next generation relative to contribution of others Natural selection can occur in 3 ways: Directional Selection Disruptive Selection Stabilizing Selection

Directional Selection: Eg Directional Selection: Eg. beak sizes of birds during wet/dry seasons in Galapagos Disruptive Selection: Eg. small beaks for small seeds; large beaks for large seeds Stabilizing Selection: Eg. average human birth weight

The key role of natural selection in adaptive evolution Examples: Cuttlefish: change color rapidly, blend into different backgraounds. Jaws of snakes: allo them to swallow prey much larger than their own head. Version of an enzyme that shows improved function in cold environments. These adaptations can arise gradually over time as natural selection increases the frequencies of alleles that enhance survival and reproduction. As the proportion of individuals that have favorable traits increases, the match between a species and its environment improves; that is , adaptive evolution occurs. Adaptive evolution is a continuous , dynamic process.

Balancing Selection Diploidy: inherit 2 alleles Recessive alleles hidden in heterozygotes Heterozygote advantage: heterozygotes have better survival Eg. heterozygotes for sickle cell anemia protected against malaria

Sexual Selection Certain individuals more likely to obtain mates Sexual dimorphism: difference between 2 sexes (size, color, ornamentation, behavior) Intra sexual selection: competition within same sex Intersexual selection: mate choice Sexual selection may lead to pronounced secondary differences between the sexes

Natural selection cannot fashion perfect organisms. Selection can only edit existing variations. Evolution is limited by historical constraints. Adaptations are often compromises. Chance, natural selection, and the environment interact.

BioFlix: Mechanisms of Evolution