Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chapter 23: The Evolution of Populations

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 23.1 Variation in a natural population

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 23.2 Individuals are selected; populations evolve

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 23.4 Mendelian inheritance preserves genetic variation from one generation to the next Generation 1 C R genotype C W genotype Plants mate All C R C W (all pink flowers) 50% C R gametes 50% C W gametes Come together at random Generation 2 Generation 3 Generation 4 25% C R C R 50% C R C W 25% C W C W 50% C R gametes 50% C W gametes Come together at random 25% C R C R 50% C R C W 25% C W C W Alleles segregate, and subsequent generations also have three types of flowers in the same proportions

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 23.5 The Hardy-Weinberg theorem Gametes for each generation are drawn at random from the gene pool of the previous generation: 80% C R (p = 0.8)20% C W (q = 0.2) Sperm C R (80%) C W (20%) p2p2 64% C R 16% C R C W 16% C R C W 4% C W qp C R (80%) Eggs C W (20%) pq q2q2 p2p2 If the gametes come together at random, the genotype frequencies of this generation are in Hardy-Weinberg equilibrium: 64% C R C R, 32% C R C W, and 4% C W C W Gametes of the next generation: 64% C R from C R C R homozygotes 16% C R from C R C W homozygotes + = 80% C R = 0.8 = p 4% C W from C W C W homozygotes 16% C W from C R C W heterozygotes + = 20% C W = 0.2 = q With random mating, these gametes will result in the same mix of plants in the next generation: 64% C R C R, 32% C R C W and 4% C W C W plants

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 23.6 Mutations are the source of all heritable variation

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 23.7 Genetic drift CRCRCRCR CRCWCRCW CRCRCRCR CWCWCWCW CRCRCRCR CRCWCRCW CRCWCRCW CRCWCRCW CRCRCRCR CRCRCRCR Only 5 of 10 plants leave offspring CWCWCWCW CRCRCRCR CRCWCRCW CRCRCRCR CWCWCWCW CRCWCRCW CWCWCWCW CRCRCRCR CRCWCRCW CRCWCRCW Only 2 of 10 plants leave offspring CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR Generation 2 p = 0.5 q = 0.5 Generation 3 p = 1.0 q = 0.0 Generation 1 p (frequency of C R ) = 0.7 q (frequency of C W ) = 0.3

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 23.8 The bottleneck effect Original population Bottlenecking event Surviving population (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. (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.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 23.9 Nonheritable variation within a population (a)Map butterflies that emerge in spring: orange and brown (b)Map butterflies that emerge in late summer: black and white

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure Geographic variation in chromosomal mutations XX XX

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure Does geographic variation in yarrow plants have a genetic component? EXPERIMENT Researchers observed that the average size of yarrow plants (Achillea) growing on the slopes of the Sierra Nevada mountains gradually decreases with increasing elevation. To eliminate the effect of environmental differences at different elevations, researchers collected seeds from various altitudes and planted them in a common garden. They then measured the heights of the resulting plants. RESULTS The average plant sizes in the common garden were inversely correlated with the altitudes at which the seeds were collected, although the height differences were less than in the plants’ natural environments. CONCLUSION The lesser but still measurable clinical variation in yarrow plants grown at a common elevation demonstrates the role of genetic as well as environmental differences. Mean height (cm) Altitude (m) Heights of yarrow plants grown in common garden Seed collection sites Sierra Nevada Range Great Basin Plateau

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure Modes of selection (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. (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. Frequency of individuals Phenotypes (fur color) Original population Original population Evolved population

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure Mapping malaria and the sickle-cell allele 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% Distribution of malaria caused by Plasmodium falciparum (a protozoan)

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure Sexual dimorphism and sexual selection

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure The “reproductive handicap” of sex Asexual reproduction Female Sexual reproduction Female Male Generation 1 Generation 2 Generation 3 Generation 4