Sex - University of Kentucky

What is sex?
Cost/Benefits of Sex
What defines the sexes?
Sexual Origins
Sexual Conflict
Male-male Competition
Female Choice
Cryptic Female Choice
*Sperm Competition
• Where is this leading?
What is Sex?
• Sex: Meiosis and Syngamy
– Meiosis - Reduction
– Syngamy - Fusion
• Fusion of gametes
• 2 kinds:
– Isogamy- protists, fungi, algae
• costs
– Anisogamy
• beneficial
Benefits of Having Sex
• Traditional: increase the rate of adaptive
evolution --> decreases extinction risk
• Creates genetic variation
• spread of advantageous traits and elimination of
deleterious genes
• Resistance to parasites (Red Queen hypothesis)
• Repair of damaged DNA via recombination
Costs of Sex
• Only one sex can bear young
– Asexual population: both “sexes” can bear
young, so population increases more
• Males and females must find each other
to mate
• Predation risk
What defines the sexes?
• Gamete size!
– Larger gamete = Female
• Increase zygote size
– Zygote fitness
– Smaller gamete = Male
• Get more for less, just like Walmart!
• This defines Anisogamy
– There is variation in gamete size though…
Giant Sperm
Drosophila bifurca
Sexual Origin
Qu i c k Ti m e ™ a n d a
TIF F (Un c o m p re s s e d ) d e c o m p re s s o
a re n e e d e d t o s e e th i s p i c tu re .
• Giardia lamblia
– Very primitive group
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Sexual Conflict
Male-Male Competition
Female Choice
Cryptic Female Choice
Sperm Competition
Male-Male Competition
• Can lead to extravagant characters and
sexual dimorphism
• Territory size and quality
• Ornaments vs. Armaments
• Mate Gaurding
Female Choice
• Females preferentially mate with males
that have larger or more intense
exaggerated characters
• Morphological characteristics, display
behaviors, territory size, nuptial gifts
• Honest indicators, sensory bias
Indirect Benefits of Mate
• Sexy sons: female preferences are
• Runaway selection
• Good Genes
• Handicap Principle
Cryptic Female Choice
• Female chooses which sperm will
fertilize her eggs after mating with
several males
• Denial of deeper genetalic access
• Discharge or digestion of sperm
• Breaking copulation
Cryptic Female Choice
• If females do these things non-randomly
then it is cryptic female choice
Sperm Competition
• Sperm from two or more males compete
inside the female
– Calopteryx
• Number of Sperm
• Sperm Quality
• Mating Order
Sperm Competition
• Sperm Displacement
– Producing more sperm
– Copulatory organ modifications
• Humans
– Genital plugging and removal
Where is this leading us?
• Mating Systems
– Monogamy
– Polyandry
– Polygyny
– Promiscuity
• Life History Evolution

similar documents