Sex Differences

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Sex Differences
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Biological Sex

• Recombination of genes
• Sexual reproduction involves multiple individuals
• Usually 2
• Male vs Female

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Having a Y Chromosome
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What About
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Definitions of Male Female

• Gender is a social categorization
• Biological categories have to apply to all forms
  of life
• Algae, humans, sea stars, plants

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Biological Definition

• Based on gamete size
• Male
• Small gametes
• Sperm (spermatozoa)
• Female
• Large gametes
• Egg (oocyte)

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Isogamy

• Gametes same size

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Mating types

• Characteristic of isogametic critters
• Algae, fungi, protozoans
• No male/ female

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Mating Types
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Clamydomonas sp
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Anisogamy

• not same gametes
• Gametes different size
• Usually 2 sizes- small large
• Rarely some species have multiple sized gametes
• Chlamydomonas euchlora
• Divides 4-64 times any inbetween
• 4 big gametes
• 64 small gametes

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Anisogamy
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Why 2 sexes?

• Multiple mating types can exist
• Ex Slime molds..15 types
• Fungi /-
• The magic number two
• More than one ensures self does not breed with
  self
• Produces variable offspring
• Inbreeding avoidance
• Smallest number possible to introduce diversity
• Minimizes time invested finding mates

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Why Two Sexes

• Cytoplasm control
• Energy investment strategies

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Cytoplasm Control

• Sperm evolved to prevent mixing of incompatible
  cytoplasm
• Sperm very little other than DNA. Mitochondria
  other organelles from sperm are typically
  excluded
• Selection against incompatibilities in cytoplasm
  fusion drive evolution of a small large gamete

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Evolution of Anisogamy

• Isogamy ancestral condition
• Anisogamy independently evolved many times
• Driving force is energy investment
• Resulted in disruptive selection

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Anisogamy via Disruptive selection
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Reproduction is an energy investment

• Different strategies exist for reproductive
  investment
• Trade-off exists between
• size number of gametes
• Lot of small
• Size quality of gametes
• Few high quality
• Results in a selection force for 2 fertility
  strategies

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The Scenario

• In a population diversity in gamete size exists
  (bell shaped curve)
• Some individuals make small nutrient poor gamete
• Some make an in-between gamete sizes
• Some make large nutrient rich gametes

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Small Nutrient Poor Gamete

• Proto-male
• Pro
• can produce lots of cheap gametes
• Increases number of reproductive events
• Greater reproductive potential
• Greater fitness more individuals in the
  population
• Con
• 2 nutrient poor gametes together reduce survival

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Large Nutrient Rich Gametes

• Proto-female
• Pro
• Offspring produced have greater probability of
  survival
• Con
• Produce fewer gametes
• Fewer reproductive events
• less prevalent in the population

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Some make an in-between gamete sizes

• No selective advantage
• Gamete not big enough to gain selective advantage
  of having lots of nutrients
• Gametes not produced in large enough numbers to
  increase reproductive success

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Proto-Male Strategy

• Once gametes begin to diverge anisogamy itself
  creates pressure that advanced differentiation
• Protomales produced in large numbers
• Success is limited by ability to find rarer
  protofemales
• Selection forces act on Proto-male to increase
  adaptations for finding proto-female

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Proto-Female Strategy

• Reproductive success driven by producing fewer
  high quality gametes

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Consequences

• Males
• Lots of gametes
• Little energy per gamete produced
• Evolution favors mechanisms for encountering
  female
• Adaptations
• Motile
• Locate- egg tracking systems
• Females
• Fewer gametes
• Lots energy per gamete produced
• Evolution favors mechanisms for being encountered
• Adaptations
• Chemical attractants
• Selection

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Coevolution of Males Females
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Male vs Female
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Batemans Principle

• Bateman 1948
• The sex which invests the most in producing
  offspring becomes a limiting resource over which
  the other sex will compete
• Male strategy
• Female strategy
• Males should be eager, females

The female, with the rarest exceptions, is less
eager than the maleshe is coy, And may often be
seen endeavoring for a long time to escape.
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Terminology

• Reproductive effort
• Proportion of the total energy budget of an
  organism that is devoted to reproduction
• Parental Investment (effort)
• Any investment made by parent to individual
  offspring to increase the survivability of the
  offspring
• Mating effort
• Any investment made in by individual to increase
  reproductive event

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Robert Trivers

• 1972
• Expanded Batemans principle to extend beyond
  gamete scope to include all aspects of
  reproductive effort
• Gestation, feeding, protection
• The sex that invests more should be passive
  discriminating

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Consequence of Anisogamy

• Male
• Increased proportion of reproductive effort
  directed into mating effort
• Female
• Increased proportion of reproductive effort
  directed into Parental effort

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Sex Conflict

• Conflict of interest as two sexes reproductive
  strategies change over the course of evolution
• Ex sponges sex strategies similar among sexes
• Vs Peacock peahen

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Effects

• Results in morphological differences
• Results in behavioral differences

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Isogamous Species
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Anisogamous Species