Chapter 23 The Evolution of Populations PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings • Overview: The Smallest Unit of Evolution • One common misconception about evolution is that individual organisms evolve, in the Darwinian sense, during their lifetimes • Natural selection acts on individuals, but populations evolve Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings • Genetic variations in populations – Contribute to evolution Figure 23.1 Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings • Concept 23.1: Population genetics provides a foundation for studying evolution • Microevolution – Is change in the genetic makeup of a population from generation to generation Figure 23.2 Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings The Modern Synthesis • Population genetics – Is the study of how populations change genetically over time – Reconciled Darwin’s and Mendel’s ideas Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings • The modern synthesis – Integrates Mendelian genetics with the Darwinian theory of evolution by natural selection – Focuses on populations as units of evolution Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings Gene Pools and Allele Frequencies • A population MAP AREA CANADA ALASKA – Is a localized group of individuals that are capable of interbreeding and producing fertile offspring Beaufort Sea Porcupine herd range N TE OR RR TH IT WE O S RI T ES Fortymile herd range ALASKA YUKON • Fairbanks • Whitehorse Figure 23.3 Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings • The gene pool – Is the total aggregate of genes in a population at any one time – Consists of all gene loci in all individuals of the population Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings The Hardy-Weinberg Theorem • The Hardy-Weinberg theorem – Describes a population that is not evolving – States that the frequencies of alleles and genotypes in a population’s gene pool remain constant from generation to generation provided that only Mendelian segregation and recombination of alleles are at work Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings • Mendelian inheritance – Preserves genetic variation in a population Generation CWCW CRCR genotype genotype Plants mate Generation All CRCW (all pink flowers) 50% CR gametes 50% CW gametes Come together at random Generation 25% CRCR 50% CRCW 50% CR gametes 25% CWCW 50% CW gametes Come together at random Generation 25% CRCR Figure 23.4 Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings 50% CRCW 25% CWCW Alleles segregate, and subsequent generations also have three types of flowers in the same proportions The Preservation of Genetic Variation • Various mechanisms help to preserve genetic variation in a population Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings Diploidy • Diploidy – Maintains genetic variation in the form of hidden recessive alleles Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings Balancing Selection • Balancing selection – Occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population – Leads to a state called balanced polymorphism Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings Heterozygote Advantage • Some individuals who are heterozygous at a particular locus – Have greater fitness than homozygotes • Natural selection – Will tend to maintain two or more alleles at that locus Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings • The sickle-cell allele – Causes mutations in hemoglobin but also confers malaria resistance – Exemplifies the heterozygote advantage Distribution of malaria caused by Plasmodium falciparum (a protozoan) Figure 23.13 Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings 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% • Frequency-Dependent Selection • In frequency-dependent selection – The fitness of any morph declines if it becomes too common in the population Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings • An example of frequency-dependent selection On pecking a moth image the blue jay receives a food reward If the bird does not detect a moth on either screen, it pecks the green circle to continue to a new set of images (a new feeding opportunity) Parental population sample Experimental group sample Phenotypic diversity 0.06 0.05 0.04 Frequencyindependent control 0.03 0.02 Plain background Patterned background Figure 23.14 Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings 20 60 40 80 Generation number 100 Neutral Variation • Neutral variation – Is genetic variation that appears to confer no selective advantage Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings Sexual Selection • Sexual selection – Is natural selection for mating success – Can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings • Intrasexual selection – Is a direct competition among individuals of one sex for mates of the opposite sex Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings • Intersexual selection – Occurs when individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex – May depend on the showiness of the male’s appearance Figure 23.15 Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings The Evolutionary Enigma of Sexual Reproduction • Sexual