Sexual Conflict and Sexual Selection

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Sexual Conflict and Sexual Selection

• Bio 378
• 4 April 2002

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Courtship and Mating the old view

• harmonious ventures
• male and female cooperate to propagate respective
  genes
• serving functions of common interest to male and
  female

• synchronize sexual arousal of the sexes
• allow species identification

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Interests (evolutionary sense) of sexes might
conflict over

• choice of mating partner
• provisioning of zygote with food
• caring for eggs and young

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Courtship and Mating sexual selection view

• Conflicts of interest between male and female
• Sexes form uneasy allliance
• Each attempts to maximize own success in
  propagating genes.
• Apparent cooperation ensues because both pass on
  genes via same progeny
• therefore each has 50 stake in survival of
  offspring.

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Exploitation Rather than Mutual Cooperation

• Understand by exploring fundamental difference
  between male and female

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Sexual reproduction gamete formation by meiosis
and fusion of genetic material from two
individuals
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Sexual reproduction gamete formation by meiosis
and fusion of genetic material from two
individuals

• Isogamy
• sexual reproduction without males and females.
• fusion of two gametes of equal size
• E.g. Paramecium.

• Anisogamy
• fusion of 2 gametes of unequal size.
• basic difference between sexes is in size of
  gametes
• Found in all plants and animals

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Female and Male Gametes

• Eggs
• females produce large, immobile, food-rich gametes

• Sperm
• males produce gametes that are tiny, mobile,
  self-propelled DNA

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Anisogamy evolution and consequences

• Anisogamy evolved from isogamy
• smaller than average gametes successfully
  parasitized those that were larger than average.
• Anisogamy determines investment in offspring
• Far-reaching consequences for sexual behavior.

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Male / Female Conflict of Interest

• Males can fertilize eggs at a faster rate than
  females produce them
• What can 5 ml human semen do?
• Opportunity to mate with females is are therefore
  a scarce resource for which males compete

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Parental Investment

• Robert Trivers 1972 relationship between
  investment of resources in gametes and other
  forms of parental care, and sexual competition.
   
• Where one sex invests considerably more than the
  other, members of the latter will compete among
  themselves to mate with members of the former.

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Parental Investment

• Parents invest in producing offspring from a
  limited pool of resources
• Allocation of resources
• to parental care (provisioning of young,
  protection)
• to mating (courtship, competition)
• Ip Em Ep

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Differential Male and Female Investment
1. Ismail the Bloodthirsty, Emperor of
Morrocco 2. 27 pregnancies
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Sex Ratio

• Usually 11 in nature.
• If one male can fertilize eggs of dozens of
  females, why not produce a sex ratio of 1 male/20
  females?

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How to explain 11 sex ratio at level of
individual?

• Fisher 1930
• Suppose population contained 20 females for every
  male.
• Then every male has 20x expected reproductive
  success of female (because there are 20
  mates/male).
• Therefore a parent whose children are exclusively
  sons can expect to have almost 20x the number of
  grandchildren produced by a parent with mainly
  female offspring.
• Possible reason for high value placed on sons in
  some societies