The Evolution of Population

Figure (similar to the prior 3 years) 1978 (after drought) Average beak depth (mm) Natural selection acts on individuals within a population, population evolve!!!

Microevolution is a change in allele frequencies in a population over generations Three mechanisms cause allele frequency change: –Natural selection –Genetic drift –Gene flow Only natural selection causes adaptive evolution © 2011 Pearson Education, Inc.

Mutation, mutation, mutation!! Variation in heritable traits is a prerequisite for evolution –Result of variations in DNA sequence –Cause variations in phenotype –Natural selection acts on phenotype Driving Force of Evolution: Genetic Variation © 2011 Pearson Education, Inc.

XX XX 6.7 Variation Between Populations

© 2011 Pearson Education, Inc. Cline: a graded change in a trait along a geographic axis as a result of natural selection Ldh-B b allele frequency Maine Cold (6°C) Latitude (ºN) Georgia Warm (21ºC)

Formation of New Alleles Mutation can cause a change in an allele Only mutations in cells that produce gametes can be passed to offspring © 2011 Pearson Education, Inc.

Three mechanisms for shuffling alleles Point mutation Chromosomal mutation Sexual reproduction –Crossing over –Independent assortment –fertilization

Rapid Reproduction The average rate of eukaryotic mutation is about one in every 100,000 genes per generation Mutations rates are often lower in prokaryotes and higher in viruses © 2011 Pearson Education, Inc.

The Hardy-Weinberg equation can be used to test whether a population is evolving key terminology: Population: a localized group of individuals capable of interbreeding and producing fertile offspring Gene pool: all the alleles for all loci in a population –A locus is fixed if all individuals in a population are homozygous for the same allele © 2011 Pearson Education, Inc.

Figure 23.6 Porcupine herd Beaufort Sea Porcupine herd range Fortymile herd range Fortymile herd NORTHWEST TERRITORIES ALASKA CANADA MAP AREA ALASKA YUKON

Allelic Frequency: the percentage of the frequency of an allele at a locus in a population -The frequency of all alleles in a population will add up to 1 -For example, p + q = 1 Hardy-Weinberg Equation

320 red flowers (C R C R ) 160 pink flowers (C R C W ) 20 white flowers (C W C W ) Calculate the number of copies of each allele: –C R  (320  2)  160  800 –C W  (20  2)  160  200 To calculate the frequency of each allele: –p  freq C R  800 / (800  200)  0.8 –q  freq C W  200 / (800  200)  0.2 The sum of alleles is always 1 –0.8  0.2  1 © 2011 Pearson Education, Inc. Practice Problem: consider a population of wildflowers that is incompletely dominant for color:

Alleles in the population Gametes produced Each egg: Each sperm: 80% chance 20% chance 80% chance 20% chance Frequencies of alleles p = frequency of q = frequency of C W allele = 0.2 C R allele = 0.8 The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation -In a given population where gametes contribute to the next generation randomly, allele frequencies will not change -Mendelian inheritance preserves genetic variation in a population

Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool Consider, for example, the same population of 500 wildflowers and 100 alleles where –p  freq C R  0.8 –q  freq C W  0.2 The frequency of genotypes can be calculated –C R C R  p 2  (0.8) 2  0.64 –C R C W  2pq  2(0.8)(0.2)  0.32 –C W C W  q 2  (0.2) 2  0.04 The frequency of genotypes can be confirmed using a Punnett square © 2011 Pearson Education, Inc.

Figure 23.8b (20%)CRCR Sperm (80%) (20%) CRCR CWCW Eggs 64% (p 2 ) C R 16% (pq) C R C W 16% (qp) C R C W 4% (q 2 ) C W 64% C R C R, 32% C R C W, and 4% C W C W Gametes of this generation: 64% C R (from C R C R plants) 4% C W (from C W C W plants) 16% C R (from C R C W plants) + + Genotypes in the next generation: 16% C W (from C R C W plants) = = 80% C R = 0.8 = p 20% C W = 0.2 = q 64% C R C R, 32% C R C W, and 4% C W C W plants CWCW (80%)

© 2011 Pearson Education, Inc. Remember that the Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation

Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other loci © 2011 Pearson Education, Inc.

Mechanisms that alters allele frequencies: © 2011 Pearson Education, Inc.

Figure Generation 1 p (frequency of C R ) = 0.7 q (frequency of C W ) = 0.3 CRCRCRCR CRCRCRCR CRCWCRCW CWCWCWCW CRCRCRCR CRCWCRCW CRCRCRCR CRCWCRCW CRCRCRCR CRCWCRCW Genetic drift

Figure plants leave off- spring Generation 1 p (frequency of C R ) = 0.7 q (frequency of C W ) = 0.3 CRCRCRCR CRCRCRCR CRCWCRCW CWCWCWCW CRCRCRCR CRCWCRCW CRCRCRCR CRCWCRCW CRCRCRCR CRCWCRCW CRCRCRCR CWCWCWCW CRCWCRCW CRCRCRCR CWCWCWCW CRCWCRCW CWCWCWCW CRCRCRCR CRCWCRCW CRCWCRCW Generation 2 p = 0.5 q = 0.5 Genetic drift

Figure plants leave off- spring Generation 1 p (frequency of C R ) = 0.7 q (frequency of C W ) = 0.3 CRCRCRCR CRCRCRCR CRCWCRCW CWCWCWCW CRCRCRCR CRCWCRCW CRCRCRCR CRCWCRCW CRCRCRCR CRCWCRCW CRCRCRCR CWCWCWCW CRCWCRCW CRCRCRCR CWCWCWCW CRCWCRCW CWCWCWCW CRCRCRCR CRCWCRCW CRCWCRCW Generation 2 p = 0.5 q = plants leave off- spring CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR Generation 3 p = 1.0 q = 0.0 Genetic drift

The Founder Effect © 2011 Pearson Education, Inc.

Figure Original population The Bottleneck Effect

Figure Original population Bottlenecking event The Bottleneck Effect

Figure Original population Bottlenecking event Surviving population The Bottleneck Effect

Case Study: Impact of Genetic Drift on the Greater Prairie Chicken Loss of prairie habitat caused a severe reduction in the population of greater prairie chickens in Illinois The surviving birds had low levels of genetic variation, and only 50% of their eggs hatched © 2011 Pearson Education, Inc.

Figure Pre-bottleneck (Illinois, 1820) Post-bottleneck (Illinois, 1993) Greater prairie chicken Range of greater prairie chicken (a) Location Population size Number of alleles per locus Percentage of eggs hatched 93 < ,000–25,000 <50 750,000 75,000– 200,000 Nebraska, 1998 (no bottleneck) (b) Kansas, 1998 (no bottleneck) Illinois 1930–1960s 1993

Effects of Genetic Drift: A Summary 1.Genetic drift is significant in small populations 2.Genetic drift causes allele frequencies to change at random 3.Genetic drift can lead to a loss of genetic variation within populations 4.Genetic drift can cause harmful alleles to become fixed © 2011 Pearson Education, Inc.

Gene Flow Population in which the surviving females eventually bred Central Eastern Survival rate (%) Females born in central population Females born in eastern population Parus major Central population NORTH SEA Eastern population Vlieland, the Netherlands 2 km

Gene flow can increase the fitness of a population Consider, for example, the spread of alleles for resistance to insecticides –Insecticides have been used to target mosquitoes that carry West Nile virus and malaria –Alleles have evolved in some populations that confer insecticide resistance to these mosquitoes –The flow of insecticide resistance alleles into a population can cause an increase in fitness © 2011 Pearson Education, Inc.

Evolution by natural selection involves both change and “sorting” - New genetic variations arise by chance -Beneficial alleles are “sorted” and favored by natural selection Natural selection is the only mechanism that consistently causes adaptive evolution © 2011 Pearson Education, Inc.

Directional, Disruptive, and Stabilizing Selection Original population Phenotypes (fur color) Frequency of individuals Original population Evolved population (a) Directional selection (b) Disruptive selection(c) Stabilizing selection

Figure Sexual Selection

Preserving genetic variation in a population: Diploidy maintains genetic variation –Heterozygotes carry recessive alleles hidden from the effects of selection Balancing selection: natural selection maintains stable frequencies of two or more phenotypic forms in a population –Heterozygote advantage –Frequency-dependent selection © 2011 Pearson Education, Inc.

Figure Distribution of malaria caused by Plasmodium falciparum (a parasitic unicellular eukaryote) Key Frequencies of the sickle-cell allele 0–2.5% 2.5–5.0% 5.0–7.5% 7.5–10.0% 10.0–12.5% >12.5%

“Left-mouthed” P. microlepis “Right-mouthed” P. microlepis Sample year ’82 ’83 ’84 ’85 ’86’87 ’88 ’89 ’90 Frequency of “left-mouthed” individuals Figure 23.18

Why Natural Selection Cannot Fashion Perfect Organisms 1.Selection can act only on existing variations 2.Evolution is limited by historical constraints 3.Adaptations are often compromises 4.Chance, natural selection, and the environment interact © 2011 Pearson Education, Inc.