CHAPTER 13 How Populations Evolve Modules 13.13 – 13.22

13.13 Variation is extensive in most populations VARIATION AND NATURAL SELECTION 13.13 Variation is extensive in most populations Phenotypic variation may be environmental or genetic in origin But only genetic changes result in evolutionary adaptation

Many populations exhibit polymorphism and geographic variation Figure 13.13

13.14 Connection: Mutation and sexual recombination generate variation Parents A1 A1 A2 A3 MEIOSIS A1 A2 A3 Gametes FERTILIZATION Offspring, with new combinations of alleles A1 A2 A1 A3 and Figure 13.14

13.15 Overview: How natural selection affects variation Natural selection tends to reduce variability in populations The diploid condition preserves variation by “hiding” recessive alleles Balanced polymorphism may result from the heterozygote advantage

13.16 Not all genetic variation may be subject to natural selection Some variations may be neutral, providing no apparent advantage or disadvantage Example: human fingerprints Figure 13.16

13.17 Connection: Endangered species often have reduced variation Low genetic variability may reduce the capacity of endangered species to survive as humans continue to alter the environment Studies have shown that cheetah populations exhibit extreme genetic uniformity Thus they may have a reduced capacity to adapt to environmental challenges Figure 13.17

13.18 The perpetuation of genes defines evolutionary fitness An individual’s Darwinian fitness is the contribution it makes to the gene pool of the next generation relative to the contribution made by other individuals Production of fertile offspring is the only score that counts in natural selection

13.19 There are three general outcomes of natural selection Original population Frequency of individuals Phenotypes (fur color) Original population Evolved population Stabilizing selection Directional selection Diversifying selection Figure 13.19

13.20 Sexual selection may produce sexual dimorphism Sexual selection leads to the evolution of secondary sexual characteristics These may give individuals an advantage in mating Figure 13.20A, B

13.21 Natural selection cannot fashion perfect organisms This is due to: historical constraints adaptive compromises chance events availability of variations

13.22 Connection: The evolution of antibiotic resistance in bacteria is a serious public health concern The excessive use of antibiotics is leading to the evolution of antibiotic-resistant bacteria Example: Mycobacterium tuberculosis Figure 13.22