C.13 ppt - sexual selection

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13.15 Sexual selection may lead to phenotypic

differences between males and females

• _{Sexual selection}_{is a form of natural selection in which individuals}

with certain characteristics are more likely than other individuals to obtain mates.

• _{In many animal species, males and females may have secondary}

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13.15 Sexual selection may lead to phenotypic

differences between males and females

• _{In some species, individuals compete directly with members of the}

same sex for mates.

• _{This type of sexual selection is called}_{intrasexual selection}_{(within the}

same sex, most often the males).

• _{Contests may involve physical combat, but are more often ritualized}

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13.15 Sexual selection may lead to phenotypic

differences between males and females

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13.15 Sexual selection may lead to phenotypic

differences between males and females

• _{What is the advantage to females of being choosy?}

• _{One hypothesis is that females prefer male traits that are correlated}

with “good genes.”

• _{In several bird species, research has shown that traits preferred by females,} such as bright beaks or long tails, are related to overall male health.

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13.16 EVOLUTION CONNECTION: The evolution of

drug-resistant microorganisms is a serious public

health concern

• _{Antibiotics are drugs that kill infectious microorganisms.}

• _{During the 1950s, some medical experts even thought the age of}

human infectious diseases would soon be over.

• _{Why didn’t that optimistic forecast come true?}

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13.16 EVOLUTION CONNECTION: The evolution of

drug-resistant microorganisms is a serious public

health concern

• _{In the same way that pesticides select for resistant insects, antibiotics}

select for resistant bacteria.

• _{Again we see both the random and nonrandom aspects of natural}

selection: the random genetic mutations in bacteria and the

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13.16 EVOLUTION CONNECTION: The evolution of

drug-resistant microorganisms is a serious public

health concern

• _{In what ways do we contribute to the problem of antibiotic}

resistance?

• _{Doctors may overprescribe antibiotics.}

• _{Patients may misuse prescribed antibiotics by prematurely stopping the} medication because they feel better.

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13.16 EVOLUTION CONNECTION: The evolution of

drug-resistant microorganisms is a serious public

health concern

• _{Each year in the United States nearly 100,000 people die from}

infections they contract in the hospital, often because the bacteria are resistant to multiple antibiotics.

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13.17 Diploidy and balancing

selection preserve genetic variation

• _{What prevents natural selection from eliminating all variation as it}

selects against unfavorable genotypes?

• _{Why aren’t less adaptive alleles eliminated as the “best” alleles are}

passed to the next generation?

• _{It turns out that the tendency for natural selection to reduce variation}

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13.17 Diploidy and balancing

selection preserve genetic variation

• _{In some cases, genetic variation is preserved rather than reduced by}

natural selection.

• _{Balancing selection}_{occurs when natural selection maintains stable}

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13.17 Diploidy and balancing

selection preserve genetic variation

• _{Heterozygote advantage}_{is a type of balancing selection in which}

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13.17 Diploidy and balancing

selection preserve genetic variation

• _{Frequency-dependent selection}_{is a type of balancing selection that}

maintains two different phenotypic forms in a population.

• _{In this case, selection acts against either phenotypic form if it becomes too} common in the population.

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Figure 13.17

“Left-mouthed”

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13.18 Natural selection cannot

fashion perfect organisms

• _{The evolution of organisms is constrained.}

1. Selection can act only on existing variations. New,

advantageous alleles do not arise on demand. 2. Evolution is limited by historical constraints.

Evolution co-opts existing structures and adapts them to new situations.

3. Adaptations are often compromises. The same structure often performs many functions. 4. Chance, natural selection, and the environment