Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
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 • 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 Where does variation come from? • Mutation and sexual recombination produce the variation that makes evolution possible – Produce the variation in gene pools that contributes to differences among individuals Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mutation Rates • Mutation rates – Tend to be low in animals and plants – Average about one mutation in every 100,000 genes per generation – Are more rapid in microorganisms Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sexual Recombination • In sexually reproducing populations, sexual recombination – Is far more important than mutation in producing the genetic differences that make adaptation possible Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Altering allele frequencies in a population • Three major factors alter allele frequencies and bring about most evolutionary change – Natural selection – Genetic drift – Gene flow Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Genetic drift – Chance events cause allele frequencies to fluctuate unpredictably from one generation to the next – Tends to reduce genetic variation – More likely in small populations 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 Figure 23.7 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Only 2 of 10 plants leave offspring CRCR CRCR Generation 3 p = 1.0 q = 0.0 The Bottleneck Effect (one example of genetic drift) • 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 a population after some environmental disaster. By chance, blue 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 genetic variation, as in these northern elephant seals in California that were once hunted nearly to extinction. Figure 23.8 B Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Founder Effect (a second example of genetic drift) • The founder effect – Occurs when a few individuals become isolated from a larger population – Can affect allele frequencies in a population Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gene 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 Natural Selection • Natural selection is the primary mechanism of adaptive evolution • Natural selection – Accumulates and maintains favorable genotypes in a population Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A Closer Look at Natural Selection • From the range of variations available in a population – Natural selection increases the frequencies of certain genotypes, fitting organisms to their environment over generations Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Directional, Disruptive, and Stabilizing Selection • Selection – Favors certain genotypes by acting on the phenotypes of certain organisms • Three modes of selection are – Directional – Disruptive – Stabilizing Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Directional selection – Favors individuals at one end of the phenotypic range • Disruptive selection – Favors individuals at both extremes of the phenotypic range • Stabilizing selection – Favors intermediate variants and acts against extreme phenotypes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The three modes of selection Original population Original population Phenotypes (fur color) Evolved population (a) Directional selection shifts the overall makeup of the population by favoring variants at one extreme of the distribution. In this case, darker mice are favored because they live among dark rocks and a darker fur color conceals them from predators. (b) Disruptive selection favors variants at both ends of the distribution. These mice have colonized a patchy habitat made up of light and dark rocks, with the result that mice of an intermediate color are at a disadvantage. Fig 23.12 A–C Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (c) Stabilizing selection removes extreme variants from the population and preserves intermediate types. If the environment consists of rocks of an intermediate color, both light and dark mice will be selected against. 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 1 Female Female Generation 2 Male Generation 3 Generation 4 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 0 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 Why Natural Selection Cannot Fashion Perfect Organisms • Evolution is limited by historical constraints • Adaptations are often compromises • Chance and natural selection interact • Selection can only edit existing variations Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings