Chapter 11 Sex: Causes and Consequences p. 328


11.1 Evolution of Sex p. 331


Why Sex? p. 331

  • Sex is not the universal rule.
  • Many species reproduce asexually, and for many others sex is optional. So why is sexual reproduction so widespread?
  1. Mechanisms of reproduction are diverse (Fig. 11.2)
    1. Reproduction
    2. Hermaphrodite
  2. Twofold cost of sex (Fig. 11.3)
    1. sexual female
    2. clonal female
  3. Consequences of sexual reproduction (Table 11.1)
    1. Disadvantages
    2. Advantages
  4. Sex creates new genotypes (Fig. 11.4)
    1. Muller's ratchet
    2. Genetic load

The Red Queen Effect: Running in Place p. 332

  • Populations are constantly evolving and adapting in relation to each other
    • when natural selection favors a change in one, no change in the others can result in extinction.
  1. Red queen effect makes sex beneficial (Fig. 11.5)
    1. Parasite/host cycles
  2. Example of red queen effect (Fig. 11.6)
    1. New Zealand snails (Lively, 1992)

A Curious Lack of Sex p. 335

  • Asexual organisms may evolve adaptations that provide benefits similar to those obtained by sexual reproduction.
  1. Persistence of asexual reproduction in Bdelloid rotifers
    1. Enter dormant phase during difficult conditions
    2. Horizontal gene transfer
    3. Shedding of parasites
  2. Key Concepts
    1. Sex is the combining and mixing of chromosomes during offspring production
      1. Meiosis and recombination
      2. Fertilization
    2. Hermaphrodites that self-fertilize reproduce sexually but do not create genetic variation
    3. In models, a strategy is a method of maximizing fitness that is contrasted with alternative methods
  3. Key Concepts
    1. Sex creates new genetic variation by mixing parental alleles
    2. The Red Queen effect has been used to explain the advantages of sex and constant coevolutionary arms races
    3. Ecological situations that require rapid evolution are likely to favor sex
    4. Asexual lineages have evolved mechanisms that compensate for lack of sex


  • Linkage disequilibrium is reduced by sexual reproduction with random mating
  • Many species are capable of both sexual and asexual reproduction (aphids, Volvox, hydra [Fig. 8.16]).
  • parthenogenesis: reproduction from unfertilized eggs

Which Reproductive Mode is Better? Sexual or Asexual

  • null model (Maynard Smith, 1978) has two assumptions
    1. number of offspring a female can produce depends only on the amount of food she can gather, not her mode of reproduction
    2. The probability that an offspring will survive to reproduce does not depend on whether that offspring was produced sexually or asexually
  • Difficulties and Costs Associated with Sexual Reproduction
    1.  costs in time, energy, increased predation risk
    2. sexual reproduction is slower than asexual. The fraction of the population that are asexual females increases every generation. [Fig. 8.17]
    3. costs of meiosis: offspring only share half of each parent's genes.  if the environment is stable the recombinant offspring may not be well adapted.
  • Maynard Smith's first assumption is violated by species in which males contribute to raising offspring. However, in most species, including mammals, males only contribute genes. Maynard Smith's first assumption is correct.
  • Maynard Smith's second assumption is not correct. Under some conditions, descendants produced by sexual reproduction have a higher fitness than descendants produced by asexual reproduction. Experiment with flour beetles shows this [Fig. 8.18].
  • sex reduces linkage disequilibrium.

Genetic Drift, in Combination with Mutation, Can Make Sex Beneficial

  1. asexual reproduction leads to accumulation of deleterious alleles (genetic load), which can cause extinction. 
  2. Mueller's ratchet (Fig. 8.20).
    1. Experimental demonstration (Fig. 8.21): bacterial populations subjected to periodic bottlenecks over 1700 generations.
    2. Muller's ratchet is long-term, requires many generations to accomplish

Selection Imposed by a Changing Environment Can Make Sex Beneficial


  • To avoid extinction, a population must constantly evolve in response to the changing environment: physical and biotic pressures (predation, parasites, competition, availability of prey or food plants, etc.).
  • Sex is a short-term adaptation to rapid environmental change.
  • Sex recreates now-favorable multilocus genotypes that were recently eliminated by selection and sexual reproduction is therefore strongly selected for.
  •  Evolutionary Arms race
    • Parasites and their hosts are engaged in a constant struggle, with the host evolving defenses and the parasite evolving mechanisms to counter the host's defenses. A parasite might select in favor of a certain multilocus genotype in one generation, and in favor of a different multilocus genotype in another generation. [Fig. 8.22] 
  • Parasites have been suggested as selective agents for sex. A higher proportion of female snails (Potamopyrgus antipodarum, in New Zealand) are of the sexual type in populations with a high incidence of parasitism from trematodes. [Fig. 8.23]

11.2 Sexual Selection p. 336

  • Cheap Sperm and Costly Eggs p. 336

    In many species, investment in gametes varies significantly between males and females.
  1. Anisogamy results in differential investment in reproduction  (Figure 11.8)
  2. Limitations on reproductive success differ for the sexes
    1. Females are limited by fecundity
    2. Males are limited by the number of mates they can obtain  (Figure 11.9)

 Unequal Investment Results in Sexual Selection p. 337

  • The sex that invests the most in the production of offspring will often become limiting, so that members of the opposite sex must compete to mate with them.
  1. Investment differences can extend past fertilization  (Figure 11.10)
  2. Uncertain paternity may explain why male parental care is rare
    1. females have certain paternity
    2. Males have uncertain paternity
      1. Parental care could be directed toward offspring not their own
  3. Asymmetrical parental care alters operational sex ratio
    1. Operational sex ratio: ratio of males to females capable of reproducing at a given time
    2. Slower rate of reproduction by females leads to male biased OSR
  4.  Sexual selection
    1. Differential reproductive success resulting from competition for mates
      1. Intrasexual selection
      2. Intersexual selection
    2. Sexual selection typically stronger on males
      1. Maximize fitness by mating multiply
      2. Male biased OSR

Male–Male Competition: Horns, Teeth, and Tusks p. 339

  • In species where the operational sex ratio is skewed, the abundant sex can try to monopolize females or resources using powerful weapons and displays.
  1.  Sexual dimorphism results from sexual selection (Fig. 11.11)
    1. Ornaments: attractive traits that increase mating success
    2. Armaments: weaponry used to outcompete other individuals
  2. Male-male competition leads to extreme variance in reproductive success (Fig. 11.13)
  3. Competing for mates is costly  (Fig. 11.14)
  4. Males may compete for territory  (Fig. 11.16)

Males Bearing Gifts p. 344

  •  Humans aren’t the only species that give nuptial gifts—males try to entice females to mate with food—sometimes at their peril.
  1. Benefits of female choice
    1. Direct benefits: benefit the female directly: e.g. food, nest sites, protection
    2. Indirect benefits: benefits that affect the genetic quality of the female’s offspring
  2.   Direct benefits  (Fig. 11.17)
  3. Females may benefit from cannibalism  (Fig. 11.8)
  4. Voluntary self-sacrifice in redback spiders  (Fig. 11.19)
  5. Direct benefits of female choice (Table 11.2)

Dancing Males and Showy Ornaments p. 345

  • Females often exhibit strong preferences for certain ornamental traits in males, preferences that get their start from preexisting sensory biases.
  1. Males may display elaborate ornaments (Fig. 11.20)
  2. Female preferences are often consistent   (Fig. 11.21)
  3. Female preferences are often consistent   (Fig. 11.22)
  4. Female preferences may arise from preexisting sensory bias  (Fig. 11.23)
  5. Indirect benefits of female choice (Table 11.3)
  1. Sexual dimorphism
    1. Males and females differ markedly in size, appearance, and behavior.
    Males and females differ markedly in size, appearance, and behavior.
  2. Asymmetries in Sexual Reproduction
    1. Parental Investment
      1. Selection pressures for females versus males.
      2. Female versus male mammals.
        1. Orangutans as an example (Fig. 11.4)
      3. Females versus male reptiles, insects, etc. 
    2. Female's reproductive success  
    3. Male's reproductive success  
    4. This is predicted to produce a fundamental asymmetry:
      1. Access to females will be a limiting resource for males,
      2. but access to males will not be a limiting resource for females.
    5. Bateman's experiments with fruit flies, Drosophila melanogaster


  1. Combat
    1. Intrasexual selection involving male-male combat
    2. Galapagos marine iguanas (Fig. 11.8 )
      1. sexual size dimorphism: Males are larger than the females. 
      2. Natural selection on body size.  (Fig. 11.9) 
      3. Mating Systems of Galapagos marine iguanas (Wikelski, Trillmich and colleagues)
        1. Combat for access to mates favors large body size in males (Fig. 11.10)
          1. Males attempt to copulate with many different females
          2. huge variation in male mating success (fig. 11.11)
          3. Before the breeding season, males compete in direct combat (Fig. 11.10) with each other for territories on basking sites (Fig. 11.11)
          4. In general, calculated selection differentials of 0.42 and 0.77 (Table 11 .1) favoring larger males on two different islands.

    Box 11.1 Alternative Mating Strategies

    1. Sneaky Males
      1. Jacking in coho salmon (Fig 11.13)


    1. Infanticide occurs in a variety of mammals, notably lions and some primates.
      1.  Conditions favoring infanticide
    2.  Case study: infanticide in African lions (Fig. 11.16 )


Female Choice in Red-Collared Widowbirds

  1. Long tail feathers are a ball-and-chain for male red-collared widowbirds ( Figure 11.18 )
  2. Female red-collared widowbirds prefer long-tailed males ( Figure 11.19 )

Female Choice in Gray Tree Frogs

  1. During the breeding season, male gray tree frogs produce four kinds of mating calls
  2. Females preferred long calls to short calls and fast calls to slow calls (Fig. 11.21 ).
  3. Two potential benefits to a choosy female include:
    1. The acquisition of good genes for her offspring.
    2. The acquisition of resources offered by males.

Choosy females may get better genes for their offspring

  1.  Gray tree frogs giving long calls are genetically superior to males giving short calls (Fig 11.22 )
  2. Five aspects of offspring performance related to fitness in offspring of long-calling versus short-calling frogs (Table 11.2 ).

Choosy females may benefit directly through the acquisition of resources

  1. Males may provide food, parental care, or some other resource
  2. Example: the hangingfly (Bittacus apicalis)  (Figs. 11.23, 11.24 ).

Choosy Females May Have Preexisting Sensory Biases

  1. Discussion
  2. Example Water mite (Neumania papillator) [Fig. 11.25 ]
    1. If there is a preexisting sensory bias hypothesis, net-stance evolved before male trembling. [Fig. 11.26 ]

 Other Explanations of Female Choice

  • Female choice may be arbitrary. 

     11.3 The Rules of Attraction p. 348

    • Male traits may signal good genes, or they may result from an arbitrary preference by females, or some combination of the two.
    1. Fisher’s runaway: preference and trait genetically correlated  (Fig. 11.24)
    2. Ornaments can serve as handicaps  (Fig. 11.25)
    3. Development of weapons can involve trade-offs  (Fig. 11.26)

    11.4 The Evolution of Mating Systems p. 354

    • Monogamous, polygynous, and polyandrous mating systems can influence the reproductive success of both males and females.
    1. Types of mating systems
      1. Monogamy: one male pairs with one female
        1. Sexual: partners mate with each other exclusively –Social: partner pair but may cheat
      2. Polygyny: males mate with multiple females
      3. Polyandry: females mate with multiple males
    2. Polyandry selects for male traits that increase paternity (Fig. 11.28)

    11.5 Sperm Wars p. 355

    • Females may have more control of the sperm that fertilizes their offspring than may be outwardly apparent
    1. Sperm competition drives evolution of larger testes (Fig. 11.29)
    2. Discriminating sperm in deer mice (Fig. 11.30)
    3. Evolution of giant sperm in Drosophila (Fig. 11.31)
    4. Key Concepts
      1. Polyandry leads to sperm competition among males

    Sperm competition

    1. Definition 
    2. Sperm competition can favor several different kinds of traits:
      1. large ejaculates
      2. mechanisms to prevent other males from inseminating female
        1. Prolonged copulation and/or other mate guarding
        2. copulatory plugs
        3. Males may apply pheromones that reduce female's attractiveness.
        4. 'sperm scoopers'  (Fig. 11.15 )
        5. Altruistic Sperm in Woodmice
          1. Sperm "hook up" into fast trains (Fig. 12.9d)

    11.6 Sexual Conflict and Antagonistic Coevolution p. 357

    • Some adaptations in males of a species reduce the fitness of females, while some of the characters that increase fitness of females reduce the fitness of males. This sexual conflict can yield extraordinary selection pressure and lead to unique adaptations.
    1. Sexual conflict results in antagonistic coevolution
      1. Sexual conflict: traits that confer a fitness benefit on one sex but cost to the other
      2. traits coevolve antagonistically
    2. Sexual conflict in ducks (Fig. 11.32)
    3. Evidence for antagonistic coevolution in Drosophila (Fig. 11.33)
    4. Key Concepts
      1. Sexual conflict leads to antagonistic coevolution between males and females




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