reproduction – Produces fewer reproductive offspring than asexual reproduction, a so-called reproductive handicap Sexual reproduction Asexual reproduction Generation Female Female Generation Male Generation Generation Figure 23.16 Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings • If sexual reproduction is a handicap, why has it persisted? – It produces genetic variation that may aid in disease resistance Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings Why Natural Selection Cannot Fashion Perfect Organisms • Evolution is limited by historical constraints • Adaptations are often compromises Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings • Chance and natural selection interact • Selection can only edit existing variations Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings [...]... of 10 plants leave offspring CRCR CRCR Generation 3 p = 1.0 q = 0.0 The Bottleneck Effect • In the bottleneck effect – A sudden change in the environment may drastically reduce the size of a population – The gene pool may no longer be reflective of the original population’s gene pool (a) Shaking just a few marbles through the narrow neck of a bottle is analogous to a drastic reduction in the size of. .. mating, these gametes will result in the same mix of plants in the next generation: Figure 23.5 64% CRCR, 32% CRCW and 4% CWCW plants Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings • If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then – p2 + 2pq + q2 = 1 – And p2 and q2 represent the frequencies of the homozygous... generation are drawn at random from the gene pool of the previous generation: 80% CR (p = 0.8) 20% CW (q = 0.2) Sperm CR (80%) CW (20%) pq CR (80%) p2 64% CRCR CW (20%) Eggs p2 16% CRCW 16% CRCW qp 4% CWCW q2 If the gametes come together at random, the genotype frequencies of this generation are in Hardy-Weinberg equilibrium: 64% CRCR, 32% CRCW, and 4% CWCW Gametes of the next generation: 16% CR from... 2pq represents the frequency of the heterozygous genotype Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings Conditions for Hardy-Weinberg Equilibrium • The Hardy-Weinberg theorem – Describes a hypothetical population • In real populations – Allele and genotype frequencies do change over time Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings • The five conditions... conditions for non-evolving populations are rarely met in nature – Extremely large population size – No gene flow – No mutations – Random mating – No natural selection Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings Population Genetics and Human Health • We can use the Hardy-Weinberg equation – To estimate the percentage of the human population carrying the allele for an inherited... Flow • Gene flow – Causes a population to gain or lose alleles – Results from the movement of fertile individuals or gametes – Tends to reduce differences between populations over time Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings • Concept 23.4: Natural selection is the primary mechanism of adaptive evolution • Natural selection – Accumulates and maintains favorable genotypes... drift – Describes how allele frequencies can fluctuate unpredictably from one generation to the next – Tends to reduce genetic variation CWCW CRCR CRCR Only 5 of 10 plants leave offspring CRCW CWCW CRCR CRCR CRCW CWCW CRCR CRCW CRCW CRCR CWCW CRCW CRCR CRCR CRCW Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3 CRCR CRCR CRCR CRCR CRCR CRCR CRCR CRCR CRCW CRCW Generation 2 p = 0.5 q = 0.5... Mutation and sexual recombination produce the variation that makes evolution possible • Two processes, mutation and sexual recombination – Produce the variation in gene pools that contributes to differences among individuals Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings Mutation • Mutations – Are changes in the nucleotide sequence of DNA – Cause new genes and alleles to arise... marbles are over-represented in the new population and gold marbles are absent Original population Figure 23.8 A Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings Bottlenecking event Surviving population • Understanding the bottleneck effect – Can increase understanding of how human activity affects other species (b) Similarly, bottlenecking a population of organisms tends to reduce... Cummings Natural Selection • Differential success in reproduction – Results in certain alleles being passed to the next generation in greater proportions Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings Genetic Drift • Statistically, the smaller a sample – The greater the chance of deviation from a predicted result Copyright © 2005 Pearson Education, Inc publishing as Benjamin Cummings ... Is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals • Relative fitness – Is the contribution of a genotype to the. .. Benjamin Cummings • The modern synthesis – Integrates Mendelian genetics with the Darwinian theory of evolution by natural selection – Focuses on populations as units of evolution Copyright ©... altitudes and planted them in a common garden They then measured the heights of the resulting plants Mean height (cm) Heights of yarrow plants grown in common garden Atitude (m) RESULTS The average plant