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Sex Differences. Biological Sex. Recombination of genes Sexual reproduction involves multiple individuals Usually 2 Male vs Female What determines male and female?. In ye old days…. Aristotle Sex of infant determined by how much heat was in a man’s sperm.

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Sex Differences

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biological sex
Biological Sex
  • Recombination of genes
  • Sexual reproduction involves multiple individuals
    • Usually 2
      • Male vs Female

What determines male and female?

in ye old days
In ye old days…
  • Aristotle
    • Sex of infant determined by how much heat was in a man’s sperm
having a 2 of the same chromosome
Having a 2 of the same Chromosome?

Edmund Beecher Wilson & Nettie Stevens 1905

chromosomal XY sex-determination system

  • Mary Lyon 1962
  • In Females each somatic cell has 2 X chromosomes
    • One X chromosome is inactivated
    • Random inactivation
  • Shortened and condensed
  • Females are genetic mosaics
  • Occurs only in females
  • X chromosome has fur color control gene
  • 2 alleles
    • Black & Orange
  • White results from genetic condition of unpigmented hair
what about
What About…
  • Genotypic sex determination
    • Birds & mammals
  • Homogametic
    • Humans female
  • Heterogametic
    • Human male
    • Bird female
temperature dependent sex determination
Temperature-dependent Sex Determination
  • Temperature eggs exposed to during development determines biological sex of offspring
  • Thermosensitive period during development
  • Males produced typically at lower incubation temperatures
    • 22.5-27C
  • Females higher temperatures
    • 30C females
  • Lizards and crocodiles opposite pattern
american alligator
American Alligator
  • Females result from eggs incubated at both high and low temps
  • Males develop at intermediate temperatures
genotypic sex determination
Genotypic Sex Determination
  • Humans exhibit heterogametic genotypic sex determination
  • All embryos have genes for both testes and ovary production
    • Female is the “Default” sex
  • Presence of a Y chromosome will stimulate the embryo to develop into a male offspring
genotypic sex determination13
Genotypic Sex Determination
  • Sex determining region of Y chromosome
    • SRY gene complex
  • Gene expression of SRY gene will result in the production of Testes determining factor (TDF)
testes determining factor
Testes Determining Factor
  • TDF triggers activation of genes found on somatic chromosomes
    • ~7 weeks after fertilization
  • Activated genes responsible for the development of testes
  • Developing testes will produce Mullerian inhibiting substance
    • Causes regression of female gonads
definitions of male female
Definitions of Male & Female
  • Gender is a social categorization
  • Biological categories have to apply to all forms of life
    • Algae, humans, sea stars, plants
biological definition
Biological Definition
  • Based on gamete size
  • Male
    • Small gametes
    • Sperm (spermatozoa)
  • Female
    • Large gametes
    • Egg (oocyte)
  • Gametes same size
mating types
Mating types
  • Characteristic of isogametic species
  • Differences in cell surfaces
  • Algae, fungi, protozoans
  • No male/ female
  • “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
why 2 sexes
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
why two sexes
Why Two Sexes
  • Cytoplasm control
  • Energy investment strategies
cytoplasm control
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
evolution of anisogamy
Evolution of Anisogamy
  • Isogamy ancestral condition
  • Anisogamy independently evolved many times
  • Driving force is energy investment
    • Resulted in disruptive selection
reproduction is an energy investment
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
the scenario
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
  • Males
    • Lots of cheap gametes
    • Little energy per gamete produced
      • Increases reproductive potential
  • Females
    • Fewer gametes lowers reproductive potential
    • Lots energy per gamete produced
      • Offspring produced have greater probability of survival
    • less prevalent in the population
  • Males
    • Evolution favors mechanisms for encountering female
    • Adaptations
      • Motile
      • Locate- egg tracking systems
  • Females
    • Evolution favors mechanisms for being encountered
    • Adaptations
      • Chemical attractants
      • Selection
what happens when there is no hope
What happens when there is no hope….
  • Fruit fly’s that mated repeatedly for several days no preference for alcohol
  • Males without access to females strongly preferred food mixed with alcohol
  • Alcohol satisfied desire for physical reward
bateman s principle
Bateman’s Principle

Mated fruit fly’s in monogamous & promiscuous groups

Male reproductive success increases with number of mates

Female reproductive success does not increase with multiple mates

bateman s principle34
Bateman’s 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
      • Promiscuous
    • Female strategy
      • Choosy
    • Males should be eager, females…

“The female, with the rarest exceptions, is less eager than the male…she is coy, and may often be seen endeavoring for a long time to escape.”

robert trivers
Robert Trivers
  • 1972
  • Expanded Bateman’s 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
  • 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
consequence of anisogamy
Consequence of Anisogamy
  • Male
    • Increased proportion of reproductive effort directed into mating effort
  • Female
    • Increased proportion of reproductive effort directed into Parental effort
sex conflict
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
  • Results in morphological differences
  • Results in behavioral differences